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67
Introduction
Although scattered in time and space, in the latter two
decades several fossil finds from the Early and mid Cretaceous
have provided increasing evidence for the divergence of the
main lineages within the crown-group Anura (e.g., Gao and
Wang 2001, Gao and Chen 2004, Báez et al. 2009, Báez 2013),
suggesting that this period of time witnessed most of the basal
cladogenetic events in this taxon. However, in many instances
the incomplete or poor preservation of remains of this age
do not allow us to determine their taxonomic placement
unambiguously, and rigorous reinterpretation of different
character states, sometimes with the inclusion of additional
finds, is crucial (e.g., Báez and Sanchiz 2007, Dong et al. 2013).
In a brief account of the latest paleontological discove-
ries between 1952–1954 published on the occasion of the
II Cursillo Internacional de Paleontología at the Sabadell
Museum of Spain (previously dated as 1956 but now
established as 1954 according to Martin et al. 2012), J.F. de
Villalta Comella announced the find of a new frog discovered
by J. Ferrer in the well-known fossiliferous lithographic
limestones at Santa Maria de Meià, Sierra de Montsec,
province of Lleida, and listed some diagnostic features. The
bearing beds were formerly assigned to the Late Jurassic but
at present are considered of Early Cretaceous age (Early
Barremian) (Ansorge 1995, Martínez-Closas and López
Morón 1995). Although the fossil material was not described
in detail at the time, it was estimated to represent a new
“primitive discoglossid” and thus named Eodiscoglossus
santonjae VILLALTA, 1954 (Villalta 1954). Almost simultane-
ously, the specimen was figured by Piveteau (1955: 269) and
a photograph with a brief comment was presented in
the Spanish edition of the work of Leonardi directed by
B. Melendez (Leonardi 1957). Later on, Hecht (1963)
made passing comments on a few features of this taxon but
subsequently published a complete description of the
holotype (Hecht 1970), which consists of an articulated
skeleton preserved on two slabs as part and counterpart that
he considered to represent a discoglossid frog. Hecht (1970)
also provided fluorescence photographs of both parts, one of
which is available for examination at the Museo Nacional de
Ciencias Naturales de Madrid (MNCN 4723, Text-fig. 1),
whereas the other one was previously in a private collection
(Colección Ferrer-Condal en Tirvia, Lleida) but its wher-
eabouts are at present unknown (Martin et al. 2012; Sanchiz,
pers. comm. 2015). Shortly afterwards, Estes and Reig
(1973) listed the main anatomical characters of E. santonjae
and presented a skeletal reconstruction in dorsal view.
REVISION OF THE SKELETAL MORPHOLOGY OF EODISCOGLOSSUS SANTONJAE,
AN EARLY CRETACEOUS FROG FROM NORTHEASTERN SPAIN, WITH COMMENTS
ON ITS PHYLOGENETIC PLACEMENT
ANA MARIA BÁEZ1, *, RAÚL O. GÓMEZ1
1Consejo Nacional de Investigaciones Cientícas y Técnicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Departamento
de Ciencias Geológicas, Pabellón 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
* corresponding author: baezanam@yahoo.com.ar
Báez, A. M., Gómez, R. O. (2016): Revision of the skeletal morphology of Eodiscoglossus santonjae, an Early Cretaceous frog
from northeastern Spain, with comments on its phylogenetic placement. – Fossil Imprint, 72(1-2): 67–77, Praha. ISSN 2533-
4050 (print), ISSN 2533-4069 (on-line).
Abstract: The holotype of the “archaeobatrachian” (i.e., non-neobatrachian) frog Eodiscoglossus santonjae VILLALTA, 1954 is
redescribed herein, with focus on some features that have been misinterpreted. The specimen is articulated and poorly preserved
as fragmentary bones and impressions, and is from the Early Barremian La Pedrera de Rúbies Lithographic Limestones Formation
at Santa Maria de Meià, Lleida Province. Other specimens from the same fossil site and unit are also examined and their former
allocation to the same species critically reconsidered, with the conclusion that more than one taxon might be represented in this
sample. Previous viewpoints on the systematic placement of E. santonjae, usually considered a discoglossid, are assessed. How-
ever, a full revision of the Jurassic and Cretaceous specimens ascribed to the genus is required in order to resolve its phylogenetic
position. Hence, E. santonjae is provisionally considered as Anura incertae sedis.
Key words: Discoglossidae, Alytidae, Cretaceous, Santa Maria de Meià, Sierra de Montsec, La Pedrera de Rúbies Lithographic
Limestones
Received: January 29, 2016 | Accepted: April 11, 2016 | Issued: August 15, 2016
FOSSIL IMPRINT • vol. 72 • 2016 • no. 1-2 • pp. 67–77
(formerly ACTA MUSEI NATIONALIS PRAGAE, Series B – Historia Naturalis)
DOI 10.14446/FI.2016.67
68
Additional specimens were subsequently collected in the
same quarry, some of which were provisionally ascribed to
this species by Vergnaud-Grazzini and Wenz (1975). These
specimens vary greatly in size and belong to different growth
stages. As discussed below, features of some of these
specimens, housed in the Museum d’Histoire Naturelle
(MNHN) in Paris, differ from the described characters of the
holotype of E. santonjae. Although some of these differences
might reflect ontogenetic variation, referral to this species
should be confirmed on the basis of a thorough revision.
