Rhexoxylon cortaderitaense (Menéndez) comb. nov.,

a species of permineralized stems newly assigned to

the Corystospermaceae, from the Triassic of

Argentina

JOSEFINA BODNAR

BODNAR, J., June, 2008. Rhexoxylon cortaderitaense (Menéndez) comb. nov., a species of permineralized stems newly
assigned to the Corystospermaceae, from the Triassic of Argentina. Alcheringa 32, 171–190. ISSN 0311-5518.

Anatomically preserved stems of Late Triassic corystosperms from the upper Cortaderita Formation in San Juan
province, Argentina are described and assigned to Rhexoxylon cortaderitaense (Menéndez) comb. nov. These
specimens were originally attributed to conifers, a group not closely related to corystosperms. The silicified axes
preserve features of the cortex, pith, and secondary vascular system. Like all Rhexoxylon species, these axes have two
discontinuous cambial rings that develop centrifugal secondary xylem (normal secondary xylem) and centripetal
secondary xylem (inverted secondary xylem). Both types of xylem are separated by a band of parenchymatous tissue
developed by a remnant cambium. The centrifugal xylem is divided by parenchymatous rays following differential
cambial activity. The secondary xylem is pycnoxylic with uniseriate rays; tracheids have mainly biseriate and
alternate bordered pits on radial and tangential walls, and one simple oval pit in cross-fields. The distinctive feature
of R. cortaderitaense is the conspicuous amount of centripetal secondary vascular tissues not forming perimedullar
bundles, this being interpreted as an apomorphic trait. This interpretation of the character modifies previous
hypothesized phylogenetic relationships of corystosperm wood taxa.

Josefina Bodnar [jbodnar@fcnym.unlp.edu.ar] División Paleobotánica, Facultad de Ciencias Naturales y Museo. Paseo
del Bosque S/N B1900FWA La Plata, Buenos Aires, Argentina and Consejo Nacional de Investigaciones Cientı́ficas y
Técnicas. Received 7.12.2006; revised 14.5.2007.

Key words: Argentina, Late Triassic, upper Cortaderita Formation, Corystospermaceae, fossil wood, Rhexoxylon.

CORYSTOSPERMS are a Mesozoic seed-
fern family that dominated most Gondwa-
nan Triassic palaeofloras. First described
from the Molteno Formation (Natal Beds)
of South Africa (Thomas 1933), the group
was reconstructed based on detached but
closely associated Dicroidium Gothan, 1912
emend. Townrow, 1957 leaves, Pteruchus
Thomas, 1933 emend. Townrow, 1957
pollen organs and Umkomasia Thomas,
1933 ovulate organs. Cuticular similarities
and the occurrence of similar pollen
grains [Alisporites Daugherty, 1941; Falcis-
porites (Leschik 1956) Klaus, 1963;

Pteruchipollenites Couper, 1958] in Pteru-
chus pollen sacs and Umkomasia ovules
confirm this association (Artabe & Brea
2003). Vegetative and reproductive remains
are well known both from permineralized
specimens and from compression/impres-
sion fossils (Artabe & Zamuner 2007).

Seven genera of Corystospermaceae
stems have been described based on permi-
neralized axes. Rhexoxylon Bancroft, 1913
emend. Archangelsky & Brett, 1961 (Argen-
tina, Brazil, South Africa and Antarctica),
Tranquiloxylon Herbst & Lutz, 1995
(Argentina and Chile), Cuneumxylon Artabe
& Brea, 2003 (Argentina) and Elchaxylon
Artabe & Zamuner, 2007 (Argentina)
exhibit anomalous development of the

ISSN 0311-5518 (print)/ISSN 1752-0754 (online)
� 2008 Association of Australasian Palaeontologists
DOI: 10.1080/03115510801928338



secondary xylem and liana-like anatomy
(Archangelsky & Brett 1961, Brett 1968,
Herbst & Lutz 1995, Chong Dı́az et al.
1997, Artabe & Brea 2003, Artabe &
Zamuner 2007). On the other hand, Kyklox-
ylon Meyer-Berthaud, Taylor & Taylor,
1993 (Antarctica) and Jeffersonioxylon Del
Fueyo, Taylor, Taylor & Cúneo, 1995
(Antarctica) have a normal cylinder of
secondary xylem (Del Fueyo et al. 1995,
Meyer-Berthaud et al. 1992, 1993, Taylor
1996). Kykloxylon was assigned to corysto-
sperms based on the presence of attached
foliar bases similar in size and structure to
Dicroidium (Meyer-Berthaud et al. 1992,
1993, Taylor 1996), whereas Jeffersonioxy-
lon was considered to belong to this group
based on its association with Dicroidium
foliage (Taylor 1996, Cúneo et al. 2003).
Another genus assigned to Corystosperma-
ceae is Antarcticoxylon Seward, 1914 (An-
tarctica, South Africa and Brazil). Walton’s
(1925) proposal to synonymize Antarcticox-
ylon with Rhexoxylon was rejected by
Archangelsky & Brett (1961), Plumstead
(1964), Mussa (1980), Meyer-Berthaud &
Taylor (1991), Meyer-Berthaud et al.
(1993), and Herbst & Lutz (1995). These
authors recommended maintaining Antarc-
ticoxylon as a morphogenus including ex-
clusively the specimen found in the Beacon
Sandstone (Permian), Antarctica. The
South African and Brazilian materials were
suggested to be referable to Corystosperma-
ceae as they share features of the pith and
secondary xylem with Rhexoxylon and
Cuneumxylon (Archangelsky & Brett 1961,
Mussa 1980, Meyer-Berthaud & Taylor
1991, Meyer-Berthaud et al. 1993, Herbst
& Lutz 1995, Artabe & Brea 2003). How-
ever, Antarcticoxylon does not show anom-
alous secondary growth (Seward 1917,
Archangelsky & Brett 1961, Mussa 1980).

