Caiman, Daudin, 1802

Scheyer, TM & Delfino, M, 2016, The late Miocene caimanine fauna (Crocodylia: Alligatoroidea) of the Urumaco Formation, Venezuela, Palaeontologia Electronica 176 (4), pp. 1-57 : 2-8

Caiman brevirostris Souza Filho, 1987

The species Caiman brevirostris was described from the Miocene Solimões Formation of Acre, Brazil based on fragmentary cranial remains carrying crushing teeth ( Souza Filho, 1987). Prior to the description the Globidentosuchus brachyrostris from Urumaco, a small form with a strong

PALAEO- ELECTRONICA.ORG crushing dentition ( Scheyer et al., 2013), most small caimanine remains were referred to C. brevirostris . It was only with the description of new material of C. brevirostris from the late Miocene Solimões Formation of Amazonas state, Brazil, that an emended diagnosis of the species was given, and detailed images and drawings of the holotype material were provided ( Fortier et al., 2014). Detailed comparison of small caimanine remains from the Urumaco Formation leads us to propose that only a single specimen, MCNC-1829, is attributable to C. brevirostris .

MCNC-1829 ( Figure 2 ) shows a well-developed durophagous dentition in the broad and short skull and robust lower jaw, which are congruent with that of the holotype of Caiman brevirostris ( Fortier et al., 2014) . A splenial symphysis is lacking in C. brevirostris ( Fortier et al., 2014) , but with the upper and lower jaw being in articulation, it cannot be determined whether the splenial participates in the symphysis or not in MCNC-1829 ( Figures 2 , 3 ). The frontal meets the nasals in MCNC-1829 ( Figure 3.1-2 ) as in the holotype of C. brevirostris (UFAC-196), whereas in Globidentosuchus brachyrostris (e.g., in the holotype AMU-CURS-222), the prefrontals meet broadly medially, thus separating the frontal from the nasals ( Figure 4 ; Scheyer et al., 2013). The latter condition is also present in the holotype of Melanosuchus fisheri (MCNC-243) and can also occur in some specimens of extant M. niger ( Mook, 1921, figure 11), whereas the prefrontals are usually separated by a frontal-nasal contact in extant Caiman latirostris ( Bona and Desojo, 2011). Furthermore the supratemporal fenestrae appear larger in a less expanded skull table (although at least partial taphonomic expansion of the supratemporal fenestrae cannot be ruled out), and the supraoccipital has a trapezoidal exposure in MCNC-1829, which appears not to exclude the parietal from reaching the posterior skull margin ( Figure 3.3-4 ). In occipital view ( Figure 5 ), the distortion of the skull is clearly visible. The squamosals and the supraoccipital form the posterior margin of the skull table, but sutures with the exoccipitals are not discernible. The foramen magnum has a dorsoventrally elongated appearance because of the mediolateral compression the specimen experienced, but was probably broad oval-shaped in life; its margins are formed by the basioccipital ventromedially and by the exoccipitals ventrolaterally, laterally, and dorsally. In the lower jaw, the angular-surangular suture enters the mandibular fenestra at about half the height of its posterior margin ( Figure 3.5-6 ). The angular-surangular suture reaches the posterior tip of the short retroarticular process in MCNC-1829, although the latter is somewhat incompletely preserved. Due to the overall strongly weathered condition of MCNC-1829, other sutures in the skull and lower jaw are partly or completely obscured and therefore yield little additional information.