Disarticulated, three-dimensionally preserved remains,
mostly postcranial bones, from the Hauterivian – Early
Barremian (Ruiz-Omeñaca et al. 2004) of the Galve re-
gion, in the central Iberian Range, were also referred to
Eodiscoglossus santonjae by Estes and Sanchiz (1982). This
assignment was based mainly on the presence of a projecting
intercotylar process on the atlas, a feature that is also present
on the atlas of one of the specimens from the type locality
provisionally ascribed to the same species figured by
Vergnaud-Grazzini and Wenz (1975, fig. 2); this material is
now considered E. cf. santonjae (Sanchiz 1998). In turn,
comparisons with the remains from Galve led Evans et al.
(1990) to refer disarticulated bones from the Bathonian of
England to the genus Eodiscoglossus VILLALTA, 1954, as
a new species: E. oxoniensis EVANS, MILNER et MUSSETT,
1990. The generic identification was based on an overall
phenetic resemblance, as well as the combination of some
similarity to the ilium of the extant Discoglossus and the
presence of presumable plesiomorphies. More recently,
Sanchiz (1998) considered Neusibatrachus wilferti SEIFFERT,
1972 from the Santa Maria de Meià fossiliferous limestones,
a junior synonym of Eodiscoglossus santonjae but a later
study did not confirm this conclusion (Báez and Sanchiz
2007). In addition, a new reconstruction of E. santonjae based
on the holotype was presented by Roček (2000), depicting
some features that differ from earlier interpretations.
First-hand examination of the holotype of Eodiscoglossus
santonjae at the Museo Nacional de Ciencias Naturales in
Madrid (MNCN 4723, Text-fig. 1) revealed features that are
in disagreement with the interpretation of most previous
authors, as stated above. Hence, we considered it important
to focus on some aspects of the anatomy of this species as
a contribution that may assist in unraveling its taxonomic
position as well as provide anatomical information that might
help elucidate the evolutionary relationships of other basal
taxa and shed light on the early diversification of crown-
group Anura.
The name Discoglossidae has been frequently applied to
the group formed by the extant genera Alytes WAGLER, 1830,
Bombina OKEN, 1816, Barbourula TAYLOR et NOBLE, 1924,
and Discoglossus OTTH, 1837 (e.g., Noble 1931, Lynch 1973,
Clarke 1988). However it has also been applied to the total
group, including a presumably monophyletic crown and the
fossil taxa placed at its stem (e.g., Duellman and Trueb 1985,
Sanchiz 1998, Gao and Wang 2001, Gao and Chen 2004).
These frogs belong to one of the earliest diverging anuran
lineages (e.g., Roelants and Bossuyt 2005, Frost et al. 2006,
Pyron and Wiens 2011) and are part of the group of taxa
usually designated as “archaeobatrachians” because its
members are perceived as “primitive” with respect to the
more derived neobatrachians, which constitute the majority
of the living anurofauna (Roelants and Bossuyt 2005). In the
last decade, several cladistic and non-cladistic studies based
on larval and adult morphological evidence have retrieved
the monophyly of the group formed by the extant genera
(e.g., Clarke 1988, 2007, Haas 2003) but no well-corrobo-
rated discoglossid adult morphological synapomorphies,
potentially diagnostic, are so far known, as discussed below.
However, other studies based on morphology-only cladistic
analyses (e.g., Ford and Cannatella 1973) retrieved
a paraphyletic Discoglossidae, with a clade formed by
Bombina and Barbourula (Bombinatoridae) diverging
earlier than the one formed by Alytes and Discoglossus
(Discoglossidae sensu stricto or Alytidae of Frost et al. 2006).
In turn, recent analyses based on molecular or combined
molecular and morphological evidence support the mono-
phyly of the group formed by Bombina, Alytes, and
Discoglossus (Roelants and Bossuyt 2005, San Mauro et al.
2005, Frost et al. 2006, Pyron and Wiens 2011, Biton et al.
2013), the Costata of Frost et al. (2006), and the group also
including Barbourula (Blackburn et al. 2010), but few
morphological characters that unambiguously optimize on
this branch have been discovered so far (Frost et al. 2006).
With regard to the taxonomic framework, it should be noted
that Discoglossidae is used as a stem-based name herein,
unless stated otherwise. In fact, the characterization of the
discoglossids has relied on a mosaic of several putative
plesiomorphic plus a limited number of derived character
states (e.g., Duellman and Trueb 1986, Roček 1994). This
makes it difficult to clarify the identity and phylogenetic
placement of several fossils that have been considered as
belonging to this lineage, including those assigned to
Eodiscoglossus santonjae.
Systematic Palaeontology
Anura FISCHER [DE WALDHEIM], 1813
Eodiscoglossus VILLALTA, 1954
Type species: E. santonjae VILLALTA, 1954
E. santonjae VILLALTA, 1954
H o l o t y p e : Articulated skeleton preserved as part and
counterpart, inv. no. MNCN PV-4723 (Museo Nacional de
Ciencias Naturales, Madrid).
Locality: quarry near Rúbies, about 30 km north of
Balaguer (fide Hecht 1970), Santa Maria de Meià, Lleida
province, northeastern Spain.
Horizon: La Pedrera de Rúbies Lithographic Lime-
stones Formation; the age of the limestones has been
determined as Early Barremian (Ansorge 1995, Martínez-
Closas and López Morón 1995).