Rhexoxylon trunks have a pycnoxylic
wood derived from a unique mode of
secondary vascular tissue production
that results in centripetal and centrifugal

polyxylic stems. Secondary xylem is com-
monly used to describe the whole secondary
vascular cylinder, as in these stems the
secondary xylem is more conspicuous and
has more chance of preservation, than
secondary phloem. All the species assigned
to this genus are characterized by a pith
surrounded by a secondary vascular cylin-
der composed of two zones: (1) an inner
zone with one or more cycles of secondary
xylem strands that are either centripetal or
both centripetal and centrifugal, i.e., the
perimedullar vascular zone or medullar
vascular system; and (2) an outer zone with
wedges of centrifugal secondary xylem
separated by parenchymatous rays, i.e., the
peripheral vascular zone or cylindrical
vascular system (Archangelsky & Brett
1961, Artabe et al. 1999, Artabe & Brea
2003). Seven species have been attributed to
Rhexoxylon: R. africanum Bancroft, 1913
emend. Walton, 1923, R. tetrapteridoides
Walton, 1923 emend. Archangelsky & Brett,
1961, R. piatnitzkyi Archangelsky & Brett,
1961 emend. Brett, 1968, R. brasilense
Herbst & Lutz, 1988, Rhexoxylon sp. Lutz
& Herbst (1992 ¼ R. kraeuselii Spalletti,
Artabe, Morel & Brea nomen nudum; see
systematic palaeobotany section below),
Rhexoxylon sp. Taylor (1992) and R. brunoi
Artabe, Brea & Zamuner, 1999. This genus
shows a wide distribution in southwest
Gondwana (Argentina, Brazil and South
Africa) and Antarctica (Taylor 1992).

A species newly assigned to Rhexoxylon
is described in this paper. The material
comes from the upper Cortaderita Forma-
tion exposed at La Tinta Creek, Barreal
area, San Juan, Argentina. These stems were
first described by Menéndez (1956) and
named as Protophyllocladoxylon cortaderi-
taense. He assigned it to the conifer genus
Protophyllocladoxylon Kräusel, 1939 based
on secondary xylem characters (pycnoxylic
wood, uniseriate rays, cross-fields with one
simple oval to circular pit, tracheids with
alternate and slightly hexagonal bordered

172 JOSEFINA BODNAR ALCHERINGA



pits). However, corystosperm axis anatomy
was poorly known at the time of Menéndez’
(1956) work. Comprehension of their com-
plex anatomy advanced in subsequent years
via the contributions of Archangelsky
(1968), Archangelsky & Brett (1961), Brett
(1968), Petriella (1981, 1983), Herbst & Lutz
(1988, 1995), Zamuner (1991), Meyer-
Berthaud et al. (1993), Brea (1995), Artabe
et al. (1999), Artabe & Brea (2003), and
Artabe & Zamuner (2007). Moreover, cor-
ystosperm trunks have pycnoxylic wood
resembling that of primitive conifers. In
particular, Kräusel (1949) recognized a
strong similarity between the secondary
xylem of Rhexoxylon and the Protophyllo-
cladoxylon-type wood. Consequently, it is
very difficult to distinguish the wood of these
groups if only isolated secondary xylem is
found. Menéndez (1956), in spite of having a
complete trunk, merely studied, described
and illustrated the secondary xylem of the
specimens, this situation resulting in later
authors’ failure to notice the corystosper-
maceous nature of these fossils. Protophyl-
locladoxylon cortaderitaense has also been
mentioned as a primitive conifer by
Schultze-Motel (1961), Vogellehner (1965),
and Serra (1966). Lepekhina (1972) consid-
ered P. cortaderitaense and the whole genus
as gymnosperm morphotaxa, not necessarily
coniferous.

The species was later revised and in-
cluded in the family ‘Protopinaceae’
by Vogellehner (1967), who recombined it
as Protocircoporoxylon cortaderitaense.
Furthermore, several revisions of the Bar-
real palaeoflora were carried out by Bonetti
(1963), Stipanicic (1972, 1979), Artabe et al.
(1995, 2001, 2003), and Zamuner et al.
(1999, 2001). These authors pointed out the
occurrence of conifer trunks in the palaeo-
flora based on Menéndez’ (1956) descrip-
tions and illustrations.

Lutz & Herbst (1992) were the first
investigators to mention the presence of
Rhexoxylon trunks in the Barreal zone, but

their description was preliminary. Pre-
viously, Archangelsky (unpublished annota-
tion on the collection label dated 1990) had
reviewed the Protocircoporoxylon cortader-
itaense type material and noticed that it
probably belonged to Rhexoxylon. That
supposition was confirmed by Bodnar
(2006). Actually, the Protocircoporoxylon
cortaderitaense type material corresponds to
the informally named Rhexoxylon species
cited by Lutz & Herbst (1992) and has
unique features within the genus.

Materials and methods
The studied fossil stems were found in the
upper Cortaderita Formation at La Tinta
creek, 8 km east of Barreal city, situated in
the southern Precordillera, San Juan pro-
vince, western Argentina. The type material
was collected by Menéndez (1956) at the
southern limit of that creek (698 230 W, 318
400 S; Fig. 1). It consists of fragments,
branches, and sprouts belonging to a larger
silicified trunk buried in rose-coloured
siltstones and sandstones with a high
proportion of pyroclastic material. Comple-
mentary samples were later collected from
the same locality and unit during a field trip
carried out by the División Cientı́fica de
Paleobotánica of the Museo de La Plata.
The stems were preserved parallel to bed-
ding, in very fine red conglomerates (sabu-
lites) with a high proportion of pyroclastic
material and silcrete development (Fig. 2).
The sediments were deposited in a sandy to
gravelly braided fluvial system (Spalletti
2001). This species is also very abundant
in the lower Cortaderita Formation (Middle
to Late Triassic). Conspecific specimens of
Rhexoxylon were erroneously ascribed to
the Barreal Formation by Lutz & Herbst
(1992), whereas they certainly derive from
the lower Cortaderita Formation.

