A NEW SPECIES OF HATHLIACYNIDAE (METATHERIA, SPARASSODONTA) FROM THE
MIDDLE MIOCENE OF QUEBRADA HONDA, BOLIVIA

ANALÍA M. FORASIEPI1,2, MARCELO R. SÁNCHEZ-VILLAGRA3, FRANCISCO J. GOIN4, MASANARU TAKAI5,
NOBUO SHIGEHARA5, and RICHARD F. KAY6

1Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, 500 S. Preston,
Louisville, Kentucky 40292, U.S.A., amfora01@gwise.louisville.edu;

2Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’ Avenida Angel Gallardo 470, Capital Federal, 1405,
Buenos Aires, Argentina, borhyaena@hotmail.com;

3The Natural History Museum, Department of Palaeontology, Cromwell Road, London SW7 5BD, England,
m.sanchez_villagra@nhm.ac.uk;

4Departamento Paleontología de Vertebrados, Museo de La Plata, Paseo del Bosque s/n, La Plata, 1900,
Buenos Aires, Argentina, fgoin@museo.fcnym.unlp.edu.ar;

5Primate Research Institute, Kyoto University, Inuyama, Aichi 484, Japan, takai@pri.kyoto-u.ac.jp;
6Department of Biological Anthropology and Anatomy, Duke University, Medical Center, Durham,

North Carolina 27710, U.S.A., rich_kay@baa.mc.duke.edu

ABSTRACT—A new species of Hathliacynidae (Sparassodonta, Metatheria), Acyon myctoderos, from the middle Mio-
cene of Quebrada Honda, Bolivia, is described. This new species is the largest known hathliacynid. Compared to the type
species of the genus, A. tricuspidatus, Acyon myctoderos differs in having: (1) longer diastemata among premolars; (2) p2
comparatively more robust, with a better developed posterior cusp and with a sharp anterior crest; (3) lower molars with
a more poorly developed anterobasal cingulum; (4) m1–m3 with hypoconulids less salient posteriorly and more vertically
oriented; and (5) larger hypoconids at least on the m2. A phylogenetic analysis including nine taxa of Sparassodonta, with
Mayulestes as the outgroup, showed that Acyon is more closely related to Cladosictis than to any other hathliacynid.

INTRODUCTION

The extinct metatherian fauna of South America was very rich
and diverse and included forms with ecological counterparts oc-
cupied by eutherians on the northern continents (Marshall,
1977a; Simpson, 1980, among others). This extinct fauna in-
cluded insectivorous to omnivorous didelphimorphians, frugivo-
rous paucituberculatans and polydolopimorphians, and carnivo-
rous sparassodonts. The ecological significance of the sparas-
sodonts is that they were the only group of mammalian
carnivores in South American during most of the Tertiary, when
this continent was isolated from other landmasses (e.g., Marshall,
1977a; Simpson, 1980).

The fossil record of the Sparassodonta extends from the early
Paleocene to the late Pliocene (Marshall, 1978; Simpson, 1980;
Muizon, 1998). Among them, the Hathliacynidae represents one
of the most generalized groups (Marshall, 1981; Muizon, 1999).
Hathliacynids were of small to medium size, with a slender body
form compared with other sparassodonts, and probably having
scansorial (i.e., partly arboreal and partly terrestrial) life habits
(Argot, 2003, 2004). Hathliacynids have been reported from Ar-
gentina, Brazil, and Bolivia (e.g., Ameghino, 1887, 1891; Paula
Couto, 1952; Marshall, 1981; Villarroel and Marshall, 1982, 1983;
Hoffstetter and Petter, 1983; Muizon, 1999, among others). The
most comprehensive review of this family (Marshall, 1981) rec-
ognized 21 species (three of them are probably nomina vana).
Except for Cladosictis patagonica and Sipalocyon gracilis (both
Miocene), which are represented by almost-complete skeletons
(Sinclair, 1906), most of the remaining recognized species are
known only by dental and fragmentary cranial remains.

The fossil-bearing Quebrada Honda Formation is exposed in
the vicinity of the settlement of Quebrada Honda, southern Bo-
livia, between 21°53�–22°05� south latitude and 65°05�–65°10�
west longitude (Fig. 1). Quebrada Honda is located in the De-

partment of Tarija, about 65 kilometers southwest of the depart-
ment capital Tarija, and 20 km north of the Argentine frontier. It
is at an elevation of about 3500 meters above sea level. The
fossil-bearing exposures in the valley of the Río Honda and its
tributaries occur principally, but not exclusively, in two drain-
ages, the Quebrada Honda and Quebrada El Rosario. The ap-
proximately 360-m-thick section of red clays and silts with inter-
calated tuffs referred to the Quebrada Honda Formation were
deposited in a small intermontane sedimentary basin just west of
the principal Andean thrust in the Bolivian Eastern Cordillera.
The sediments rest on strongly folded and partially metamor-
phosed Paleozoic (Cambrian) basement (Troëng et al., 1993).

MacFadden et al. (1990) reported two conventional K/Ar
dates and a local magnetic polarity stratigraphy for the Que-
brada Honda section. A preferred age of 12.83 +/− 0.07 Ma for
the tuff at the top of the fossil-bearing portion permits correla-
tion of the fossil-bearing N1 and R1 local polarity intervals to
Chrons C5Ar.3r and C5Ar.2n, calibrated to 12.991–12.775 Ma
using the GMPTS of Berggren et al. (1995). Thus, based on
published K/Ar ages and local magnetic polarity stratigraphy,
the fossil mammals from Quebrada Honda have an extrapolated
age of between 13.0 and 12.8 Ma, corresponding to the Laventan
South American land mammal age in Colombia (Madden et al.,
1997).

The relatively rich mammalian assemblage recovered in Que-
brada Honda includes the following groups: Metatheria (Borhy-
aenidae, Caenolestidae, Argyrolagidae), Xenarthra (Dasypodi-
dae, Glyptodontidae, Mylodontidae, Megalonychidae), Notoun-
gulata (Hegetotheriidae, Interatheriidae, Mesotheriidae,
Toxodontidae), Astrapotheria (Astrapotheriidae), Litopterna
(Protherotheriidae, Macraucheniidae), and Rodentia (Octodon-
tidae, Echymidae, Capromyidae, Chinchillidae, Caviidae) (Hoff-
stetter, 1977; MacFadden and Wolff, 1981; Marshall and Sem-
pere, 1991). The occurrence of sparassodonts in Quebrada

Journal of Vertebrate Paleontology 26(3):670–684, September 2006
© 2006 by the Society of Vertebrate Paleontology

670

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Honda was first mentioned in a faunal list by MacFadden and
Wolff (1981); this record was later repeated by Marshall and
Sempere (1991), but no description of a specimen was provided.
Here we describe a new specimen, different from that mentioned
previously, consisting of a well-preserved skull and partial post-
cranial skeleton belonging to a new species of the genus Acyon
(Hathliacynidae), and analyze its phylogenetic relationships.

The nomenclature in the descriptions is largely after Muizon
(1998, 1999; for dental, cranial, and postcranial terminology),
and Wible (2003; for cranial terminology). The comparisons with
other sparassodonts are based on the specimens cited in Appen-
dix 1 and in the referred literature. The phylogenetic analysis,
largely based on characters mentioned by Muizon (1999), is re-
stricted to sparassodonts and focused on hathliacynids.

Abbreviations

CORD-PZ, Museo de Paleontología, Facultad de Ciencias
Exactas, Físicas y Naturales de la Universidad Nacional de Cór-
doba, Córdoba, Argentina; FMNH, Field Museum of Natural
History, Chicago, Illinois; MACN A, Ameghino collection, Mu-
seo Argentino de Ciencias Naturales ‘Bernardino Rivadavia,’
Buenos Aires, Argentina; MLP, Museo de La Plata, Buenos
Aires, Argentina; MNHN-Bol, Museo Nacional de Historia
Natural, La Paz, Bolivia; SALMA: South American land mam-
mal age; UCMP, University of California Museum of Paleontol-
ogy, Berkeley, California. Capital and lower case letters, I/i, in-
cisor, C/c, canine, P/p, premolar, and M/m, molar, refer to upper
and lower teeth, respectively. Incisor nomenclature followed
here is I1, 2, 3, 4, and i1, 2, 3 from front to back, without con-
sidering the homologies proposed by Hershkovitz (1982).

SYSTEMATIC PALEONTOLOGY

Subclass METATHERIA Huxley, 1880
Order SPARASSODONTA Ameghino, 1894

Family HATHLIACYNIDAE Ameghino, 1894
ACYON Ameghino, 1887

Acyon Ameghino, 1887:8.
Anatherium Ameghino, 1887:8 (partim).
Hathliacynus Mercerat, 1891 (nec Ameghino, 1887).
?Ictioborus Ameghino, 1894 (partim).
Cladosictis Cabrera, 1927 (nec Ameghino, 1887) (partim).