Emended diagnosis based on the holotype: Frog having
a premaxillary with high, distally notched alary process;
dentate maxillary arcade; quadratojugal extending along
about 50% of pterygoid fossa; nasals in medial contact at
least anteriorly; sphenethmoid exposed dorsally between
nasals and frontoparietals; frontoparietals lacking supra-
orbital flanges; zygomatic ramus of squamosal not articulated
with maxilla; pterygoid with robust anterior and posterior
rami, the former articulating with maxilla at orbital
midlength; angulosplenial with single bladelike coronoid
69
process; eight presacral vertebrae; sacral diapophyses slightly
expanded distally; short free ribs on vertebrae II–IV; scapula
relatively short dorsoventrally and with straight leading
margin; cleithrum non-forked at distal end; clavicle with long
acuminate acromial end overlapping scapula anteriorly;
carpus lacking torsion and with at least six free elements
(probably “Morphology A” of Fabrezi 1992); prepollex
hypertrophic with three elements and associated with
epidermic pads; ilium relatively long, bearing a dorsal crest
at least along the proximal (posterior) one third (the presence
of a dorsal protuberance is uncertain); femur only slightly
shorter than tibiofibula; proximal tarsals slightly more than
half the length of tibiofibula.
Comments: This emended diagnosis is based on the
holotype, redescribed below; it consists of a combination
o
f characters, most of which are common among
“archaeobatrachians” and their evolutionary polarity is still
ambiguous. The original diagnosis by Villalta (1954)
included a few general features including the relatively short
and wide skull, expanded sacral diapophyses, and bicondylar
sacro-urostylar articulation; as discussed below, we consider
that the latter two traits have been either misinterpreted or
overinterpreted. Hecht (1970) also provided a diagnosis for
E. santonjae (as a diagnosis for the genus, which was
monotypic at that time) but the supposedly diagnostic
absence of teeth on the maxillary arches of the holotype was
subsequently denied by Vergnaud-Grazzini and Wenz (1975)
and by us. The rest of his diagnostic features, presence of
nuptial pads and columella, are insufficient to identify this
species.
Annotated redescription of the type specimen: The only
part of the holotype of this species available for examination
consists of an articulated skeleton lacking most of the left
hind limb, exposed in dorsal view. Some skeletal elements
are preserved as impressions and others as fragmented bones;
also evident are impressions of the body outline, eye
pigments, and other soft-tissue structures (Text-fig. 1). The
specimen has a total length of approximately 27.5 mm; the
well-ossified carpus and digits, including a robust prepollex
formed by three elements and the impression of pads,
indicate that it belongs to a postmetamorphic individual,
probably a male.
The premaxillae are flipped anteriorly but it is clear that
they bear well-developed, distally notched alary processes
that were posterolaterally orientated when in natural position;
other parts of these bones are poorly preserved (Text-fig. 2a).
Anteriorly, the pars facialis of each maxilla seems to have
overlapped the corresponding premaxilla, although the shape
of the anterior terminus of the former bone is not discernable,
whereas posteriorly the maxilla, when in natural position,
articulated with the delicate quadratojugal. The latter element
is clearly visible adjacent to the angulosplenial on both sides
of the skull; it overlapped the maxilla for about 50% of the
pterygoid fossa length (Text-fig. 2b). The left maxilla is
flipped laterally exposing a regular series of triangular
impressions along the anterior portion of its ventral margin
(Text-fig. 2a); this suggests the presence of teeth, as was
already noted by Vergnaud-Grazzini and Wenz (1975, pl. III,
fig. 3). Likewise, impressions of dentate bones are
discernable lateral to the nasals on both sides of the skull;
they correspond to the displaced vomers. Originally, Hecht
(1970) had considered the maxillae as edentulous bones,
a diagnostic feature of the species, a conclusion later
provisionally accepted by Estes and Reig (1973). The
maxillary pars facialis is moderately high anteriorly,
diminishing in height along the orbit and becoming slightly
higher posteriorly but lacking a zygomatic process. Adistinct
pars palatina with a rounded cross-section is visible in
the left maxilla, but other features in this aspect are not
discernable. The nasals, preserved mostly as impressions, are
extensive and in contact with one another along the midline
anteriorly whereas posteriorly they appear slightly separated;
their overall shape, however, is difficult to reconstruct due to
poor preservation. A midline contact between the nasals
was considered by Dong et al. (2013) to be lacking in
E. santonjae. The anterior margin of each nasal has a shallow
indentation next to a narrow, well-defined rostral process;
this indentation probably marks the location of the external
nare in life; a well-defined maxillary process appears to
be absent. Posteriorly, the nasals are separated from the
frontoparietal by the sphenethmoid, which is exposed
dorsally. The sphenethmoid, whose paired or single condition
is not clear, circumscribes the anterior margin of the
frontoparietal fenestra. The frontoparietals are incompletely
preserved and crushed against the cavum cranii, precluding
description of their overall configuration. Amedian suture is
Text-fig. 1. Eodiscoglossus santonjae VILLALTA, 1954. General
aspect of the holotype (MNCN 4723).
70
not evident, although this might be the consequence of
postmortem crushing; this is probably why Hecht (1970)
described the frontoparietal as if it were azygous. In contrast,
Estes and Reig (1973) figured the frontoparietals as paired
bones in contact along the midline for most of their lengths.