Stems were preserved by siliceous cellu-
lar permineralization. Preservation of all the
material is poor. Polished surfaces were

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 173



obtained from new specimens. The three
types of thin-sections (transverse, radial and
tangential) were prepared from these fossils
for microscopic examination and also
from the type material, because some of
Menéndez’ (1956) original slides are lost.
The standard terminologies of Greguss
(1955), Boureau (1956), Fahn (1990) and
IAWA Committee (2004) were used to
describe the fossil woods. The complemen-
tary material enabled improvement of the
original diagnosis and description of the
species.

The specimens and their microscopic
slides are deposited in the Paleobotanical
National Collection, Museo Argentino de
Ciencias Naturales ‘Bernardino Rivadavia’
(BAPb and BAPbPm), and Paleobotanical
Collection, División Cientı́fica de Paleobotá-
nica, Museo de La Plata, Universidad
Nacional de La Plata (LPPB and pmLPPB).

Systematic palaeobotany

Class GYMNOSPERMOPSIDA Stewart &
Rothwell, 1993
Order CORYSTOSPERMALES Petriella,
1981
FamilyCORYSTOSPERMACEAEThomas,
1933

Rhexoxylon Bancroft, 1913 emend. Arch-
angelsky & Brett, 1961

Type species. Rhexoxylon africanum
Bancroft, 1913 emend. Walton, 1923

Rhexoxylon cortaderitaense (Menéndez, 1956)
Bodnar comb. nov. emend. (Figs 3– 7)

Fig. 2. Sedimentary log of the upper section of
Cortaderita Formation at La Tinta Creek including
the fossiliferous level with silicified trunks.

Fig. 1. Location map of Barreal area, San Juan
province, Argentina. A, Geographical map of Argen-
tina showing the location of San Juan province. B,
Geographical map of San Juan province showing
Barreal fossiliferous locality.

174 JOSEFINA BODNAR ALCHERINGA



Basionym. Protophyllocladoxylon cortaderi-
taense Menéndez, 1956, p. 274, pls. I – II.

Synonymy
1967 Protocircoporoxylon cortaderitaense
Vogellehner, p. 41.

1992 Rhexoxylon sp. Lutz & Herbst, p. 74 –
76, fig. 1.

1999 Rhexoxylon kraeuselii Spalletti,
Artabe, Morel & Brea, nomen nudum.

2001 Rhexoxylon sp. A (R. kraeuselii?) Lutz
& Herbst in Zamuner, Zavattieri, Artabe
& Morel, p. 163, fig. 6.21.

2001 Rhexoxylon kraeuselii Morel, Artabe,
Zavattieri & Bonaparte, nomen nudum.

2003 Rhexoxylon kraeuselii Artabe & Brea
nomen nudum.

2003 Rhexoxylon sp. (R. kraeuselii?) Lutz &
Herbst in Artabe & Brea, p. 11.

Type material. Lectotype specimen BAPb
5410; BAPbPm 5410, 505, 506, 507 and 520.
Paralectotypes BAPb 5372, 5378, 5382 and
5417; BAPbPm 5433; LPPB 13083, 13084,
13085, 13086, pmLPPB 1358, 1359, 1360,
1361, 1362, 1363, 1364.

Additional material. LPPB 13087, 13088,
13089, 13090.

Type locality. La Tinta Creek, Barreal, San
Juan; upper Cortaderita Formation, Late
Triassic (Spalletti et al. 1999).

Emended diagnosis. Columnar, eccentric, and
polyxylic gymnospermous stems, 10 – 45 cm in
diameter. Oval and heterogeneous pith, 3 –4
cm61.4 –3.2 cm in diameter, with parenchy-
ma cells, polyhedral idioblasts, sclerotic nests,
and secretory cavities. Vascular secondary
system with centripetal and centrifugal xylem
and phloem, divided in wedges by parenchy-
matous rays 0.07 – 0.45 cm wide. Conspicuous
and thick centripetal secondary xylem, sepa-
rated by a thin band of parenchymatous tissue
from centrifugal secondary xylem. Secondary
xylem tracheids rectangular to pentagonal in
cross-section, with mainly biseriate and alter-
nate bordered pits on radial and tangential
walls. Cross-fields with one simple and large
pit, circular to oval in shape. Rays uniseriate,
1 – 15 cells high. Axial parenchyma and ray
tracheids absent. Cortex with an inner zone of
vascular strands and a poorly preserved outer
zone composed of centrifugal secondary
phloem and cork.

Description. The trunks are eccentric and
oval in cross-section with a major diameter

Fig. 3. Rhexoxylon cortaderitaense (Menéndez) Bodnar comb. nov. A, Polished surface showing the general aspect of
the stem in transverse section, BAPb 5410 (lectotype). Scale bar: 2 cm. B, Transverse section of the same stem
showing secondary vascular cylinder pattern, BAPbPm 5410 (lectotype). Scale bar: 2 cm. C, Diagram of the same
stem showing the secondary vascular cylinder organization: pith (p), centripetal secondary xylem (cp), centrifugal
secondary xylem (cf) and parenchymatous rays (r), BAPb 5410 (lectotype). Scale bar: 2 cm.