Type Species—Acyon tricuspidatus Ameghino, 1887:8.
Included Species—A. tricuspidatus Ameghino, 1887, and A.

myctoderos sp. nov.; Acyon herrerae (Marshall, 1981) could rep-
resent a junior synonym of A. tricuspidatus (see below).

Distribution and Age—South America (Argentina and Bo-
livia). Early to middle Miocene (Colhuehuapian, Santacrucian,
and Laventan SALMA).

Diagnosis—(Modified from that of Anatherium in Marshall,
1981): largest known hathliacynid (length of skull up to 180 mm;
for other hathliacynids see Marshall, 1981); dentary long, low,
and gracile; large diastemata separating p1 from canine and p2;
premolars strongly laterally compressed; talonids of m1–m3
without entoconid (autapomorphy, character 45).

Comments—The early Miocene (Santacrucian SALMA) spe-
cies, Acyon tricuspidatus, was described by Ameghino (1887)
based on a right mandibular fragment (MLP 11-64) with the last
premolar and almost complete molars. Mercerat (1891) referred
MLP 11-64 to Hathliacynus tricuspidatus, whereas Cabrera
(1927) allocated the same specimen to Cladosictis tricuspidatus,
as listed in the old catalog of the Museo de La Plata. In his review
of the hathliacynids, Marshall (1981) regarded this species as a
junior synonym of Anatherium defossus Ameghino, 1887. He
included the following specimens in the hypodigm of Anatherium
defossus: MACN A 669 (type of Anatherium defossus), MLP
11-64 (type of Acyon tricuspidatus), MACN A 9 (type of Acyon
?bardus), MACN A 5988 (type of Ictioborus destructor), and
MACN A 646. Marshall (1981:103) also recognized a new spe-
cies, Anatherium herrerae, on the basis of only one specimen
(FMNH P 13521) from Colhuehuapian beds (early Miocene) at
Gran Barranca (Chubut Province, Argentina).

Review of the specimens mentioned above leads us to con-
clude that: (1) specimens MACN A 9, MACN A 646, and
MACN A 669 (type of Anatherium defossus) are referable to
Cladosictis sp., and, consequently, the genus Anatherium repre-
sents a junior synonym of Cladosictis; (2) specimen MACN A
5988 is only tentatively referable to Acyon tricuspidatus; and (3)
Anatherium herrerae Marshall, 1981, is referable to the genus
Acyon (see the generic diagnosis of Acyon), and is very close or
probably conspecific with Acyon tricuspidatus. Therefore, we
recognize Acyon as a valid genus, including two species, A. tri-
cuspidatus Ameghino, 1887 (Santacrucian SALMA), and the
new species from the Laventan SALMA described below.

ACYON MYCTODEROS, sp. nov.
(Figs. 2–9)

Type and Only Specimen—MNHN-Bol-V-003668, a skull
with associated lower jaws and postcrania. The left side of the
skull is almost complete, including a portion of the left petrosal;
in contrast, the right side of the skull is partially broken. Both
lower jaws are preserved: the left is mostly complete except for
the incisor crowns and part of the coronoid process. The post-
cranial elements include the atlas, axis, four isolated vertebral
bodies, cuboid, ectocuneiform, a probable entocuneiform, and
five fragmentary metapodials.

Locality and Age—Papachacra, Quebrada Honda, Bolivia.
Laventan SALMA, middle Miocene (between 13 and 12 Ma;
MacFadden et al., 1990; Madden et al., 1997). The specimen was
collected in 1995 by a joint expedition of the Kyoto University
and the Museo Nacional de Historia Natural of La Paz led by M.
Takai and bears the field number ‘Takai 3668.’

Derivation of Name—From the Greek mykter, nose, and
deros, long, in reference to the long snout.

Diagnosis—Largest species of the genus (A. myctoderos ap-
proximately 5% larger than A. tricuspidatus, based on measure-
ments of the dentition; Table 1; Marshall, 1981:Table 25) and
largest know hathliacynid; differs from A. tricuspidatus (Cabrera,
1927:fig. 6) in having longer diastemata among premolars; p2
comparatively more robust and with a better-developed poste-
rior cusp; less-developed anterobasal cingulum on lower molars;
hypoconulid less salient posteriorly and more vertically oriented
on m1–m3; hypoconids larger, at least on m2.

FIGURE 1. Geographic location of Quebrada Honda, in southern
Bolivia.

FORASIEPI ET AL.—NEW HATHLIACYNID FROM BOLIVIA 671

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https://www.researchgate.net/publication/284682760_Review_of_the_Hathlyacyninae_an_extinct_subfamily_of_South_American_dog-like_marsupials?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==
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DESCRIPTION

Skull—The skull is preserved in several separate portions
(Figs. 2–7). The main fragment of the skull is formed by the left
portion of the snout with the dentition and part of the associated
right nasal bone, the almost complete posterior hard palate, left
orbital region, part of the left zygomatic arch, left glenoid cavity,
and the anterior portion of the basicranium. Most of the roof of
the choanal region and the lateral side of the skull are fractured;
hence, anatomical features and sutures between bones are diffi-
cult to distinguish in these areas. The other fragments of the skull
preserve the right posterior portion of the premaxilla associated
with the right maxilla and the canine-postcanine dentition, the
endocranial portion of the left petrosal bone, and a portion of the
occiput with the left occipital condyle and the surrounding basi-
cranium.

The skull is fox-like in shape, with the preorbital and postor-
bital regions particularly elongated, as is characteristic of the
Hathliacynidae (Figs. 2–6). The snout is long and narrow; at the

level of the diastema between P1 and P2 there is a shallow con-
striction, clearly seen in dorsal view. Posterior to P2 the snout
widens to the level of M3–M4, where the base of the zygomatic
arch is located. The postorbital processes are prominent and
placed almost at the middle of the anteroposterior length of the
skull. The postorbital constriction is very pronounced and lo-
cated on the posterior two-thirds of the skull. The braincase is
narrow, being at its most posterior extent as wide as the skull at
the level of the postorbital processes.

Both nasals are preserved; the left is almost complete but the
right is broken posteriorly. The nasal contacts laterally (from
anterior to posterior) the premaxilla, maxilla, lacrimal, and fron-
tal. There is a wide contact between the nasal and the lacrimal
bones. The suture between the nasals and frontals has a wide ‘W’
shape, with an anterior mid-sagittal wedge of the frontals (Fig.
2). The anterior two-thirds of the nasals are narrow with parallel
lateral borders. At a point approximately dorsal to P2, they flare
laterally, reaching their maximum width at the level of the suture
with the lacrimal. The anterior-most edges of the nasals are bro-

TABLE 1. Measurements (in mm) of the upper and lower postcanine dentition of Acyon myctoderos sp. nov. MNHN-Bol-V-003668.

LP1 WP1 LP2 WP2 LP3 WP3 LM1 WM1 LM2 WM2 LM3 WM3 LM4 WM4

6.40 2.70 8.40 3.00 10.05 4.35 11.85* 7.45* 10.6* 3.05 10.85 3.15 5.50 7.20
6.20 2.90 8.00 3.00 10.04 4.30 10.04* 5.07* — — — — — —

Lp1 Wp1 Lp2 Wp2 Lp3 Wp3 Lm1 Wm1 Lm2 Wm2 Lm3 Wm3 Lm4 Wm4

7.75* 2.85 7.95 3.40 9.95 4.05 8.80 3.40 10.05 4.45 10.70 4.85 12.80 5.65
7.70 3.00 — 3.55 9.85 3.85 9.60 3.60 9.70 4.45 11.50 5.15 12.10 5.65

*Indicates that the measurement is approximate. First row of measurements is the left dentition; the second row of measurements is the right
dentition. Abbreviations: L, anteroposterior length; W, mediolateral width.

FIGURE 2. Acyon myctoderos, MNHN-Bol-V-003668. Stereopair photographs and line-drawing reconstruction of the skull in dorsal view. Ab-
breviations: ap, ascending process of the premaxilla; FR, frontal; JU, jugal; LA, lacrimal; lc, lambdoidal crest; MX, maxilla; NA, nasal; PA, parietal;
pn, precanine notch; pop, postorbital process; PX, premaxilla; sc, sagittal crest; sf, subsquamosal foramen; SQ, squamosal; tc, temporal crest. Scale
bar equals 50 mm.

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 3, 2006672



ken; based on the condition observed in most sparassodonts
(e.g., Cladosictis, Borhyaena, and Arctodictis), they probably did
not overhang the external nasal aperture (versus the condition in
didelphids).