Biton et al. (2013) also scored the frontoparietals as paired
in Eodiscoglossus even though their cited source of
information for this taxon does not include the configuration
of this bone. A short anterior fontanelle formed by the
divergent anteromedial margins of the frontoparietals might
be present, but it is clear that at least in the posterior orbital
region these bones roofed the braincase completely. The
lateral margins of the frontoparietals are slightly divergent
posteriorly; supraorbital flanges are absent. The presence and
shape of posterolateral expansions investing the epiotic
eminences are unknown due to breakage on both sides of
the skull. The outline of the posterior margins of the
frontoparietals, located slightly anterior to the foramen
magnum, is not clearly discernible. The dorsal surface of the
frontoparietal is shallowly pitted, as is that of the few
preserved bony fragments of the nasals. The prootics and
exoccipitals form the otic capsules, although the crushed
condition of this part of the skull prevents confident
assessment as to whether these bones were fused. The
T-shaped squamosals are preserved on both sides of the skull.
The free-ending zygomatic ramus seems to be somewhat
shorter than the otic ramus, whereas the ventral ramus is the
longest of the three rami of the bone. There is no evidence of
contact between the zygomatic ramus of the squamosal and
the maxilla (Text-fig. 2a, b) as depicted in the restoration
based on the holotype by Roček (2000), whereas this contact
was scored as absent in the genus by Biton et al. (2013). The
well-ossified pars media plectri is clearly visible between the
otic capsule and the squamosal, presumably applied to the
oval foramen on the left side of the skull, whereas the
contralateral element is isolated and poorly preserved. The
plectrum has a robust shaft and a slightly expanded footplate.
The pterygoid is partially visible ventral to the squamosal;
the anterior ramus of this palatal bone is relatively wide and
articulates with the maxilla at the level of the midlength
of the orbit. Each half of the lower jaw is composed of
an angulosplenial and dentary; the presence of a mento-
meckelian cannot be determined because the anterior part of
the mandible disappears under the nasals. The angulosplenial
has a dorsally open Meckelian groove (Text-fig. 2b) and
a single well-developed, laminar coronoid process exposed
on the right bone (see also Hecht 1970, fig. 3), contra the
opinion of Roček (1994) who considered the coronoid
process as absent. The articulation for the mandible is located
at almost the level of the occiput.
The axial skeleton is composed of eight presacral
vertebrae, a single sacral vertebra, and the urostyle. The type
of articulation between successive vertebrae cannot be
determined; in this regard we agree with Hecht (1970) and
Estes and Reig (1973) but not with Gao and Wang (2001)
who considered that the opisthocoelous condition of the last
three presacrals can be ascertained in this specimen. The
atlantal neural arch has a straight anterior margin and lacks
transverse processes; on the right side, the ventrally directed
pedicle projects laterally suggesting that the cotyles for the
occipital condyles were transversely elongated and with axes
forming a shallow angle and thus mostly located ventral to
the neural canal. The following three presacrals (II, III, and
IV) have distally expanded transverse processes that
articulate with short ribs; the most anterior pair of these
processes is slightly anteriorly directed whereas the
succeeding two pairs are laterally orientated. The ribs that
articulate with Vertebra III are the longest; these ribs as well
Text-fig. 2. Eodiscoglossus santonjae VILLALTA, 1954. Holotype (MNCN 4723), a: aspect of the skull; b: Close-up of the right suspensorial
region, with interpretive drawing below. Abbreviations: ang – angulosplenial; mx – maxilla; pro – prootic; pt – pterygoid; qj – quadratojugal;
sq – squamosal.
71
as those on Vertebra IV are expanded proximally and distally
but the proximity of the clavicles partially obscures
observation of the distal ends of the most anterior pair of ribs.
The four succeeding vertebrae (presacrals V to VIII) have
short transverse processes with blunt distal ends that might
have terminated in cartilage. The most anterior pair of these
processes is nearly perpendicular to the longitudinal axis of
the vertebral column, the following pair is slightly anteriorly
directed, and the two most posterior ones are distinctly
anteriorly directed forming a relatively wide angle with
respect to the axis of the vertebral column. The medial
regions of the neural arches of vertebrae II to V project
posteriorly into spinous processes that reach the respective
succeeding vertebra, whereas the posterior margins of the
arches on both sides of the processes are markedly concave,
as also shown by Hecht (1970), with oval patches of sediment
posterior to these concavities that partially obscures the
anterior margin of the corresponding succeeding vertebra
(Text-fig. 3). This suggests that the neural canal might have
been partially exposed dorsally (contra Estes and Reig 1973
who depicted the neural arches strongly imbricated). The
neural arches of vertebrae VI and VII are incompletely
preserved; fine longitudinal ridges along the midline borne
by vertebrae V and VI are clearly visible. The neural arches
of the last presacral (VIII) and sacral vertebra (IX) are
missing whereas their respective centra are only represented
by scraps of bone. The interpretation of the configuration of
the sacral diapophyses is confusing due to distinct
hatchet-shaped, whitish areas that are in contact with the ilia
(Text-fig. 3). Areas of similar color however also occur near
the transverse processes along the vertebral column and
might be due to the deposits in the endolymphatic system
related to the maintenance of the acid-base homeostasis, as
suggested by Hecht (1970, see also Warren and Jackson
2005). Shallow impressions of posteriorly deflected, proxi-
mally narrow and slightly distally expanded diapophyses that
reach the corresponding ilia are clearly discernible on both
sides. Moreover, illustrations of this region under polarized
light (Hecht 1970) were interpreted to depict narrow sacral
diapophyses preceded by an artifact of preservation contra
to the opinion of Villalta (1954) and Gao and Wang (2001)
who scored the sacral diapophyses of this taxon as expanded.