Caiman latirostris ( Daudin, 1802) and Caiman wannlangstoni ( Salas-Gismondi, Flynn, Baby, Tejada-Lara, Wesselingh, and Antoine, 2015)

The species Caiman lutescens was originally described as a species of Alligator from the Miocene “capas del Paraná” in northeastern Argentina ( Rovereto, 1912) based on two specimens (see below) and was sometimes proposed to be a junior synonym (see Riff et al., 2010 for discussion) of the modern broad snouted caiman Caiman latirostris . Bona et al. (2013b, c) reanalysed the type material of C. lutescens , and identified and figured MACN PV 13551 (a skull table) as the holotype. The second type specimen, MACN PV 5416 (partial right rostrum) was reassigned by Bona et al. (2013b, 2013c) to the extant species C. latirostris . In dorsal view MACN PV 5416 shows a strong lateral festooning of the skull margin, although this is a feature which is less prominent in the extant species ( Bona and Desojo, 2011). Langston (1965) also described a fragmentary skull (UCMP 39978) from La Venta fauna, Colombia, as Caiman cf. C. lutescens based on comparison with MACN PV 5416 - the partial right rostrum. Some of the characteristics given by Langston (1965) for UCMP 39978, which according to ( Bona et al., 2013b) lack data comparable to the holotype of C. lutescens , are: triangular head shape in dorsal view with blunted tip of snout; bulbous appearance of skull in lateral view in the region of the external naris; large undivided external naris that is a bit wider than long; nasals that do not enter the external naris; strong facial canthi across the rostrum as in spectacled caimans; and an upper tooth row comprising 18 alveoli, five of which are in the premaxilla. As in MACN PV 5416, the fourth, ninth, and fourteenth maxillary alveoli are the largest in the series, but UCMP 39978 does not show strong lateral festooning of the skull margin in dorsal view ( Rovereto, 1912; Langston, 1965). Salas-Gismondi et al. (2015) erected a new caimanine species, Caiman wannlangstoni , from the late middle Miocene Pebas Formation of the Iquitos area in Peru, the holotype of which is a partial skull (MUSM 2377) with lateral margins that are strongly sinuous and distinctly diverging posteriorly in dorsal view. The authors indicate that specimen UCMP 39978 shows some affinities to C. wannlangstoni , but they treat its taxonomic status for the time being as a “La Venta Caiman ” and a “distinct entity of uncertain taxonomic affinities.”

AMU-CURS-49 from Urumaco was previously identified as belonging to Caiman lutescens (Aguilera, 2004) . It consists of a fragmentary right premaxilla and maxilla, comprising 17 alveoli in total (premaxillary alveoli 2-5, the first alveolus is not preserved; maxillary alveoli 1-13), hosting three (partially) preserved teeth, as well as a sliver of the right nasal entering the external narial opening ( Figure 6 ). The posterior suture of the nasal with the premaxilla and maxilla is not determinable with confidence. The specimen was figured both in Aguilera (2004) and Sánchez-Villagra and Aguilera (2006), and a short description in Spanish was given in the former work. As noted by Aguilera (2004), the lateral margin of the specimen is festooned in dorsal view, with a series of convexities that coincide with the presence of the largest alveoli in the series (the fourth premaxillary alveolus and the fourth and tenth of the preserved maxillary alveoli), one of the characters of the Argentinian (MACN PV 5416) and the Colombian material (UCMP 39978). Salas-Gismondi et al. (2015, page 6) refer AMU-CURS-49 to the recently erected species C. wannlangstoni , based on its “strong sinuous rostral margins and robust globular posterior teeth.” According to Bona and Desojo (2011) in modern C. latirostris the lateral margins of the rostrum are more triangular and not wavy/festooned in dorsal view, whereas in ventral view, slight convexities also coincide with the largest alveoli in the premaxilla and maxilla (as Langston, 1965 described in UCMP 39978). Aguilera (2004) described the ninth and tenth maxillary alveolus in AMU-CURS-49 as being confluent (= the thirteenth and fourteenth in Aguilera, 2004). Similarly, the eighth and ninth alveoli could be confluent as well, with the teeth standing very close to each other and the intermediate alveolar bone wall being very thin, if present at all. Given the weathering the specimen experienced in this area, this anatomical aspect remains inconclusive, however. Two large occlusion pits are present, the first is only partially preserved at the level of the second and third premaxillary alveolus, and the second lies medially to the fifth premaxillary and the first maxillary alveolus (corresponding to a small diastema). Two faint impressions of smaller occlusion pits are situated anteromedial and posteromedial to the fourth premaxillary alveolus. The number of teeth, as well as shape of the alveoli and occlusion pits are thus very similar to those in modern C. latirostris ( Bona and Desojo, 2011). The maxillary-premaxillary suture is visible laterally and dorsolaterally on the specimen ( Figure 6 ), but gets increasingly obscured medially (both on the dorsal and ventral surface). Size and shape of the external narial aperture and the orbit remains ambiguous in AMU- CURS-49 because it is too fragmentarily preserved in these areas.