The left premaxilla is almost complete, but the right one is
known only by its most posterior portion. In lateral view the
premaxilla is seen to contact the nasal dorsally and the maxilla
laterally; in ventral view, it contacts the maxilla posteriorly and
the opposite premaxilla in the midsagittal plane. In lateral view
(Figs. 4, 6A), the premaxilla has a roughly triangular shape, wide
in the alveolar border and acute dorsally; both anterior and pos-
terior borders are slightly concave posteriorly. The ascending
process of the premaxilla (sensu Muizon, 1998) is thin and inter-
posed between the nasal and the maxilla. It projects posteriorly
to the level of the posterior border of the canine. The paracanine
fossa (Figs. 4, 6A) is formed exclusively by the premaxilla as in
other Sparassodonta, but differs from that of Mayulestes and
didelphids in which the anterolateral process of the maxilla con-
tributes to the formation of this fossa (Muizon, 1998, 1999). A
low ventrally concave crest borders the paracanine fossa dor-
sally. In dorsal view (Fig. 2), the precanine notch (i.e., the con-
striction between the posterior incisor and the canine; Muizon,
1999) is well developed, as in Cladosictis and unlike Sipalocyon
among Hathliacynidae (Muizon, 1999). The anterior border of
the premaxilla is flat and almost straight, differing at least from
Mayulestes (Muizon, 1998) and Sipalocyon (see MACN A 691–
703 and 5439–5449), where it is parabolic. Only three complete
alveoli for the incisors are preserved in the anterior border of the

premaxilla; however, there is a significant space between the
medial-most preserved alveolus and the sagittal plane. In addi-
tion, on the medial-most border of the preserved part of the
premaxilla there is a small concavity facing anteriorly that sug-
gests the presence of the alveolus for a fourth tooth closer to the
mid-sagittal plane. In ventral view (Fig. 3), the palatine portion
of the premaxilla forms approximately the anterior 10% of the
hard palate. The most posteromedial portion of the left premax-
illa is broken, but the information provided by the right element
helps in the reconstruction of this area (Fig. 3). The incisive
foramen is oval and anteroposteriorly elongated. It is bordered
by the premaxilla anteriorly, laterally, and medially, and by the
maxilla posteriorly. The medial palatine process of the premax-
illa is a narrow bridge of bone interposed between the incisive
foramen and the midsagittal plane. The most posterior portion of
this process is broken on both sides of the skull; therefore, its
extension and the exact position of the suture with the maxilla
can not be located.

The maxilla is known by the left and right elements. In lateral
view (Figs. 4, 6A), the maxilla contacts the premaxilla anteriorly,
the nasal dorsally, and the lacrimal and jugal posteriorly. The
maxilla does not contact the frontal, as occurs in primitive met-
atherians (Rougier et al., 1998). In ventral view (Fig. 3), the
maxilla contacts the premaxilla anteriorly, the palatine posteri-
orly, and the maxilla of the opposite side in the mid-sagittal
plane; in the orbital region the sutures are difficult to distinguish.
In lateral view (Figs. 4, 6A), the maxilla is considerably longer
than high, similar to other hathliacynids, but differing from

FIGURE 3. Acyon myctoderos, MNHN-Bol-V-003668. Stereopair photographs and line-drawing reconstruction of the skull in ventral view.
Abbreviations: AS, alisphenoid; atp, alisphenoid tympanic process; BO, basioccipital; BS, basisphenoid; eam, external acoustic meatus; EC, ecto-
tympanic bone; fm, foramen magnum; fo, foramen ovale; gf, glenoid fossa; hf, hypoglossal foramen; if, incisive foramen; jf, jugular foramen; JU, jugal;
mpf, minor palatine foramen; mpp, medial palatine process; MX, maxilla; oc, occipital condyle; pcp, paracondylar process; pgf, postglenoid foramen;
PL, palatine; pp, protoconid pit; PX, premaxilla; SQ, squamosal. Scale bar equals 50 mm.

FORASIEPI ET AL.—NEW HATHLIACYNID FROM BOLIVIA 673

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https://www.researchgate.net/publication/248616585_Marsupial_skulls_from_the_Deseadan_Late_Oligocene_of_Bolivia_and_phylogenetic_analysis_of_the_Borhyaenoidea_Marsupialia_Mammalia?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==


borhyaenids, which have an anteroposteriorly short but deep
maxilla. The infraorbital foramen is large and faces anteriorly.
The border of the infraorbital foramen is ‘U’-shaped and located
above the posterior border of P3. This border protrudes con-
spicuously laterally; therefore, the roof of the most anterior por-
tion of the infraorbital canal can be seen in dorsal view. The root
of the canine defines a low prominence on the lateral side of the
maxilla, which extends nearly to the dorsal border of this bone.
The zygomatic process of the maxilla is short and restricted to
the base of the zygomatic arch. In other sparassodonts, as well as
other basal metatherians (Rougier et al, 1998), the maxilla

behind the infraorbital foramen (including the zygomatic pro-
cess) flares laterally, forming ‘cheeks,’ a feature that is most
clearly seen in ventral view. Acyon myctoderos lacks ‘cheeks’
(Fig. 3). The anterior half of the palatal process is perforated by
numerous small foramina probably for the palatine arteries,
veins, and branches of the maxillary division of the trigeminal
nerve, structures that in didelphids and some other mammals
pass through the major palatine foramen (Evans, 1993; Wible,
2003). The largest of these foramina is located between the ca-
nines, as in other Sparassodonta (e.g., Borhyaena, Arctodictis,
Cladosictis, and Sipalocyon). There is no major palatine fora-
men, nor any palatal vacuities, a resemblance to other Sparas-
sodonta. In Mayulestes, a major palatine foramen is probably
present (Muizon, 1998:fig. 6, identified as medial palatine fora-
men), but the palatine vacuities are absent. The minor palatine
foramen is a small oval aperture anterolaterally-posteromedially
elongated, which opens between the maxilla and the palatine.
Lateral to the minor palatine foramen and between M3 and M4
there is a deep, round pit, which houses the tall protoconid of m4
when the jaws are closed. The anterior-most contact of the max-
illa and the palatine bone is at the level of M1, as in Cladosictis.
In dorsal view (Fig. 2), the maxilla contributes to the floor of the
orbit and probably the floor of the infraorbital canal. The floor
of the orbit bears numerous fractures, but some natural grooves
and small foramina can be distinguished. These foramina and the
corresponding grooves probably transmitted branches of the
maxillary artery, vein, and nerve to supply the posterior teeth
(Evans, 1993).

Both palatines are preserved in the specimen described here,
but the left is better preserved than the right. In ventral view
(Fig. 3), the horizontal process of the palatine forms approxi-
mately 35% of the most postero-medial length of the hard palate.
Anterolaterally the palatine contacts the maxilla by a rounded

FIGURE 5. Acyon myctoderos, MNHN-Bol-V-003668. Photographs of
the left dentary in lateral (A) and medial (B) views. Scale bar equals 50
mm.

FIGURE 4. Acyon myctoderos, MNHN-Bol-V-003668. Stereopair
photographs of the skull in lateral view. Scale bar equals 50 mm.

FIGURE 6. Line-drawing reconstruction of the skull of Acyon myc-
toderos, MNHN-Bol-V-003668 in lateral view (A), left dentary in lateral
(B), and medial (C) views. Abbreviations: (as for Figs. 2, 3) anp, angular
process; cd, condyle; corp, coronoid process; DEN (ar), ascending ra-
mus; DEN (hr), horizontal ramus; EO, exoccipital fr, foramen rotun-
dum; iff, infraorbital foramen; lf, lacrimal foramen; lt, lacrimal tubercle;
masf, masseteric fossa; mc, masseteric crest; menf, mental foramina; mf,
mandibular foramen; pf, paracanine fossa; pgp, postglenoid process; pp,
post-tympanic process; sy, symphysis. Scale bar equals 50 mm.

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 3, 2006674

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suture and the palatine of the opposite side in the midsagittal
plane. As was mentioned above, the minor palatine foramen
opens in the suture between the palatine and the maxilla. The
palatine extends posteriorly to the posterior edge of M4 as in
Cladosictis, Sipalocyon, and Mayulestes. It differs from other
sparassodonts (e.g., Borhyaena) where it is anterior or only
reaches the posterior edge of M4. The posterior rim of the choa-
nae is thick and double-arched, but lacks the postpalatine torus
found in Mayulestes and didelphids (Muizon, 1998, 1999). Pos-
terior to the level of the rim of the choanae, the palatine extends
as a thin laminar bone forming the choanal walls.