The anterior portion of the urostyle is longitudinally
sectioned whereas its posterior two thirds are preserved as
an impression of the round cross-sectioned hypochord. At
the anterior part of the urostyle one pair of irregular swell-
ings on each side of the bone suggests the participation of
a pair of postsacral neural arches in the formation of the
coccyx. Transverse processes and a dorsal crest are not
discernible on the urostyle, as noted previously by Hecht
(1970).
Components of the pectoral girdle are exposed in dorsal
aspect. The cleithrum is a well-developed, elongate bone that
formed a groove for the anterior margin of the suprascapular
cartilage (Text-fig. 4). The available evidence does not
support the extensive, bifurcated cleithrum depicted by Estes
and Reig (1973). Loss of the proximal part of this bone on
the right side has exposed the scapula, whereas the anterior
shifting of the left cleithrum has made part of the scapula
visible on this side. The scapula has a straight leading edge
due to the presence of an anterior crest that extends from the
pars acromialis to the suprascapular margin; the dorsoventral
width of the shaft is similar to that of the glenoidal region.
The acromial and glenoidal parts are nearly equally
developed and they appear to be separated by a medial
(ventral) notch. The glenoidal ends of the clavicles are visible
on both sides; right and left clavicles have long, tapering ends
that must have been broadly articulated with the respective
Text-fig. 3. Eodiscoglossus santonjae VILLALTA, 1954. Holotype (MNCN 4723), close-up of posterior presacral vertebrae and sacral
region, with interpretive drawing.
72
pars acromialis of the scapula (Text-fig. 4). The dorsal ends
of both coracoids are preserved; they are expanded with
a relatively narrow diaphysis. The ventral halves of the
bones disappear beneath the axial column but it is clear
that their long axes were posteromedially directed. The
humerus is robust and bears a well-developed ventral ball;
an ample deltoid crest extends along the proximal half of
the humeral length. Impressions of several carpal elements
are preserved in both forelimbs. The proximal row is
composed of two large bones that are in contact with the
radioulna; these are the radiale and ulnare respectively.
Distal to the former bone, the impression of Element Y is
visible, whereas distal to the ulnare there is another large
element that probably corresponds to the Distal Carpal 5.
On the preaxial side of the latter and next to the base of
Metacarpal IV, a smaller bone might be identified as Distal
Carpal 4. Another distal carpal, probably Distal Carpal 3,
appears between the bases of metacarpals II and III.
A hypertrophied prepollex that is composed of three
elements is clearly visible on the preaxial side of Element
Y. The metacarpals are proximally and distally expanded.
The phalangeal formula is 2-2-3-3, the digital lengths (in
ascending order) being II<III<V< IV. The terminal
phalanges have elongate, narrow distal ends.
The ilia and ischia are the only visible components of
the pelvic girdle. The ilia are preserved mostly as imprints.
The iliac shaft bears a narrow dorsal crest that is broader
along the proximal (posterior) third of the shaft length. The
ischia are fused to one another. Other features of the pelvic
girdle are not clearly visible owing to the position in which
this skeletal part is exposed. The femur is distinctly
sigmoid and slightly shorter than the tibiofibula; a femoral
crest is not discernible. The tibiale and fibulare are fused
proximally and distally; their lengths represent about 57 %
the length of the tibiofibula. A tiny triangular prehallical
element is visible next to the wide base of Metatarsal I and
on the preaxial side of a slightly larger tarsal bone that can
be identified as Element Y. Other distal tarsals are not
preserved indicating that they might have been cartilagi-
nous. The phalangeal formula is 2-2-3-4-3. The terminal
phalanges are long and have simple distal ends.
Other specimens from the type locality referred to
E. santonjae
As noted earlier, several specimens from the type locality
but from two different fossiliferous levels were described
by Vergnaud-Grazzini and Wenz (1975) and referred
provisionally to Eodiscoglossus santonjae, whose holotype
had been thoroughly described previously by Hecht (1970)
a
nd later briefly by Estes and Reig (1973). Vergnaud-
Grazzini and Wenz (1975) considered that all these
specimens were discoglossids owing to the presence of at
least three of the following features in each specimen: 1)
eight, probably opisthocoelous, presacral vertebrae; 2) free
ribs on vertebrae II–IV; 3) biconvex sacral vertebrae, with
two posterior condyles; 4) one pair of transverse processes
on the urostyle; 5) short scapula, apparently lacking a medial
notch; and 6) dentate maxillary arches and vomers. These
authors also concluded that, despite the different sizes and
proportions, all these specimens belong to the same species.
They also attributed the difference in disposition of the different
elements to postmortem deformation. These conclusions were
subsequently tacitly accepted by other authors and comments
either on Eodiscoglossus or E. santonjae as well as scoring for
these taxa have been based on the evidence provided by this
collection of specimens (e.g., Evans et al 1990, Gao and Wang
2001, Biton et al. 2013). Moreover, the diagnosis of the genus
by Evans et al. (1990) includes the count of premaxillary and
maxillary teeth not discernible in the holotype of the type
species.