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 175



of 10 – 30 cm and a minor diameter of
8 – 24 cm. Some specimens reach 45 cm in
diameter (Menéndez 1956). The specimens

are characterized by the presence of pith,
secondary xylem, secondary phloem, and
cortex. This trunk is polyxylic because of

Fig. 4. Rhexoxylon cortaderitaense (Menéndez) Bodnar comb. nov. A, Perimedullar region of a mature stem, showing
the pith (P) and centripetal secondary vascular tissues: centripetal phloem (PH) and centripetal xylem (XY), LPPB
13083 (paralectotype). Scale bar: 10 mm. B, Detail of centripetal phloem (PH) and centripetal xylem (XY) in the
same stem, LPPB 13083 (paralectotype). Scale bar: 5 mm. C, Cross-section showing less developed centripetal
phloem (PH) towards the pith (P) in an immature stem, BAPbPm 5410 (lectotype). Scale bar: 1 mm. D, Cross-section
of the pith with polyhedral idioblasts (ID) and secretory cavities (SC), pmLPPB 1358 (paralectotype). Scale bar:
300 mm. E, Radial section of the pith with polyhedral idioblasts (ID), secretory cavities (SC) and sclerotic nests (SN),
BAPbPm 5410 (lectotype). Scale bar: 300 mm. F, Detail of a secretory cavity (SC) surrounded by idioblasts (ID) in
radial section, BAPbPm 5410 (lectotype). Scale bar: 250 mm.

176 JOSEFINA BODNAR ALCHERINGA



two discontinuous anomalous cambial rings
that start developing centripetal secondary
xylem and phloem towards the pith and
centrifugal secondary xylem and phloem
towards the cortex (Figs 3A, B, C; 4A, B, C).

The pith is 3 – 4 cm61.4 – 3.2 cm in dia-
meter, oval and heterogeneous (Figs 3B, C;
4A), with spherical parenchyma cells, poly-
hedral idioblasts with dark contents, sclero-
tic nests, secretory cavities, and probably
vascular strands (Fig. 4D, E, F).

Centripetal secondary phloem is 0.1 –
(0.65) – 1.1 cm thick. Owing to its poor
preservation, anatomical features of the
phloem cannot be determined in detail.
However, it seems to include phloem

parenchyma, phloem fibres and sieve cells.
Phloem fibres are clustered in groups
separated by bands of crushed and de-
formed cells (Fig. 4A, B, C).

Centripetal secondary xylem is 0.24 –
(1.05) – 1.4 cm thick. Each centripetal xylem
segment is 0.37 – (0.94) – 1.3 cm wide and is
separated from centrifugal secondary xylem
by parenchymatous tissues of cells similar to
those of the medulla (¼tangential parench-
yma) arising from a remnant cambium
(Fig. 5A).

Primary xylem, which should be located
between centripetal and centrifugal second-
ary xylem, is not discernible. Despite this, it
can be inferred to be mesarch.

Fig. 5. Rhexoxylon cortaderitaense (Menéndez) Bodnar comb. nov. A, Segment of centripetal secondary xylem in
cross-section. BAPbPm 5410 (lectotype). Scale bar: 2 mm. B, Centrifugal secondary xylem wedge in cross-section.
BAPbPm 5410 (lectotype). Scale bar: 4.5 mm. C, Ring-like growth developed inside the centripetal xylem. GR
symbolizes the ring-like boundary. pmLPPB 1358 (paralectotype). Scale bar: 1 mm. D, Ring-like growth developed
inside the centrifugal xylem wedges. GR symbolizes the ring-like boundary. pmLPPB 1361 (paralectotype). Scale bar:
400 mm.

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 177



Centrifugal secondary xylem is 3.3 –
(5.73) – 11.5 cm thick (Fig. 5B). It is ar-
ranged in wedges as a result of differential
cambial activity around the circumference
of the axes. There are cambium strands that
only produce ray-parenchyma, so the whole
secondary wood is divided by parenchyma-
tous rays. Such rays may also develop
locally within the xylem wedges (Figs 3B,
C; 5B). Each centrifugal xylem wedge is
1.1 – (2.81) – 6.5 cm wide. Parenchymatous

rays are 0.07 – (0.19) – 0.45 cm wide and
consist of tissues similar to those of the
medulla. In addition, tangential fragmenta-
tion inside xylem wedges is evident in some
specimens. It is a consequence of remnant
cambium activity, which produces parench-
ymatous tissue (¼tangential parenchyma).

Ring-like growth is developed inside the
centripetal and centrifugal secondary xylem,
with an abrupt diminution of the radial
diameter of tracheids towards the ring’s

Fig. 6. Rhexoxylon cortaderitaense (Menéndez) Bodnar comb. nov. A, Detail of tracheids in transverse section,
BAPbPm 505 (lectotype). Scale bar: 200 mm. B, General view of secondary xylem in radial section, pmLPPB 1359
(paralectotype). Scale bar: 100 mm. C, Biseriate radial pitting of tracheids, BAPbPm 506 (lectotype). Scale bar: 30 mm.
D, Fenestriform cross-fields (CF) and triseriate radial pitting of tracheids (TP), pmLPPB 1359 (paralectotype). Scale
bar: 40 mm. E, Detail of biseriate radial pitting of tracheids showing its slightly hexagonal shape and circular pore,
BAPbPm 506 (lectotype). Scale bar: 10 mm. F, Uniseriate homocellular rays (R) of secondary xylem in tangential
section, pmLPPB1363 (paralectotype). Scale bar: 150 mm.