Only the left lacrimal is preserved. The facial process contacts
the frontal and nasal dorsally, and the maxilla anteriorly; the
zygomatic process contacts the jugal laterally, posteriorly, and
medially (Figs. 2, 4, 6A). Unfortunately, the orbital process of
the lacrimal is mostly broken. The facial process of the lacrimal
is semicircular, with an anterior round border that extends to the
level of the M1–M2 embrasure. Among other sparassodonts,
Arctodictis and Borhyaena express the same anterior extension
of the lacrimal as does the specimen described here, but in Cla-
dosictis and Sipalocyon the lacrimal reaches the level of M1. In
lateral view (Figs. 4, 6A), the rim of the orbit extends to the level
of the middle of M2. An elongate lacrimal tuberosity is devel-

oped on the lateral face of the lacrimal bone, delimiting the
orbital margin anteriorly. There is only one lacrimal foramen,
which opens within the orbit. The presence of a lacrimal tuber-
osity and a single lacrimal foramen opening inside the orbit is
also characteristic of other basal metatherians (Rougier et al.,
1998). The zygomatic process of the lacrimal is small and rect-
angular, and scarcely contributes to the anterior base of the
zygomatic arch. It extends back to the level of the posterior root
of M2.

Only the left zygomatic arch is preserved, but this element is
broken on its most posterior portion. The anterior part of the
zygomatic arch is formed by the zygomatic processes of the max-
illa and lacrimal, as described above; the jugal forms its main
portion, and the zygomatic process of the squamosal forms its
posterior part. The jugal is a relatively flat medially curved bone.
In lateral view (Figs. 4, 6A), this bone contacts the lacrimal
dorsally and the maxilla anteriorly. The jugal-maxilla suture is
obliquely oriented; the most anterior point of this suture reaches
the M2 level, while the posterior one is located just dorsal to M4.
The anterodorsal margin of the jugal is concave and forms the
ventral rim of the orbit. Behind this area there is a low triangular
protuberance located just below the postorbital process of the
frontal bone (see below), where the postorbital ligament would
be attached delimiting the orbit posteriorly (Evans, 1993). The
ventral border of the jugal is concave. In lateral view (Figs. 4,
6A), dorsal and almost parallel to the ventral border, there is a
low crest that limits the ventral one-third of this bone, corre-
sponding to the area of attachment of the masseter muscle (pars
profunda) (based on Didelphis; Turnbull, 1970).

The frontal is the major component of the skull roof (Fig. 2)
medial and immediately posterior to the orbits. In living marsu-
pials, the frontals are two independently ossified dermal bones
(Clark and Smith, 1993), but these components are co-ossified in
the Quebrada Honda specimen; therefore, it is described as a
single unit. In dorsal view (Fig. 2), the frontal contacts the nasal
anteriorly and the lacrimal anterolaterally. The suture with the
right and left nasals define: a wide ‘W’-shape with the lateral
moieties longer than the medial one. Posteriorly, the frontal con-
tacts the parietal by mean of an acute ‘V’-shaped suture. In
lateral view (Figs. 4, 6A), the skull is fractured making it difficult
to distinguish the sutures. Anteriorly, the frontal forms the con-
cave dorsal orbital rim. The postorbital process is weaker than
that of Cladosictis and Sipalocyon, but similar to Lycopsis,
Borhyaena, and Arctodicits. Temporal crests arise from the post-
orbital processes and extend posteromedially joining just poste-
rior to the postorbital processes and forming a sagittal crest. The
sagittal crest continues posteriorly on the frontal and parietal
bones becoming higher posteriorly. There are numerous small
grooves and muscle scars on both sides of the sagittal crest that
indicate the attachment area of the temporal musculature (based
on Didelphis; Turnbull, 1970). Behind the frontal, there is only a
single triangular bone on the posterior cranial roof (Fig. 2),
which extends from the level of the postorbital constriction to
the lamboidal crests. This bone is referred here as the parietal,
but it could also represent the fusion of the interparietal and the
parietals, as was assumed in Mayulestes (Muizon, 1998, 1999). A
differentiation between the interparietal and parietal bones is
clearly shown in some hathliacynids (e.g., Cladosictis and Sipalo-
cyon), but in the other Sparassodonta examined (Borhyaena,
Arctodictis, Lycopsis) there exists only a single bone.

The sutures between bones forming the lateral wall of the skull
and mesocranium are difficult to distinguish owing to the exten-
sive damage on this part of the skull. The only feature clearly
identifiable in the lateral wall of the skull is the foramen rotun-
dum (Figs. 4, 6A); this is a circular foramen opening anteriorly.
In living marsupials (Clark and Smith, 1993; Wible, 2003), as well
as other metatherians (e.g., Mayulestes, Muizon, 1998), the fora-

FIGURE 7. Acyon myctoderos, MNHN-Bol-V-003668. Stereopair
photographs and line-drawing reconstruction of the left petrosal in cer-
ebellar view. Abbreviations: amf, anteromedial flange; ca, cochlear
canaliculus; cp, crista petrosa; iaf, inferior acoustic foramen; iam, internal
acoustic meatus; ips, sulcus for the inferior petrosal sinus; saf, superior
acoustic foramen; sf, subarcuate fossa; sps, sulcus of the prootic sinus; va,
vestibular aqueduct. Scale bar equals 5 mm.

FORASIEPI ET AL.—NEW HATHLIACYNID FROM BOLIVIA 675

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men rotundum opens entirely in the alisphenoid; unfortunately
the sutures in our specimen are not clearly delineated.

Only the left squamosal bone has been preserved in the speci-
men from Quebrada Honda. The zygomatic process of the squa-
mosal is a relatively lateral flat bridge of bone that contributes to
the posterior portion of the zygomatic arch. This process is bro-
ken anteriorly and its contact with the jugal is not preserved. In
ventral view (Fig. 3), and anterior to the glenoid fossa, there is a
triangular area on the zygomatic process with irregular texture;
this feature suggests that, as in other metatherians, the jugal
extends back below the squamosal forming the preglenoid pro-
cess. The squamous portion of the squamosal bone is roughly
circular and located on the postero-ventral side of the skull (Figs.
4, 6A). It contacts the parietal dorsally, probably the alisphenoid
anteriorly (although this suture is not clearly observable), and
the occipital posteriorly. Unfortunately, one feature that is not
observable in this specimen, but is noteworthy among sparas-
sodonts, is the contact between the frontal and squamosal bones.
As noted by Muizon (1999), hathliacynids (e.g., Sallacyon, No-
togale, Cladosictis, and Sipalocyon), and didelphids in general,
have an alisphenoid-parietal contact, whereas in other sparas-
sodonts (e.g., Borhyaena, Prothylacynus, and Arctodictis), the
two bones are separate and the frontal contacts the squamosal.
The squamous process is poorly developed; by comparison, in
Borhyaena and Arctodictis it reaches nearly to the sagittal crest.
Near the suture with the occipital bone the squamosal becomes
flatter, contributing to the formation of the most ventral portion
of the lambdoidal crest. The subsquamosal foramen (sensu
Muizon, 1998, 1999; suprameatal foramen sensu Wible, 2003) is
large, oval, and opens posteriorly. It is located directly above the
dorsal edge of the external acoustic meatus. The external acous-
tic meatus is a wide ‘U’-shaped excavation, ventrally concave,
and developed entirely in the squamosal bone. It is bordered
posteriorly by the post-tympanic process of the squamosal and
anteriorly by the postglenoid process of the same bone. The
post-tympanic process of the squamosal is a thick process that
extends ventrally to the level of the basicranium in lateral view
(Figs. 4, 6A); its most ventral portion is curved anteriorly making
a contribution to the tympanic bulla. A similar anteriorly curved
post-tympanic process occurs in other hathliacynids (Muizon,
1999; as pars mastoidea of the petrosal bone in Patterson, 1965),
but is absent in borhyaenids where this process is almost vertical.
The glenoid fossa (Fig. 3) is oval, with its transverse width ap-
proximately twice the anteroposterior length. It is bordered an-
teriorly by a weak preglenoid squamosal process and posteriorly
by a prominent postglenoid process. As mentioned above, the
preglenoid process of the jugal is not preserved in the specimen
from Quebrada Honda, but it was probably present, as in other
Sparassodonta. The postglenoid process is relatively narrow
and high as in Cladosictis. The postglenoid process of Borhyaena
and Arctodictis is considerable wider and lower than that of
A. myctoderos, whereas that of didelphids (e.g., Didelphis,
Monodelphis) is narrower and higher. The posterior wall of the
postglenoid process forms a slightly convex and anteriorly in-
clined anterior wall of the external acoustic meatus (Fig. 3). A
postglenoid foramen opens on the medial side of the postglenoid
process, at the level of the medial-most edge of the glenoid fossa;
this foramen is oval and faces posteroventrally. The glenoid pro-
cess of the alisphenoid (sensu Wible, 2003, the triangular lateral
expansion of the alisphenoid located at the anteromedial angle
of the glenoid fossa, called entoglenoid process by Muizon, 1998,
1999, among others) is absent, as in other sparassodonts, and
differing from Mayulestes and didelphids where it is clearly pres-
ent.

The basicranium is preserved in separate portions, one as part
of the main fragment of the skull and the other associated with
the occipital area (reconstructed in Figs. 3, 6A). This portion of

the skull is formed by the squamosal, alisphenoid, occipital, ec-
totympanic, and petrosal bones.