Independently from the validity of each of these
characters and/or their combination to diagnose this anuran
lineage, it is pertinent to evaluate the evidence for the re-
ferral of all or some of these specimens to E. santonjae.
Comparisons, however, are problematic because these speci-
mens are preserved in aspects different from the holotype,
thereby exposing different bones, the lack of apomorphic
characters in the holotype to avoid plesiomorphic phenetic
resemblance, and the different ontogenetic stages to which all
these specimens belong. Despite these shortcomings, and
considering size as a proxy for the developmental stage,
firsthand examination of some of these specimens makes it
possible to point out some morphological and comparative
features.
Text-fig. 4. Eodiscoglossus santonjae VILLALTA, 1954. Holotype (MNCN 4723), close-up of left half of pectoral girdle with interpretive
drawing.
73
Specimens MNHN MSE 1 and MSE 3a, b belong to
individuals smaller (their total lengths are estimated as about
16 and 21 mm, respectively) and younger than the holotype
of E. santonjae. The former, exposed in ventral aspect, has
the shallow impression of a short tail, the hypochord is still
not synostotically fused to the coccyx, and the terminal
phalangeal elements are not ossified; it probably belongs to
a metamorphosing individual. MSE 3a and b belong to an
individual preserved mostly ventrally, the premaxillary and
maxillary teeth are pedicellate, the vertebral column is
lacking the centra, which seem to have been weakly united
to the well-developed pedicles. We did not find any feature
preserved in these two specimens that would make their
referral to E. santonjae questionable, although a few
similarities can be ascertained apart from the elongate,
notched alary processes borne by the premaxillae, the dentate
condition of the maxillary arches and the vomers, presence
of distally expanded free ribs in vertebrae II–IV, anteriorly
directed and relatively short transverse processes on the last
three presacrals, and slightly expanded sacral diapophyses.
Specimen MNHN MSE 2 is from a slightly lower level
than the holotype-bearing horizon (Vergnaud-Grazzini and
Wenz 1975); it most likely belongs to an adult that had a to-
tal length about twice that of the holotype. The partially
articulated skeleton, exposed in ventral view, is incomplete;
some elements such as the distal portion of the urostyle and
parasphenoid alae are somewhat covered by sediment so they
are not as short as depicted by Vergnaud-Grazzini and Wenz
(1975). The vertebral column is composed of eight presacrals
that show the scars of the recent synostotic fusion of pedicles
and centra. There is also partial fusion of the expanded ends
of ribs and transverse processes borne by vertebrae II–IV.
The centra of vertebrae II–VIII are longer than they are wide
and strongly protuberant and probably with a rounded cross
section at the level of articulation with adjacent vertebrae;
the nature of these articulations is not discernable. Few
elements can be compared with the same elements in the
holotype. The atlantal centrum bears cotyles closely spaced
and with axes forming a wide, obtuse angle. The sacral
vertebra bears posteriorly slanted, moderately expanded
diapophyses whose distal lengths are about twice their
proximal lengths. A shallow groove at the posterior end of
the sacral centrum is the only indication that a bicondylar
articulation between sacrum and urostyle might have existed.
The leading margin of the scapula is broken; partes
acromialis and glenoidalis are well developed and separated
by a shallow notch. The extended ventral contribution of the
sphenethmoidal ossification to the floor of the nasal capsules,
the long cultriform process of the parasphenoid, the ventrally
directed flange along the anterior ramus of the pterygoid, as
well as the partially fused ribs and transverse processes might
be due to the probable advanced age of this individual.
Transverse processes of vertebrae VII–VIII are short but
horizontally orientated, unlike the anteriorly orientated pro-
cesses of these vertebrae in MSE 1, 3, and MNCN 4723.
MNHN MSE 5 is preserved as disarticulated bones; the
tibiofibula is about 12 mm long, i.e. nearly the same length
as that of MNCN 4723, so we assume that it belongs to an
individual of about the same total length. Left and right ilia
are relatively well preserved and exposed in acetabular or
slightly dorsolateral aspect, although the most anterior
portions of the shafts are missing in both elements (Text-fig.
5). In each bone, the main portion of the shaft has an oval
cross-section; a depression separates this portion from
a dorsal crest whose distal half is distinctly deflected
medially. Slightly anterior to the level of the anterior margin
of the acetabulum, a distinct oval dorsal protuberance occurs
on the dorsal crest projecting slightly from its dorsal margin.
Posterior to the protuberance, is a marked oval depression
previously described as a supraacetabular fossa (the latter
fossa was erroneously identified as a depression dorsal to the
acetabulum in E. oxoniensis by Evans et al 1990; see also
Gómez and Turazzini 2015). The acetabulum is clearly
demarcated ventrally by a moderately developed rim that
does not reach the ventral margin of the bone, whereas
dorsally it is less defined. There is a broad dorsal acetabular
expansion but posteriorly it does not project beyond the
dorsal margin of the shaft (Text-fig. 5).
MNHN MSE 6 is an articulated, partially preserved
specimen (Vergnaud-Grazzini and Wenz 1975, pl. III, fig. 4).