178 JOSEFINA BODNAR ALCHERINGA



outer margin (Fig. 5C, D). This ring-like
growth does not constitute true continuous
rings because the boundaries are interrupted
by the parenchymatous rays; this results in
unconformity with those of the adjacent
xylem wedges. The ‘latewood’ band is 1 – 3
cells thick. Secondary xylem tracheids are
rectangular to pentagonal in transverse
section. The radial diameter of tracheids is

16.33 – (37.11) – 65.31 mm and the tangential
diameter of tracheids is 28.57 – (40.77) –
58.77 mm (Figs 5D, 6A). The thickness of
the double wall between two tracheids is
4.16 – (7.99) – 24.49 mm in radial section and
4.16 – (7.70) – 20.41 mm in tangential section.
Tracheids show mainly biseriate, less com-
monly triseriate and uniseriate, bordered
pits on both radial and tangential walls

Fig. 7. Rhexoxylon cortaderitaense (Menéndez) Bodnar comb. nov. A, General view of the outer zone of centrifugal
xylem wedges (W) and cortex (C), its inner zone composed of vascular strands (VS). A probable branch trace (BT) is
also visible departing from centrifugal xylem, LPPB 13086 (paralectotype). Scale bar: 10 mm. B, Detail of the branch
trace (BT) and the inner cortex zone composed of vascular strands (VS), LPPB 13086 (paralectotype). Scale bar: 5
mm. C, Detail of cortex vascular strands (VS), LPPB 13086 (paralectotype). Scale bar: 3 mm. D, Cross-section of the
outer zone of centrifugal xylem wedges (W) and cortex vascular strands (VS), pmLPPB 1364 (paralectotype). Scale
bar: 600 mm.

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 179



(Fig. 6B, C, D, E). Tangential pits are very
poorly preserved. Pits are circular or slightly
hexagonal, with contiguous or compressed
arrangement in alternate pit rows. Pit pores
are circular or elliptical (Fig. 6E). On both
radial and tangential tracheid walls, pits are
10.40 – (13.67) – 18.72 mm high and 11.44 –
(14.10) – 22.88 mm wide. Cross-fields have
one, sporadically two, simple pits with
circular or oval outline, 10.4614.56 mm–
(16.89623.44 mm) – 29.7633 mm in dia-
meter, placed in vertical rows (Fig. 6B, D).

Secondary xylem rays are homocellular;
uniseriate, rarely biseriate (Fig. 6F). The
rays are short with rectilinear trajectory,
and are 48.98 – (198.65) – 448.98 mm and 1 –
(mode 3; mean 5.78) – 15 cells high. They are
composed of rectangular parenchyma cells
with thin walls, and 20.41 – (31.31) –42.9 mm
high and 12.24 – (24.96) – 36.73 mm wide.
Ray cell length cannot be measured because
of poor preservation. Axial parenchyma
and ray tracheids are absent.

The cortex is 1 – 3 cm wide and consists
of two zones (Fig. 7A). The inner zone
shows continuity with centrifugal secondary
and parenchymatous tissues (Fig. 7B, C). It
consists of numerous vascular strands sur-
rounded by parenchyma cells, polyhedral
idioblasts and probably sclerotic nests
(Fig. 7D). The outer zone is poorly pre-
served and consists of centrifugal secondary
phloem and cork. Primary phloem is not
preserved.

Remarks. A lectotype (BAPb 5410) was
chosen from the syntypes described by
Menéndez (1956). The specimen assigned
as lectotype is the most complete and best
preserved of Menéndez’ samples. Two
original slides (transverse and radial) of
BAPb 5410 material are identified by the
collection number: BAPbPm 5410. The new
slides prepared from BAPb 5410 specimen
are labelled with the collection numbers:
BAPbPm 505, 506, 507, and 520. Menéndez’
other specimens (BAPb 5372, 5378, 5382

and 5417; BAPbPm 5433) are indicated as
paralectotypes. Some of the complementary
materials (LPPB 13083, 13084, 13085,
13086) were essential for the emended
diagnosis, and so they are also considered
paralectotypes. The rest of the complemen-
tary fossils are indicated as additional
material (LPPB 13087, 13088, 13089,
13090).

Rhexoxylon cortaderitaense is also abun-
dant in the lower Cortaderita Formation
(Artabe et al. 1995). It is the same species
that Lutz & Herbst (1992) described as
Rhexoxylon sp. and erroneously ascribed to
the Barreal Formation. After the original
description of the species, several authors
mentioned it under different names: Rhex-
oxylon kraeuselii and Rhexoxylon sp. A (R.
kraeuselii?); based on the intention of Lutz
& Herbst (1992) to dedicate this species to
Dr Richard Kräusel. Rhexoxylon kraeuselii
is a nomen nudum because it was published
without a description or diagnosis (ICBN,
McNeil et al. 2006). On the other hand,
Rhexoxylon sp. A (R. kraeuselii?) is not an
appropriate reference to the species because
such a citation is applied to a taxon doubt-
fully ascribed to the legitimately named
species between parentheses, this not being
the case here.

Comparisons and discussion
Corystospermaceae stems share some
anatomical features with medullosans
and cycads (Archangelsky & Brett 1961,
Archangelsky 1996). Deviations from the
typical pattern of secondary growth are
present in these groups. Each segment of
primary xylem in the vascular system of
medullosans is surrounded by a complete
cylinder of secondary xylem (Namboodiri &
Beck 1968, Basinger et al. 1974), which
has centrifugal and centripetal portions
developed by normal and inverted cam-
bium, respectively. The presence of two

180 JOSEFINA BODNAR ALCHERINGA



discontinuous anomalous cambial rings is
considered as polyxyly.

Recently, Césari et al. (2005) described
Late Carboniferous stems from Argentina
with anomalous secondary xylem comple-
tely surrounding each segment of primary
xylem, which is the first record of this kind
of axis for the Palaeozoic of Gondwana.