The most conspicuous feature of the alisphenoid in the basi-
cranium is its contribution to the formation of the tympanic bulla
(Fig. 3). The tympanic process of the alisphenoid is a concave
projection placed in the antero-ventral corner of the middle ear
cavity. In some hathliacynids (e.g., Cladosictis) the tympanic pro-
cess of the alisphenoid and the post-tympanic process of the
squamosal contact to form the auditory bulla, and the ectotym-
panic is situated within the bulla (Patterson, 1965). In the speci-
men described here, a small fragment of the distal portion of the
alisphenoid tympanic process is broken, leaving a small space
ventrally where the ectotympanic is visible. The foramen ovale
opens anterolateral to the alisphenoid tympanic process. This
foramen is oval, enclosed entirely by the alisphenoid, and faces
anteroventrally. There is no transverse canal (sensu Muizon,
1999; transverse canal foramen sensu Wible, 2003), whereas
among sparassodonts a transverse canal is observed in Notogale,
Sipalocyon, Lycopsis, Prothylacynus, and some specimens of
Cladosictis. Anterior to the foramen ovale, there is a tiny
foramen of uncertain homology. In ventral view (Fig. 3), in the
most medial border of the alisphenoid, there are small shallow
grooves that run posterolaterally–anteromedially from the level
of the alisphenoid tympanic process to the level of the foramen
ovale. In some metatherians, a groove medial to the alisphenoid
tympanic process supports the anteromedial flange of the petro-
sal. Other grooves observed in the area in other metatherians
connect the middle ear and the oral cavities via the auditory
tube.

The occipital bone is preserved separately from the rest of the
skull. The occipital is formed by the supraoccipital, basioccipital,
and exoccipital bones. The sutures among these bones are not
observable in the specimen from Quebrada Honda, therefore,
the occipital will be described as a single unit. The left occipital
condyle (the only one preserved) is convex and oval with its
main axis in the anterodorsal-posteroventral orientation. The
entire surface of the condyle bears a smooth articular facet for
the atlas. Anterior to the occipital condyle, there is a single large
hypoglossal foramen, in contrast with the condition of two fo-
ramina typical of the crown group Marsupialia (Sánchez-
Villagra, 1998, among others). A deep sulcus extends anterolat-
erally from the hypoglossal foramen. In better-preserved hath-
liacynids (i.e., Cladosictis and Sipalocyon) this sulcus is also
conspicuous, bordered laterally by sharp crests, and reaches the
level of the foramen for the inferior petrosal sinus. This sulcus is
probably associated with a vein since the inferior petrosal sinus
is venous and because a vein is transmitted with the hypoglossal
nerve in marsupials (e.g., Didelphis and Monodelphis, Wible,
2003) as well as in other mammals (e.g., Homo, Jungers et al.,
2003). In the lateral margin of the occipital bone, posterolateral
to the anterior end of the described sulcus, there is a small notch
that would correspond to the posteromedial border of a small
jugular foramen. The size of this foramen suggests that, as occurs
in living marsupials, the internal jugular vein was probably a
minor conduit for drainage of the dural venous sinuses (Dom et
al., 1970); its small size represents a plesiomorphic condition
retained in metatherians (Rougier and Wible, 2006). The para-
condylar process (sensu Evans, 1993; paraoccipital process sensu
Muizon, 1998, 1999, among others) constitutes the ventral pro-
jection of the exoccipital lateral to the condyle (Wible, 2003);
it is relatively wide and low. In this specimen, it occurs as in
sparassodonts (clearly observed in Borhyaena, Arctodictis, Cla-
dosictis, Sipalocyon, and Lycopsis) where the paracondylar pro-
cess has the same ventral development as the post-tympanic pro-
cess of the squamosal. It differs from didelphids (e.g., Didelphis
and Monodelphis) where the paracondylar process is larger than
the post-tympanic process.

The posterior part of the skull is formed in metatherians gen-

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 3, 2006676

https://www.researchgate.net/publication/17661993_The_venous_system_of_the_head_and_neck_of_the_opossum_Didelphis_virginiana?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==
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erally (e.g., Didelphodon, Mayulestes, didelphids; Clemens, 1966;
Muizon, 1998; Wible, 2003, respectively) by the occipital, squa-
mosal, and petrosal bones. In sparassodonts, the petrosal is ex-
cluded from the occiput and only the squamosal and occipital
bones form this area (Muizon, 1999). The only exception is Cal-
listoe, a proborhyaenid from the Eocene of Argentina, which
according to Babot et al. (2002) shows the plesiomorphic condi-
tion. Unfortunately, in the present specimen the sutures in the
occipital area are not clearly observable. The nuchal plane of the
occiput exhibits numerous scars for the attachment of the cervi-
cal musculature (Evans, 1993), and small foramina, probably re-
lated to venous sinus drainage (Evans, 1993). A large mastoid
foramen is observed in the nuchal plane above the occipital con-
dyle. A dorsolateral portion of the lambdoidal crest has been
preserved; this crest gives a roughly circular contour to the pos-
terior view of the skull. The most conspicuous feature in poste-
rior view is the foramen magnum. The right border is missing,
but remaining parts suggest that it was oval with the main axis
horizontal.

The left ectotympanic is a ‘U’-shaped bone located in the inner
side of the floor of the tympanic bulla (Figs. 4, 6A). The ecto-
tympanic contacts the squamosal at both ends of the ‘U,’ medial
to the postglenoid foramen. In addition to the squamosal con-
tact, in other metatherians (e.g., Cladosictis, Didelphis, Mono-
delphis) the ectotympanic abuts the alisphenoid tympanic pro-
cess; this specimen has a broken alisphenoid tympanic process,
so contact between these two bones is assumed based on the
proximity of the remaining preserved structures. In ventral view,
the ectotympanic is roughly triangular with the anterior crus
wider than the posterior one. As in other hathliacynids (Patter-
son, 1965; Muizon, 1999), the posterior crus contacts the post-
tympanic process of the squamosal (Figs. 4, 6A). In some living
marsupials (e.g., Monodelphis; Wible, 2003) a different contact is
formed with the posterior crus of the ectotympanic abutting the
rostral tympanic process of the petrosal, which in turn forms the
posterior wall of the tympanic bulla (Segall, 1969).

Only the endocranial portion of a left petrosal is available for
study (Fig. 7). The orientation of this element in the description
is arbitrary. In recent marsupials, the main axis of the petrosal is
placed 45º obliquely with respect to the main axis of the skull;
this is assumed to be the position in Acyon myctoderos. The
anterior expansion of the promontorium, the anteromedial
flange (sensu Wible, 2003), is elongated and narrows anteriorly.
The anteromedial flange is similar to Sipalocyon among sparas-
sodonts, but different from Borhyaena and Lycopsis because it is
extremely reduced. On the posterior edge of the anteromedial
flange there is a groove that runs lateromedially (according to
the orientation in this description) and is here interpreted as a
sulcus for the inferior petrosal sinus. The internal acoustic me-
atus is a deep depression located in the anterior half of the
petrosal. Two well-defined foramina open inside the internal
acoustic meatus: the superior acoustic foramen on the lateral side
and the larger inferior acoustic foramen on the medial one. No
other internal structure within the internal acoustic meatus can
be observed. On the medial margin, the petrosal has two small
openings, the cochlear canaliculus and vestibular aqueduct. The
cochlear canaliculus is an elongate fissure that opens in a depres-
sion at the level of the internal acoustic meatus. The vestibular
aqueduct is smaller than the canaliculus and opens at the level of
the posterior border of the subarcuate fossa. The crista petrosa
(the crest that separates the middle and posterior crania fossae;
Wible, 1990) is low and almost indistinct. The subarcuate fossa is
deep, but less so than in Sipalocyon, and without a clearly de-
fined dorsolateral border. A deep subarcuate fossa represents
the primitive condition of Metatheria (Rougier et al., 1998;
Sánchez-Villagra, 2002; Sánchez-Villagra and Wible, 2002).