Only part of the postcranial skeleton, mostly in dorsal view,
is present; it belongs to an adult individual about twice the
size of MNHN 4723, estimated according to the lengths of
femur and tibiofibula (22 mm and 24 mm, respectively). The
posteriormost six presacral vertebrae (III–VIII?) are exposed
in dorsal view although only their right sides are preserved;
no free ribs are discernable. The longest transverse processes
are those on the two most anterior vertebrae, especially the
one borne by Vertebra III. The following two vertebrae have
posteriorly directed transverse processes, whereas the two
last presacrals have anteriorly directed processes. The sacral
vertebra has narrow, posteriorly directed diapophyses. The
long disarticulated urostyle (18.5 mm) is exposed on its right
side; along the anterior two thirds of its length it bears
a dorsal spinous process whose height diminishes posteriorly.
Anteriorly this process is as high as the ventral part of the
bone; transverse processes are not discernable. The ischia
have rounded posterior margins and are not fused to one
another. The left ilium is exposed in medial view whereas the
contralateral element is visible in acetabular view. The
morphology of this bone is similar to that of MSE 5 described
above and we consider that these two specimens represent
the same species. However, based on the morphology and
Text-fig. 5. MNHN MSE 5. Drawing of right ilium in dorso-
lateral view with reconstructed cross section. Abbreviations: acf
– acetabular fossa; acr – acetabular rim; dae – dorsal acetabular
expansion; dc – dorsal crest; dpt – dorsal protuberance; ish –
iliac shaft; saf – supraacetabular fossa; vae – ventral acetabular
expansion.
74
orientation of the presacral transverse processes and the
shape of the sacral diapophyses, MSE 6 is not referable to
the taxon represented by MSE 2, an adult of about the same
total length and, thus, wholly comparable with it.
In summary, the holotype of E. santonjae was probably
an adult male, as inferred by the hypertrophied prepollex and
the associated nuptial pads (Hecht 1970). Only a few features
included in the diagnosis presented herein can be ascertained
in each of the specimens reviewed above. However, none of
these features unequivocally diagnoses this species, as most
have been described in other taxa and many are probable
plesiomorphic. Moreover the probable presence of more than
one taxon in this sample aggravates this problem. Therefore,
the taxonomic allocation of these specimens awaits critical
comparison with other material, especially that of Mesozoic
age and ascribed to “Discoglossidae”.
The Phylogenetic Position of E. santonjae
Since its discovery, the holotype of Eodiscoglossus
santonjae, type species of the genus, has been considered to
represent a discoglossid species (e.g., Villalta 1954, Hecht
1970, Estes and Reig 1973). This taxonomic placement
was based mainly on the presence of eight, supposedly
opisthocoelous presacral vertebrae, free ribs on presacrals
II−IV, and allegedly imbricate neural arches. However, from
the foregoing description it is clear that the opisthocoelous
nature of the vertebral centra cannot be unambiguously
assessed, free ribs are plesiomorphic at this level of ge-
nerality, and the neural canals are dorsally partially exposed
in this specimen. Acceptance of the referral of several
additional specimens to E. santonjae (Vergnaud-Grazzini and
Wenz 1975), as noted above, added other “discoglossid”
features, particularly those present in the Discoglossus group.
Hence, some authors emphasized that E. santonjae might be
more closely related to species of Discoglossus than to other
living discoglossids, based mainly on the shared presence of
a dorsal crest and an elongated dorsal protuberance on the
ilium (Estes and Sanchiz 1982, Evans et al. 1990, Roček
2000). Although not common amongst “archaeobatrachians”,
a well-developed dorsal crest on the iliac shaft is present in
some Early Cretaceous pipimorphs (Jones et al. 2003, Báez
2013) whereas it also occurs in several groups of neo-
batrachians, implying that this structure might have arisen
independently several times in anuran history. In addition, it
is noteworthy that if E. santonjae is closer to Discoglossus
than to Alytes, the lineages represented by these extant taxa
must have diverged at least earlier than the appearance of
E. santonjae in the Barremian, but this is at odds with several
recent estimates that position this divergence later in the
Cretaceous (Wiens 2007, Blackburn et al. 2010, Ruane et al.
2011, Gómez-Mestre et al. 2012).
Most discussions of the taxonomic position of
Eodiscoglossus have been based on the results of parsimony
analyses in which the scoring for this taxon was probably
composite and without an explicit and critical examination
of the content of the genus. The position was first tested in
a cladistic context by Wang et al. (2000) by including
Eodiscoglossus as a terminal taxon in their examination of
discoglossid interrelationships, incorporating some extinct
taxa in the analysis largely based on the data matrix of Báez
and Basso (1996). They recovered Eodiscoglossus as sister
to crown-group Discoglossidae, a relationship supported by
two synapomorphies of the pectoral girdle: coracoid with
limited expansion anteromedially and forked cleithrum. The
former condition is common amongst frogs, even in groups
that were not included in their analysis such as pipoids and
many neobatrachians. Based on our interpretation of the
holotype of E. santonjae, the presence of a forked cleithrum
in this species is unwarranted, whereas the condition in the
putative Jurassic member of the genus E. oxoniensis is
unknown (Evans et al. 1990). Subsequently, Eodiscoglossus
was incorporated in the similarly conducted cladistic analyses
by Gao and Wang (2001), Gao and Chen (2004), and Wang
(2006), also emerging as a member of the discoglossid
lineage. Among these studies, in the only analysis in
which a discussion of the synapomorphies supporting the
discoglossid node is accessible to us (Gao and Wang 2001)
this relationship is justified by the following features: vomer
bearing a postchoanal process that forms a narrow angle with
the anterior part of the bone, presence of anterior overlap of
clavicle on scapula, coracoid little expanded medially, and
trigeminal and facial foramina separated by the prefacial
commissure. However, some of these features have been
erroneously scored in several of their terminal taxa. In
addition, in the case of Eodiscoglossus, the entire set of
character-states for this genus does not agree with those of
any of the species that have been included in the genus. For
example, Gao and Wang (2001) scored several features
according to their interpretation of a photograph of the
holotype of E. santonjae, such as the expanded sacral
diapophyses and the poorly developed iliac dorsal tubercle,
even though in other specimens attributed to the species that
they used for scoring other features the condition is different.