Permian medullosans have a more com-
plex vascular cylinder with two distinct
systems: a cylindrical one or ‘Plattenringe’,
and medullar one or ‘Sternringe’ (Weber &
Sterzel 1896, Worsdell 1906, Bancroft 1914).
Both systems, also present in cycad and
corystosperm stems, are considered to be
homologous, as the medullar arrangement
is interpreted to derive from the cylindrical
one (Worsdell 1906, Artabe & Stevenson
1999, Artabe & Brea 2003, Artabe et al.
2005). Within medullar and cylindrical
systems, there may be more than one cycle
of secondary vascular tissues produced by
supernumerary cambia.

Most Permian medullosans show an
increase in complexity and regularity of
cylindrical system. They have one or two
cycles of centripetal – centrifugal secondary
xylem and secondary phloem segments,
which constitute an almost complete ring
by tangential fusion (Weber & Sterzel 1896,
Bancroft 1914, Petriella 1981). Conse-
quently, some species of Medullosa Cotta,
1832 have been observed to possess included
phloem (Weber & Sterzel 1896). The cylind-
rical system in cycads shows a progressive
loss of the centripetal secondary xylem,
which becomes relictual in extant genera
(Worsdell 1896, 1906). Centripetal second-
ary xylem is also absent in the cylindrical
system of some corystosperm genera
(Artabe & Brea 2003).

As mentioned above, the medullar sys-
tem develops from centripetal secondary
xylem of the cylindrical system (Worsdell
1906, Artabe & Stevenson 1999, Artabe &
Brea 2003, Artabe et al. 2005). Centripetal
xylem splits from the cylindrical system,

and subsequently, centrifugal xylem begins
to grow at the outer edge leading to
perimedullar bundle formation. The medul-
lar system in medullosans and cycads
consists of many concentric and double
(centripetal – centrifugal) bundles with an
irregular arrangement (Weber & Sterzel
1896, Worsdell 1896, 1906, Bancroft 1914).
In contrast, corystosperms have perimedul-
lar double bundles organized in a regular
pattern (Artabe & Brea 2003).

Although medullosans and cycads share
structural features with corystosperms, their
secondary xylem histology differs from each
other, since, in the first two groups, it is
manoxylic and in the last one, pycnoxylic.
The presence of anomalous secondary
xylem production yielding pycnoxylic wood
is unique within gymnosperms.

Within the Corystospermaceae, Kyklox-
ylon, Jeffersonioxylon and Antarcticoxylon
possess a normal solid secondary vascular
cylinder equivalent to the cylindrical system
described above. Tranquiloxylon and Cu-
neumxylon only show an anomalous vascu-
lature in the form of polyxylic centrifugal
secondary xylem dissected by parenchyma-
tous rays. They do not produce centripetal
xylem or perimedullar double bundles, so
they lack a medullar system (Herbst & Lutz
1995, Artabe & Brea 2003). Elchaxylon has
an undivided cylindrical system, but it is a
polyxylic stem because it possesses both
centrifugal and centripetal secondary xylem
around the mesarch primary xylem; how-
ever, it does not develop perimedullar
bundles (Zamuner 1991, Artabe et al.
1999, Artabe & Brea 2003, Artabe &
Zamuner 2007).

Rhexoxylon shows the most complex
vascular cylinder pattern within corystos-
perms. It has a polyxylic stem owing to the
presence of a centrifugal (normal) cambium
and a centripetal (inverted) cambium. These
stems can develop more than one centrifu-
gal cambium, generating a centrifugal poly-
xylic anatomy composed of secondary

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 181



xylem and phloem concentric rings in the
cylindrical system. In addition, the medullar
system develops from more than one
cambium, i.e. centripetal polyxyly, charac-
terized by one or more cycles of perimedul-
lar bundles. Each perimedullar bundle
consists of a centripetal vascular section
(xylem þ phloem) towards the pith and a
centrifugal portion (xylem þ phloem) to-
wards the cortex. The formation of tangen-
tial parenchyma (from a remnant cambium)
separates both centrifugal – centripetal sec-
ondary xylems triggering bundle fragmenta-
tion. In brief, Rhexoxylon stems have two
secondary vascular systems: cylindrical or
peripheral and medullar or perimedullar.
Most of the species of Rhexoxylon express
variations on the pattern described above.

In R. tetrapteridoides (Molteno Forma-
tion, Stormberg Group, Middle to Late
Triassic; Natal, South Africa), the cylindri-
cal vascular system incorporates quite com-
pact centrifugal secondary xylem wedges
with a small quantity of parenchymatous
tissues (tangential parenchyma and parench-
ymatous rays) and scarce development of
centripetal secondary xylem towards the
medulla (Archangelsky & Brett 1961). The
medullar system is composed of one com-
plete ring of centripetal – centrifugal bundles
(Archangelsky & Brett 1961, Artabe & Brea
2003). In these bundles, the centripetal
xylem is significantly thicker than the
centrifugal xylem (Zamuner 1991).

In R. piatnitzkyi (Ischigualasto Forma-
tion, Agua de La Peña Group, early Late
Triassic; San Juan province, Argentina), the
dissected, cylindrical vascular system con-
sists of several centrifugal xylem segments
with a discrete quantity of centripetal secon-
dary xylem separated by a narrow zone of
tangential parenchyma (Archangelsky &
Brett 1961, Archangelsky 1968, Brett 1968,
Zamuner 1991). Parenchymatous tissues are
abundant. The medullar system is more
conspicuous than in R. tetrapteridoides. It
is constituted by one complete ring of

centripetal – centrifugal bundles, in which
centrifugal parts show a more marked
development than in the R. tetrapteridoides
medullar system.

Rhexoxylon brasilense (Caturrita Forma-
tion, Rosário do Sul Group, Late Triassic;
Rio Grande do Sul, Brazil) possesses a
cylindrical vascular system with a ring of
centripetal – centrifugal wedges, in which
centripetal xylem is thicker than in pre-
viously mentioned species and is closely
associated with the medullar system (Herbst
& Lutz 1988). The medullar system has a
complete ring of centripetal – centrifugal
bundles.