Dentary—The left dentary is almost complete, missing only
the tip of the coronoid process. The right dentary is missing

almost the entire ascending ramus. The horizontal ramus of the
dentary is long and slender (Figs. 5, 6B, C), similar to that of
Acyon tricuspidatus, Lycopsis longirostrus, L. viverensis, and
some specimens of Cladosicits. The ventral border of the dentary
is slightly curved from the incisive alveoli to the condyle, as in
other sparassodonts, except Prothylacynus in which the ventral
and posterior borders form a right angle. The symphysis (Figs.
5B, 6C) is not as rugose as in other sparassodonts (i.e., it has
relatively few and low ridges where the dentaries would have
joined). This feature varies among sparassodonts from a rela-
tively smooth surface present in hathliacynids in general, to the
ossified symphysis found in some borhyaenids (e.g., Prothylacy-
nus and Arctodictis munizi). The posterior-most portion of the
symphysis extends to a point below the anterior border of the
last premolar, as is typical for the Hathliacynidae (Marshall,
1981). In lateral view, (Figs. 5A, 6B) three mental foramina are
observed: in the left dentary there is a large one below the pos-
terior border of p1, the other two are below the middle of m1 and
the posterior root of m2, respectively. In the right dentary the
foramina are placed below the p1–p2 embrasure, below the pos-
terior root of p3, and below the middle of m2. In the right den-
tary of the type of Acyon tricuspidatus there are at least two
foramina: the largest and most anterior one opens at the level of
p1, whereas the second opens at the level of the anterior root of
m2. The number and relative position of mental foramina varies
among species, specimens, and sometimes between the right and
left dentary of the same individual. For example, many speci-
mens of Cladosictis (MACN A 6289, A 5927, Sinclair, 1906:fig.
LIX) have only two mental foramina, but the specimen illus-
trated by Sinclair (1906:fig. LVI) has four foramina. Prothylacy-
nus has two (MACN A 5926) or three (Sinclair, 1906:fig. XLVII)
foramina; Lycopsis longirostrus has six foramina; and the speci-
men MACN A 9342 of Borhyaena tuberata has three mental
foramina in one dentary and four in the other. The coronoid
process is broad anteroposteriorly and concave laterally where a
relatively deep masseteric fossa is developed. The masseteric
crest (posterior shelf of the masseteric fossa sensu Marshall and
Muizon, 1995, and Wible, 2003) protrudes laterally defining the
ventral and posterovental borders of the masseteric fossa. The
anterior border of the coronoid process forms an angle of ap-
proximately 120º with respect to the horizontal axis of the den-
tary; this is a more obtuse angle than that of Cladisictis where it
inclines approximately 110º. There is a relatively long retromolar
space between the last molar and the anterior border of the
coronoid process, larger than in Cladosictis. The condyle is oval
and placed slightly above the alveolar margin in lateral view. The
angular process is broken on its medial edge, but apparently was
broad and inflected as in other sparassodonts (type two, inter-
mediate angular process of Sánchez-Villagra and Smith, 1997).
The mandibular foramen (Figs. 5B, 6C) is oval, faces posteriorly,
and is located nearer to the base of the condyle than to the
anterior base of the coronoid process; this is slightly more pos-
terior than the condition observed in Cladosictis, Borhyaena,
Arctodictis, and Prothylacynus.

Dentition—The dental formula is 4.1.3.4/?3.1.3.4. (for mea-
surements see Table). The upper incisor tooth row (Fig. 3) is
almost straight following the morphology of the anterior border
of the premaxilla. All the incisors have single roots. I2 and I4 are
mesiodistally compressed. The crown of the distal-most upper
incisor is approximately twice as large as those of similar sized
I2–I3 (the upper central incisor is missing). The single-rooted
upper canine is relatively slender, slightly larger and more pro-
cumbent than the lower one. Diastemata between the canine and
P1 and between P1 and P2 are long and nearly equal in length.
The diastema between P2 and P3 is nearly one-third that of the
more mesial diastema. There is no diastema between P3 and M1.
The two-rooted upper premolars are sharp, laterally compressed,
and single cusped. The premolars increase in size posteriorly; P3

FORASIEPI ET AL.—NEW HATHLIACYNID FROM BOLIVIA 677

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is much larger than P2. The main cusp of P1 is asymmetrical with
the anterior crest almost straight and the posterior one concave;
P2–P3 have a main cusp with symmetrically arranged anterior
and posterior crests. An anterior cusp is absent on all upper
premolars, but a minute posterior cuspule is present on P2–P3
(larger on P2). The upper molars (Fig. 8A) are arranged in size
M1 < M2 < M3 >> M4. M1 to M3 have two labial and one lingual
root, but M4 has only one labial and one lingual root. In M1–M3
the bases of the paracone and the metacone are twinned, as in
most sparassodonts, being more widely separated only in the
most plesiomorphic tooth pattern of the group (i.e., Patene, All-
qokirus, and Mayulestes). The metacone is the largest cusp on
M1–M3. It increases abruptly in size posteriorly. In contrast, on
M4 the metacone is extremely reduced, and is represented as a
small prominence on the posterior slope of the paracone. On all
the molars the protocone is much lower than the metacone and
the paracone. The protocone is distinctly basined and broader
than in Cladosictis. In M1–M3 the protocone is lingual to the
paracone; in contrast, in M4 it is posterolingual to the paracone.
The postmetacrista is strongly developed, a characteristic feature
of carnivorous mammals (Muizon and Lange-Badré, 1997). The
length of the postmetacrista increases from M1 to M3, and
changes from an almost straight crest (in occlusal view) on M1 to
a posteriorly convex one on M3. M4 lacks any vestige of a post-
metacrista. There is no carnassial notch visible on the postmeta-
crista of M1–M3, but this absence may be related to the ad-
vanced stage of wear. A stylar shelf is absent in M1–M2 and M4,
and is only present on M3 as a posterolabial lobe. The parastylar
corner is rounded in all the molars; there is not an individual
stylar cusp, but a crest surrounds the anterolabial corner of the
tooth. The only stylar cusp recognizable is the cusp B on M2
where it forms a small swelling posterolabial to the paracone.
Cusp B on M3 forms an ectocingulum (sensu Marshall, 1978)
continuous with the crest running mesially from the parastyle.

The crowns of the lower incisors are not preserved. There are
only two mesiodistally compressed incisor roots preserved. A
third lower incisor is postulated based on the condition observed
in other hathliacynids. The lower canines (Fig. 5) are slender,
long, and vertically oriented; they are suboval in cross section
and single rooted. Diastemata are present between the canine
and p1 and between successive premolars. These diastemata are

longer than in Acyon tricuspidatus. The longest diastema is be-
tween c and p1; these spaces diminish in length along the pre-
molar series. The premolars are strongly laterally compressed
and become progressively taller and longer from p1 to p3. All
premolars are two rooted and the main axis is aligned with the
anteroposterior axis of the dentary, as in other hathliacynids. In
other sparassodonts (e.g., Prothylacynus) p1 is slightly oblique
with the anterior root slightly labial and the posterior root
slightly lingual. This condition is most extreme in borhyaenids
(e.g., Arctodictis), where p1 is situated transverse to the antero-
posterior axis of the dentary. As in the upper dentition, p1 has an
asymmetrically arranged main cusp, while in p2–p3 this cusp is
almost symmetrical. The p1 lacks an anterior cusp, but has a long
posterior portion that ends in a minute cuspule. Premolars p2–p3
have small anterior and posterior cuspules; the posterior cus-
pules are larger than the anterior ones. Acyon myctoderos differs
from A. tricuspidatus in having p2 comparatively more stout with
a better-developed posterior cusp. The molars increase in size
posteriorly (Fig. 8B), but not as abruptly as in borhyaenids. The
trigonids have only two cusps, the paraconid and protoconid;
there is no vestige of a metaconid, as in other hathliacynids. The
paraconid increases sharply in size along the tooth row, from a
minute cusp in m1 to a taller one in m4. The anterolingual corner
of the paraconid of m2–m4 is flat and protrudes anteriorly, form-
ing the interlocking mechanism of the lower molars. The lower
first molar lacks this feature and has the paraconid rounded
anteriorly. There is an extremely reduced anterobasal cingulum
in all molars, restricted to the base of the paraconid, and even
more reduced than in Acyon tricuspidatus. The protoconid is the
largest cusp of the molars; it is roughly circular in cross section on
m1–m2 and more triangular on m3–m4. The paracristid and pre-
protocistid are particularly sharp on m3–m4, with a carnassial
notch developed between them. The paraconid, protoconid, and
talonid are aligned in m1; in the other molars they form an
obtuse angle. The talonid is present on all the molars. Lower first
to third molars have wide talonids not fully basined due to the
absence of an entoconid, as do the other species of the genus.
Only the right m2 displays a shallow crest at this point that is
reminiscent of a talonid border. The talonid of m4 is consider-
ably reduced; a sharp crest surrounds the lingual and labial bor-
ders forming a complete basin. The hypoconid is clearly distin-
guished on m1–m3 at the buccal edge of the talonid (m4 has a
crest in this position). The hypoconids are larger than in Acyon
tricuspidatus; this feature is clearly shown on the talonid of m2.
The hypoconulid in all the molars is relatively large and quite
vertical, differing in this regard from the posteriorly projecting
hypoconulids of Acyon tricuspidatus.