More recently, Eodiscoglossus has been included as a termi-
nal taxon in the phylogenetic analyses of Dong et al. (2013)
and Báez (2013) based on morphological traits, of which only
in the latter was the scoring based exclusively on the holotype
of E. santonjae. The results of these analyses deviated to
some extent from previous hypotheses regarding the
systematic position of this taxon. The reanalysis of the
published data matrix of Dong et al. (2013) with TNT
v. 1.5-beta resulted in 16 most parsimonious trees (MPTs) of
178 steps, the strict consensus of which is identical with the
tree depicted by these authors (Dong et al. 2013, fig. 8).
A monophyletic Discoglossidae is not recovered in the
majority of these MPTs, whereas Eodiscoglossus and extant
discoglossids occur in a polytomy that also includes a clade
containing pipoids and pelobatoids. In contrast, Báez (2013)
did obtain a monophyletic Costata sensu Frost et al. (2006)
and recovered E. santonjae as sister group of Lalagobatrachia
(i.e., all anurans other than leiopelmatids sensu Frost et al.
2006). In her analysis, the latter clade is supported by two
putative synapomorphies, sacral diapophyses moderately
expanded and cleithrum forked, which occur in the respective
plesiomorphic conditions in the holotype of E. santonjae.
Additionally, most putative synapomorphies of the crown
group are either in their plesiomorphic states or unde-
terminable in the holotype of E. santonjae. In turn, Biton et
al. (2013) scored 22 skeletal characters for several extinct and
extant putative discoglossid taxa, including Eodiscoglossus,
the scoring for which was obviously based on all the
75
specimens previously assigned to the genus. The Bayesian
tree, based on three mitochondrial and three nuclear genes
and the morphological characters, was rooted on Ascaphus
truei STEJNEGER, 1899 placing Eodiscoglossus as sister to the
clade that includes Alytes and Discoglossus as well as several
extinct taxa. It is noteworthy, however, that this analysis did
not test the inclusion of all these extinct taxa as members of
the discoglossid lineage as only discoglossids are considered
with the exception of Ascaphus.
A phylogenetic placement of Eodiscoglossus santonjae
crownward to leiopelmatiids but outside Lalagobatrachia
clearly explains its overall plesiomorphic morphology,
including the configuration of the skull, eight presacrals, free
ribs, atlantal cotyles horizontally orientated, sacrum with
poorly expanded diapophyses, relatively short scapula with
a straight leading margin, narrow cleithrum, generalized
proportions of the limbs, and carpus lacking torsion. This
position is also consistent with the estimated divergent times
for Anura, Lalagobatrachia, and crown-group Discoglossi-
dae based either on some molecular clock methods or the
paleontological record (Blackburn et al. 2010, San Mauro
2010, Ruane et al. 2011, Marjanović and Laurin 2014).
However, the monophyly of the genus has not yet been
unambiguously corroborated, although it has been used as
a calibration point that serves as a minimum boundary for
the clade Lalagobatrachia (e.g., Ruane et al. 2011), or even
for the crown-group Discoglossidae (Blackburn et al. 2010;
Costata therein), thus directly impinging on the estimated
ages of these clades. To exemplify this point, Blackburn et
al. (2010) obtained an estimated divergence time for Costata
25 million years younger by removing this single calibration
point.
From the foregoing, we consider being as explicit and
conservative as possible in evaluating the systematic
placement of fossil taxa such as E. santonjae to be of the
utmost importance. Given that the placement of this taxon is
not unambiguously resolved at present and that a full revision
of the Jurassic and Cretaceous specimens already ascribed to
this genus is still pending (Estes and Sanchiz 1982, Evans et
al. 1990, Báez 2013), at this point we regard Eodiscoglossus
as Anura incertae sedis.
Acknowledgements
The authors are pleased to honor the valuable work of
Dr. Z. V. Špinar on fossil frogs. They express their thanks to
Celia M. Santos and Borja Sanchiz (Museo Nacional de
Ciencias Naturales, Madrid), and Damien Germain (Museum
National d’Histoire Naturelle, Paris) for the loan of fossil
material in their care. Thanks are also extended to Jesús
Muñoz and Borja Sanchiz (Museo Nacional de Ciencias
Naturales, Madrid) for the photographs of the holotype of
Eodiscoglossus santonjae. This work was funded by the
Agencia Nacional de Promoción Científica de Argentina,
PICT 1895/2011. We are grateful to the reviewers, Márton
Venczel (Ţării Crişurilor Museum, Oradea) and Yuan Wang
(Chinese Academy of Sciences, Beijing), for their valuable
comments and to Jan Wagner (National Museum, Prague) for
his careful editorial work.
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