In R. brunoi (Los Colorados Formation,
Uspallata Group, Late Triassic; Mendoza
province, Argentina), the cylindrical vascu-
lar system comprises wedges with centripe-
tal – centrifugal secondary xylem, and the
centripetal xylem is also closely linked to the
medullar system. The medullar system has a
complete cycle of 16 centripetal – centrifugal
bundles and one cycle of centripetal xylem
(Artabe et al. 1999).

The stem of R. africanum (lower Elliot
Formation? or ‘red beds’, Stormberg
Group, Late Triassic to Early Jurassic?,
Cape Province, South Africa) possesses a
cylindrical vascular system with well-
developed centrifugal wedges of secondary
xylem and centripetal secondary xylem. The
medullar system contains one or more
concentric series of centripetal – centrifugal
bundles (Bancroft 1913, Walton 1923,
Kräusel 1956, Archangelsky & Brett 1961).

The specimen with Rhexoxylon-like sec-
ondary anatomy from the Triassic of
Antarctica is characterized by a cylinder of
secondary xylem divided into sectors by
large parenchymatous rays and the devel-
opment, within the pith, of some secondary
xylem strands of uncertain structure (Taylor
1992). As the medullar system arrangement
is unknown, this stem is not included in
the following discussion and phylogenetic
considerations.

182 JOSEFINA BODNAR ALCHERINGA



In the seventh species of the genus, R.
cortaderitaense, the cylindrical vascular sys-
tem is composed of one cycle of large
wedges of centrifugal secondary xylem
separated by tangential parenchyma from
one cycle of centripetal secondary xylem
and secondary phloem. The medullar vas-
cular system is not developed, since R.
cortaderitaense lacks perimedullar bundles,
but centripetal tissues are very well devel-
oped, and they could be described as a
‘perimedullar vascular zone’ for practical
purposes. Despite this, R. cortaderitaense
shows a strong resemblance to R. tetra-
pteridoides as the secondary vascular cylinder
of smaller R. tetrapteridoides specimens is
composed of only one cycle of centrifugal –
centripetal xylem, without perimedullar cen-
tripetal – centrifugal bundles (Archangelsky
& Brett 1961, Zamuner 1991).

Ontogenetic and phylogenetic
considerations
The cauline vascular system ontogeny of three
species of Rhexoxylon was studied: R. tetra-
pteridoides, R. piatnitzkyi, and R. africanum
(Archangelsky & Brett 1961, Zamuner 1991).
Developmental features expressed within these
species can be extrapolated to other species
within the genus (R. brunoi, R. brasilense). In
the juvenile axes, the secondary vascular
cylinder consists of only one centrifugal –
centripetal xylem cycle, with uniform devel-
opment of both parts (Fig. 8A). In the
following stages, cambium activity becomes
differential with more prominent growth of
centrifugal tissues than of centripetal tissues
(Fig. 8B). Later, the centripetal xylem splits
from the centrifugal one due to parenchyma
development between them (Fig. 8C). In the
late phases, centrifugal xylem begins to grow
at the outer edge of the centripetal one,
this leading to the double (centripetal –
centrifugal) bundle formation. Furthermore,
a discrete quantity of centripetal xylem is

developed on the inner edges of centrifugal
wedges. These three processes, centripetal
xylem growth, separation of centrifugal and
centripetal parts, and double bundle forma-
tion, may take place more than once. Conse-
quently, themature trunk ofRhexoxylon has a
vascular cylinder with a cycle of perimedullar
centripetal – centrifugal bundles and a cycle of
centrifugal – centripetal peripheral wedges
(Fig. 8D). On the other hand, when the first
centripetal xylem splits from centrifugal xylem
in R. cortaderitaense axes (Fig. 9A, B), a new
centrifugal xylem is not generated at the outer
edge. Conversely, more centripetal tissues
continue to develop (Fig. 9C). This is a distinc-
tive feature (¼ autapomorphy) of this species,
and the differences in xylem production have
important phylogenetic implications.

It was suggested that the corystosperm
secondary vascular cylinder evolved along
two principal lineages (rhexoxyloid and
cuneumxyloid) from a medullosan or simi-
lar precursor (Artabe & Brea 2003, Artabe
& Zamuner 2007). Cuneumxylon and Tran-
quiloxylon characterize the cuneumxyloid
line. These taxa show anomalous vas-
culature only in the centrifugal secondary
vascular tissues because they do not
produce centripetal xylem or perimedullar
bundles. The rhexoxyloid line includes
Rhexoxylon and Elchaxylon and is
characterized by: mesarch primary xylem,
centrifugal wedges of secondary xylem,
centripetal secondary xylem and perimedul-
lar bundles. According to Artabe &
Brea (2003), the Argentinean species of
Rhexoxylon fit into a developmental series
(R. cortaderitaense, R. piatnitzkyi, and
R. brunoi) showing an increase in anom-
alous vascular tissue development with
formation of successive cycles of perime-
dullar bundles inside the trunk and the
progressive acquisition of larger stems by an
increase in the amount of centrifugal
secondary conducting tissues. Elchaxylon
represents a branch that initiates with R.
cortaderitaense (Artabe & Zamuner 2007).

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 183



Fig. 8.Development sequence of secondary vascular system in R. tetrapteridoides. Lined pattern represents secondary
xylem; dotted pattern symbolizes tangential parenchyma and secondary phloem. At the first stage (A), two
discontinuous cambial rings give rise to equal amounts of centrifugal secondary xylem and centripetal secondary
xylem around primary xylem. At the next phase (B), cambial rings show preferential development of their centrifugal
part. In late stages, centripetal xylem splits from centrifugal xylem due to parenchyma development between them
(C). At the adult phase of R. tetrapteridoides (D), centrifugal xylem grows at the outer edge of centripetal xylem
generating a cycle of double (centripetal – centrifugal) bundles. Furthermore, centripetal xylem is developed in the
inner edges of some centrifugal wedges.