Postcrania—The atlas (Fig. 9A, B) is robust, with the inter-
centrum fused to the rest of the vertebra as in Prothylacynus and
Cladosictis. The anterior and posterior borders of the neural arch
are straight and there is no neural spine. The atlantal foramen
(Fig. 9A) is limited anteriorly by a bony bridge as in Sipalocyon,
Cladosictis, Prothylacynus, and Lycopsis. In dorsal view (Fig.
9A), there is a foramen on the posterior portion of the dorsal
arch, which probably transmitted a small branch of the vertebral
artery. This foramen is also present in Sipalocyon and Borhy-
aena, but it is absent in Prothylacynus, Arctodictis, and probably
Mayulestes (Muizon, 1998:fig. 10). The neural canal is rounded,
similar to other Sparassodonta. The anterior articular facet for
the occipital condyle is strongly concave and faces anteromedi-
ally; its dorsal border is distinctly curved and nearly reaches to
the level of the neural arch. In dorsal view, the lateral border of
the condylar facet is almost parallel to the anteroposterior axis of
the vertebra. The posterior articular facet for the axis articula-
tion is flatter than those for the condyles and faces posterome-
dially, as in other Sparassodonta. In ventral view (Fig. 9B), the
axial facets of both sides form a wide ‘U’-shaped concavity that

FIGURE 8. Acyon myctoderos, MNHN-Bol-V-003668. Stereopair
photographs of the occlusal view of the left upper (A) and right lower
(B) molars. Scale bar equals 5 mm.

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 3, 2006678

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enclosed the anterior portion of the axis. The transverse process
has a roughly rounded outline; its anterior edge does not ex-
tend beyond the level of the atlantal foramen and its posterior
edge reaches at least the level of the axial facet. A small foramen
opens at the junction of the transverse process and the neural
arch on both sides of the vertebra. The left foramen opens at
the level of the anterior border of the transverse process, while

the right one opens at the level of its mid-portion. There is no
transverse foramen.

The axis (Fig. 9C, D) is relatively long anteroposteriorly. The
neural arch is broken anteriorly, including the spinous process.
This process is elongate in Sparassodonta (Sinclair, 1906;
Muizon, 1998; Argot, 2003, 2004). The postzygapophyses are
rounded and protrude posteriorly. The articular facets are oval
and face posterolaterally. The odontoid process is broken and
only its robust base is preserved. In dorsal view (Fig. 9D), there
are two small foramina immediately posterior to the odontoid
process. In one specimen of Prothylacynus (MACN A 5931–
5937), these foramina are similar in size to those of the specimen
described here, but in another specimen (MACN A 706–720) the
foramina are larger, with a median crest between them. As in
extant marsupials (e.g., Didelphis), these differences may actu-
ally represent individual variation (pers. obs.). In ventral view,
the anterior border of the axis, which contains the prezyg-
apophyses, is rounded, even more than in other sparassodonts.
The prezygapophyses and the dens have a clear border between
them, and do not form a continuous surface as in some marsu-
pials (character 12 of Horovitz and Sánchez-Villagra, 2003). One
feature present in the Quebrada Honda specimen that is shared
with other sparassodonts is the strongly developed ventral me-
dian crest of the axis (Fig. 9C), which increases in height poste-
riorly (Sinclair, 1906; Argot, 2003, 2004). This crest is present;
the axis and the remaining anterior cervical vertebrae (see be-
low). The suture between the axial and atlantal (anterior cotyles
and odontoid) portions of the axis is partially visible; in dorsal
view, it describes a wide ‘V’ opening anteriorly, but in ventral
view it shows a wide ‘U’ opening posteriorly. This sutural mor-
phology is like that of other marsupials (e.g., Didelphis). The
transverse process is largely damaged and the transverse fora-
men is not preserved.

Four other partially preserved vertebrae have been recovered;
they are mainly represented by fragmentary bodies. The bodies
are robust and anteroposteriorly elongate. On their dorsal sur-
faces open one or two foramina of relatively large caliber. Three
of the elements are possible cervical vertebrae due to the strong
ventral median crest. This crest ends in a bifid tubercle in two of
the three preserved vertebral bodies. A median crest is com-
monly present in the cervicals of Sparassodonta, ending in a
large single or bifid posterior tubercle. The anterior and poste-
rior surfaces of the body are inclined anteriorly, as occurs in
general in the cervicals of mammals (Slijper, 1946). The anterior
surfaces are rectangular in shape as in Prothylacynus. The fourth
and smallest vertebra has only a small part of the body pre-
served; its identification is difficult due to its fragmentary con-
dition. The ventral side of the body has two parallel crests run-
ning anteroposteriorly.

Three tarsal elements have been recovered, including a right
cuboid, right ectocuneiform, and, probably, an entocuneiform. In
anterior view the cuboid has the medial border longer than the
lateral one, and the proximal articular surface (calcaneal facet)
convex and oblique with respect to the distal one. This latter
feature, present also in other sparassodonts (Sinclair, 1906; Ar-
got, 2003, 2004), modifies the otherwise strictly cubic form. The
anterior face of the cuboid is flat, but the posterior face bears a
strong plantar tubercle. The distal articular surface has a smaller
lateral facet for the fifth metatarsal and a larger medial facet for
the fourth metatarsal; there is no crest separating the facets.

The ectocuneiform is laterally flat and rectangular in cross
section. The plantar tubercle is well developed near the dorsal
border of the plantar surface. In medial and lateral views, the
bone is roughly quadrangular. On the proximal portion of both
sides it has articular facets: in medial view, a kidney-shaped facet
for the articulation with the mesocuneiform, and in lateral view,
a rectangular facet for the articulation with the cuboid. In distal

FIGURE 10. Cladogram showing the relationships of the sparas-
sodonts considered in this study. Acyon represents a hathliacynid related
closer to Cladosictis than to other Sparassodonta. The numbers indicate
the nodes; the diagnosis of the nodes is based on unequivocal characters
(character states, between parentheses, follow the character number):
node 1: 3(1), 5(1), 6(1), 8(1), 10(1), 11(1), 12(1), 13(1), 17(1), 20(1), 29(1),
30(1), 36(1), 42(1), 44(1); node 2: 31(1), 32(1); node 3: 18(1), 33(1), 34(1),
37(1), 45(2); node 4: 16(1), 23(1), 36(2), 38(1), 41(1); node 5: 15(1), 22(1);
nodes 6 and 7 are supported by an ambiguous character, see the discus-
sion, and node 8: 39(2).

FIGURE 9. Acyon myctoderos, MNHN-Bol-V-003668. Photographs of
the atlas in dorsal (A), and ventral views (B), and the axis in lateral (C)
and dorsal (D) views. Scale bar equals 20 mm.

FORASIEPI ET AL.—NEW HATHLIACYNID FROM BOLIVIA 679

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https://www.researchgate.net/publication/284497292_Mayulestes_ferox_a_borhyaenoid_Metatheria_Mammalia_from_the_early_Palaeocene_of_Bolivia_Phylogenetic_and_palaeobiologic_implications?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==
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view the entire surface of the ectocuneiform has a concave rect-
angular facet for the articulation with metatarsal III.

The third preserved element, here identified as a probable
entocuneiform, is high, flat, and triangular in cross section. The
distal articular facet for metatarsal I is oblique. These features
are similar to the entocuneiform of Lycopsis longirostrus, and
this is the basis for its identification.

Five proximal portions of metapodials are preserved; the ele-
ments are relatively slender. Compared to those of Arctodictis
sinclairi, the preserved metapodials of the specimen from Que-
brada Honda are median ones: they lack the prominent medial
and lateral processes characteristic of metapodials I and V, re-
spectively, both in the hand and the foot. A more precise iden-
tification can not be given at present.

PHYLOGENETIC ANALYSIS

To examine the phylogenetic relationships of Acyon myc-
toderos and test the monophyly of Hathliacynidae, we conducted
a parsimony analysis of 47 characters and ten taxa using the
program NONA version 2.0 (Goloboff, 1993). Mayulestes ferox,
from the Paleocene of Bolivia, is used for outgroup comparison
following Muizon (1998, 1999). The ingroup is composed of Sal-
lacyon, Notogale, Cladosictis, Sipalocyon, Lycopsis, Prothylacy-
nus, Borhyaena, Arctodictis, and Acyon. The coding of these taxa
or the modification of previous scores is based on the specimens
listed in Appendix 1.

Mayulestes was considered a member of the Sparassodonta
(Borhyaenoidea) by Muizon (1998, 1999, among others). Never-
theless, in an analysis of mammaliaform taxa, Rougier et al.
(1998) found the sparassodonts (as Borhyaenidae) to be basal to
Mayulestes, the latter appearing as a sister group to a clade in-
cluding Jaskhadelphys, Pucadelphys, Andinodelphys, and the
crown group Marsupialia. Whatever the relationship of Mayu-
lestes to sparassodonts, we include this taxon as the outgroup
because it has one of the best-preserved skeletons of a taxon
phylogenetically close to the clade of interest in this analysis.
Only one outgroup was selected because the monophyly of the
ingroup is assumed based on previous studies (e.g., Marshall et
al., 1990; Muizon, 1999). Twenty ambiguous and unambiguous
character states differentiate Mayulestes from the ingroup (char-
acters 3, 5, 6, 8, 10–13, 17, 20, 25–27, 29, 30, 36, 39, 42, 44, 46,
Appendix 2). Since Mayulestes could be more closely related to
the crown group Marsupialia than to sparassodonts (Rougier et
al., 1998), these characters may not be synapomorphies of Spar-
assodonta. For example, the absence of a postpalatine torus
(character 12) and a glenoid process of the alisphenoid (charac-
ter 17), shown as derived for the sparassodonts in this tree, could
be plesiomorphic for the group (see Rougier et al., 1998).