184 JOSEFINA BODNAR ALCHERINGA



Fig. 9. Development sequence of secondary vascular system in R. cortaderitaense. Lined pattern represents secondary
xylem; dotted pattern symbolizes tangential parenchyma and secondary phloem. The two early stages (A and B) are
the same as those of R. tetrapteridoides. In later stages, centripetal xylem splits from centrifugal xylem due to
parenchyma development between. At the adult phase of R. cortaderitaense (C), centripetal xylem keeps on growing
towards the pith.

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 185



Elchaxylon, like this last species, has
centripetal secondary xylem but does not
produce perimedullar bundles (Fig. 10A).

An alternative phylogeny to the Artabe
& Brea (2003) model can be hypothesized
by incorporating the characters of the

Fig. 10. Conjectural phylogenetical trees showing the hypothesized relationships between corystosperm stem taxa.
A, Hypothesized relationships between Argentinean corystosperm stem taxa considering the lack of a medullar
system primitive (adapted from Artabe & Brea 2003; Artabe & Zamuner 2007). B, Hypothesized relationships
between all the corystosperm axis morphotaxa considering the lack of a medullar system derived (as suggested in
this paper).

186 JOSEFINA BODNAR ALCHERINGA



non-Argentinean taxa and interpreting all
the characters from a different point of
view. If Permian medullosans are consid-
ered as the probable ancestor of corysto-
sperms, then all their stem features must be
assumed as plesiomorphic in the corysto-
sperm axis phylogenetic analysis. Thus, the
rhexoxyloid lineage, characterized by traits
present in Permian medullosans (mesarch
primary xylem, centrifugal wedges of sec-
ondary xylem, centripetal secondary xylem,
and perimedullar bundles) is not a natural
group. On the other hand, the cuneumxy-
loid lineage could be a monophyletic group,
given that its diagnostic characters (endarch
primary xylem, loss of medullar system) are
apomorphic. Rhexoxylon tetrapteridoides is
proposed as basal in the corystosperm axis
phylogeny, given that it has perimedullar
bundles in the adult stage showing, at the
same time, the simplest vascular pattern,
features that are reiterated in the ontogeny
of most of Rhexoxylon species and Permian
medullosans. The other species of the genus
could be arranged in a lineage that shows a
trend towards increasing anomalous vascu-
lar tissue development: R. tetrapteridoides,
with one complete ring of centripetal –
centrifugal bundles; R. piatnitzkyi, with a
more conspicuous ring of centripetal –
centrifugal bundles and a discrete quantity
of centripetal xylem in the cylindrical
system; R. brasilense and R. brunoi, with a
complete ring of centripetal – centrifugal
bundles and a thicker centripetal xylem in
the cylindrical system closely linked to the
medullar system; and R. africanum, with
two complete rings of perimedullar bundles.
In contrast, R. cortaderitaense and Elchax-
ylon would be placed closer to the cuneum-
xyloid line, because they lack a medullar
system and show a reduction of the centri-
petal (inverse) xylem (Fig. 10B). In this
hypothesis, corystosperm stems with
normal secondary vascular cylinder (Kyklox-
ylon, Jeffersonioxylon, and Antarcticoxylon)
should be related to the cuneumxyloid group

given that the acquisition of a non-anom-
alous vascular cylinder is secondary. How-
ever, until a thorough phylogenetic analysis
is carried out with a character matrix
analyzed by a cladistic software program,
these relationships remain largely untested.

Conclusions
A species of Corystospermaceae, Rhexox-
ylon cortaderitaense, is described from
the Late Triassic upper Cortaderita Forma-
tion of Argentina. As R. cortaderitaense
had been originally described as the ‘proto-
pinacean’ conifer Protophyllocladoxylon
(¼Protocircoporoxylon) cortaderitaense,
the new combination has important impli-
cations for the composition and biostrati-
graphy of the southwestern Gondwanan
Triassic flora, taking into account that
Protocircoporoxylon cortaderitaense has
been considered a biostratigraphic index
characterizing one of the Assemblage
Biozones proposed for the Triassic of
Argentina (Spalletti et al. 1999).

The anatomically preserved stems, like all
Rhexoxylon species, have a polyxylic anat-
omy with two discontinuous anomalous
cambial rings, which develop both centrifu-
gal secondary xylem and centripetal second-
ary xylem. Rhexoxylon cortaderitaense adult
stems have conspicuous amounts of centri-
petal tissues that do not constitute centripe-
tal – centrifugal perimedullar bundles. This
character is considered apomorphic, placing
R. cortaderitaense in a derived position
within the corystosperm stem phylogeny.
This new interpretation of the character
modifies the previous hypothesized phyloge-
netic relationships of corystosperm axis
morphotaxa.

Acknowledgements
I acknowledge Sergio Archangelsky, Georgi-
na del Fueyo and Luis Lezama for the loan
of specimens from Paleobotanical National

ALCHERINGA ARGENTINE CORYSTOSPERM WOOD 187



Collection and allowing the preparation of
new slides. I thank Analı́a Artabe, Eduardo
Morel, Ari Iglesias and Daniel Ganuza for
their cooperation during fieldwork, and for
the important exchange of opinions and ideas
during the investigation. I am most grateful
to Amanda Zamuner for improving the
English version of the manuscript. I espe-
cially thank the reviewers for their construc-
tive comments. This work was supported by
theAgencia Nacional de Promoción Cientı́fica
y Tecnológica (PICT 07-08467).

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190 JOSEFINA BODNAR ALCHERINGA

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