The character matrix (Appendices 2 and 3) is based largely on
that of Muizon (1999). We took from his matrix all the characters
that vary among sparassodonts or between Mayulestes and the
other sparassodonts (27 characters). Other important references
are Rougier et al. (1998) and Horovitz and Sánchez-Villagra
(2003). The scores of the taxa were modified only in five cases.
First, the precanine notch was coded as present in Prothylacynus
and Sipalocyon by Muizon (1999, character 33), but based on our
observations was scored as absent. Second, the ectotympanic was
supposed to be enclosed by the squamosal in Borhyaena and
Prothylacynus based on the presence of ridges and grooves
(character 4 of Muizon, 1999; Archer, 1976). However, on the
specimens we observed there is a wide area medial to the exter-
nal acoustic meatus in the squamosal bone in which we assume
the ectotympanic was only attached in life by ligaments (i.e., it is
not ventrally support by bone). Third, the transverse canal was
scored as present in Cladosictis by Muizon (1999, character 8),
but in the only specimen examined by us it is absent. It is well
known that some metatherians are polymorphic for the pres-

ence/absence of a transverse canal (Marshall 1977b; Sánchez-
Villagra and Wible, 2002), and this may also be the case in Cla-
dosictis. The transverse canal was scored absent in Sipalocyon
and Prothylacynus by Muizon (1999). However, it was reported
to be present by Archer (1976) in both taxa and it is also present
in the specimen of Prothylacynus we examined; for that reason it
is scored here as present. Fourth, the foramen ovale of Borhy-
aena was considered confluent with the foramen lacerum me-
dium (foramen for greater petrosal nerve sensu Wible, 2003) by
Muizon (1999, character 7). Here we considered only the con-
formation of the foramen ovale, and for that reason Borhyaena
is coded as for Mayulestes. Another independent character
should consider the variation of the foramen lacerum medium
(not included in this study until more specimens are analyzed).
Finally, the incisor row was codified as transverse in Sipalocyon
by Muizon (1999, character 36), but based on our observations
we code it as parabolic.

The characters in the matrix are organized as follows: 1–32,
skull; 33–34, dentary; 35–45, teeth; 46–47, postcranial skeleton.
Forty-one characters are binary, while six characters are mul-
tistate; five of these are ordered (characters 7, 33, 36, 42, and 45)
and one unordered (character 39). All the characters are given
equal weight.

The data matrix was analysed with NONA version 2.0 (Golo-
boff, 1993); this program of parsimony was used for heuristic
searches. The Wagner trees obtained were subject to branch
swapping with bisection-reconnection. The analysis resulted in
one tree (Fig. 10) with a length of 71 steps, CI � 0.74, and RI �
0.74. There are two groups. One is formed by the Borhyaenidae
(sensu Marshall et al., 1990), supported by two unambiguous
characters: the lack of a crest separating the anterior border of
the epitympanic recess from the posterior region of the alisphe-
noid hypotympanic sinus (character 31) and the presence of a
shallow subarcuate fossa (character 32). The second group is
formed by the Hathliacynidae, supported by two unambiguous
characters: the presence of pneumatisation in the squamosal
bone (character 15) and an ectotympanic enclosed posteriorly by
the squamosal (character 22). Lycopsis is often considered a
member of the Prothylacyninae (Marshall, 1979; Marshall et al.,
1990); nevertheless, in this analysis and in that of Muizon (1999),
it is placed as the sister group of Prothylacynus + Borhyaeninae.
The inclusion of another species, ordinarily considered as be-
longing to Prothylacyninae, should test the monophyly of this
subfamily.

The general relationship of the ingroup is very similar to that
presented by Muizon (1999), without considering the proborhy-
aenids and thylacosmilids, and differing only in the arrangement
of the hathliacynids. In that paper, Sallacyon is the sister taxon of
a natural group formed by (Sipalocyon (Cladosictis-Notogale)).
In our most parsimonious tree, Sallacyon appears also at the
base of the Hathliacynidae, but Sipalocyon and Notogale are
sister taxa, and Cladosictis is the sister taxon of Acyon. The
group formed by Sipalocyon and Notogale is supported by only
one ambiguous character, the presence of the transverse canal
(character 24); whereas Cladosictis plus Acyon are grouped by
one unambiguous character, stylar cusp B on M2 extremely re-
duced to absent (character 39), and by three ambiguous features,
a long skull (character 1), a conspicuous precanine notch (char-
acter 4), and the intercentrum of the atlas fused to the rest of the
vertebra (character 46). Finally, Acyon shows one autapomor-
phic, unambiguous character, the presence of two cusps (i.e.,
hypoconid and hypoconulid) in the talonid of the lower teeth
(character 45). As a final remark we note that the Hathliacyni-
dae, as well as their internal branches, have low branch support
(Bremer support of 1 in nodes 5, 6, and 7; and 2 in node 8) and
are justified only by a few mostly ambiguous features in either
analysis (the present review and Muizon, 1999); therefore, the
topology of this family could be unstable.

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 3, 2006680

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https://www.researchgate.net/publication/13433535_Rougier_G_W_J_R_Wible_and_M_J_Novacek_Implications_of_Deltatheridium_specimens_for_early_marsupial_history_Nature?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==
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https://www.researchgate.net/publication/248616585_Marsupial_skulls_from_the_Deseadan_Late_Oligocene_of_Bolivia_and_phylogenetic_analysis_of_the_Borhyaenoidea_Marsupialia_Mammalia?el=1_x_8&enrichId=rgreq-69370b89-367d-45bb-a425-06c8c6c44d8e&enrichSource=Y292ZXJQYWdlOzIzMjY4Nzc5MztBUzoxMDA5MjkxMzkzODAyMjRAMTQwMTA3NDc5MDkyNA==


CONCLUSIONS

Since the end of the Nineteenth century, the hathliacynids
have been consistently considered as a monophyletic group (e.g.,
Ameghino, 1894). Recent cladistic reviews also support this
grouping of the family (Marshall et al., 1990; Muizon, 1999,
among others). Hathliacynids are also recognized as a natural
group in our analysis by two derived, unambiguous characters,
the presence of pneumatization in the squamosal bone and an
ectotympanic enclosed posteriorly by the squamosal.

The discovery of an almost-complete skull and associated
postcranial elements of a new species, Acyon myctoderos, from
the middle Miocene beds (Laventan SALMA) of Quebrada
Honda (Bolivia), prompted the analysis of the systematics of this
genus and its inclusion in a phylogenetic framework. Acyon myc-
toderos is the largest known hathliacynid. Differences with the
type species of the genus, A. tricuspidatus, include longer diaste-
mata among premolars, p2 comparatively more robust with a
better developed posterior cusp and with a sharp anterior crest,
lower molars with a more poorly developed anterobasal cingu-
lum, m1–m3 with hypoconulids less salient posteriorly and more
vertically oriented, and larger hypoconids at least on m2.

Our phylogenetic analysis, including nine taxa of Sparas-
sodonta and Mayulestes as the outgroup, showed that Acyon is
more closely related to Cladosictis than to any other hathlia-
cynid. Acyon and Cladosictis share a long skull, a conspicuous
precanine notch, the stylar cusp B on M2 being extremely re-
duced to absent, and an intercentrum of the atlas fused to the
rest of the vertebra.

ACKNOWLEDGMENTS

We thank A. Kramarz (MACN), M. Reguero and S. Bargo
(MLP), and W. Simpson (FMNH) for access and assistance to
specimens under their care; J. R. Wible, A. Candela, and A.
Winkler for the extensive review of the manuscript; G. W. Rou-
gier and A. G. Martinelli for comments on the original manu-
script; Shawn Zack for help with fossil preparation; M. Hohloch
for help with photographic work; J. Blanco for help with the
figures; F. Anaya-Daza, R. Mamani, and B. Mamani for their
cooperation in the field work in Quebrada Honda. Work of
AMF was supported by CONICET and NSF (grant DEB 010971
to G. W. Rougier); MRSV by the Deutsche Forschungsgemein-
schaft (SA 883-4-1); MT and NS by the Overseas Scientific Re-
search Funds (no. 07041136) from the Ministry of Education,
Science and Culture of Japan; and RFK by the NSF (grant EAR-
00-87636).

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