Another rostral fragment, MCNC-URU-145- 72V (=MCNC-URU-2002-145), from the Urumaco Formation was previously identified as belonging to Caiman lutescens as well (Aguilera, 2004). It comprises of the left premaxilla, the anterior portion of the left maxilla, and the anterior portion of the left nasal bone ( Figure 7 ). The sutures of the three bones are clearly visible dorsally and ventrally. The premaxilla forms the anterior, lateral, and posterolateral portions of the external naris, and the nasal enters the external naris posteriorly. The external naris is undivided, slightly constricted posteriorly, with slightly concave borders in the posterior half and more strongly concave (crescent-shaped) borders in the anterior half. Ventrally, the tooth row of the premaxilla and the preserved anterior maxillary portion contains nine alveoli in total (four in the premaxilla and five in the maxilla), with the fourth alveolus in the premaxilla being the largest. There is a large gap between maxillary alveoli 3 and 4, with the alveolar bone showing a generally rough irregular structure here. In comparison with AMU- CURS-49, its lateral margins are not highly sinuous, but reminiscent of the condition seen in C. latirostris . We propose that one maxillary alveolus (either the third or the fourth) has not been developed in MCNC-URU-145-72V, thus creating the larger distance between the alveoli here. Premaxillary alveolus 4 is the only one containing a tilted tooth fragment. There are four distinct depressions identified as occlusion pits that accommodate the teeth of the lower jaw, one large and deep occlusion pit posterior to premaxillary alveolus 1 (which is partially broken anteriorly) and 2, followed by two small occlusion pits anteromedial and posteromedial to premaxillary alveolus 4. The fourth occlusion pit, again wide and deep, lies on the premaxillarymaxillary suture between the fourth premaxillary and first maxillary alveolus. Posteromedial, the deep occlusion pit is confluent with a much smaller pit, which, in comparison with extant Caiman latirostris skulls, opens into a large neurovascular foramen, the premaxilla-maxilla foramen that houses part of the maxillary branch of the trigeminal nerve. Posteriorly to the depressions and medial to maxillary alveoli 1 to 3 lies a row of eight small dental foramina. There are about three more foramina present medial to maxillary alveoli 4 and 5, but here the secondary palate portion of the maxilla is partially eroded posteromedially, so that the internal trabecular support structures of the maxilla are exposed. Medially, the ventral side of the nasal bone is visible, forming the roof of the nasopalatine duct. The posterior margin of the specimen is formed by the ventral exposure of the dorsal compacta of the maxillary bone.

Figures

FIGURE 2. Photographs of Caiman brevirostris (MCNC-1829). 1, right dorsolateral view. 2, left ventrolateral view. 3, right lateral view. Note that the specimen is strongly crushed and folded in on itself.

FIGURE 3. Photographs (1, 3, 5) and interpretative drawings (2, 4, 6) of Caiman brevirostris (MCNC-1829). 1, 2, close-up of the fronto-nasal contact in right dorsolateral view. 3, 4, close-up of the skull roof in dorsal view. 5, 6, closeup of the craniomandibular articulation in slanted right dorsolateral view. Abbreviations: an, angular, ar, articular; d, dentary; f, frontal; j, jugal, itf, infratemporal fenestra; mf, mandibular fenestra; n, nasal; o, orbit; oc, occipital condyle; po, postorbital; prf, prefrontal, q, quadrate; qj, quadratojugal; sa, surangular; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra.

FIGURE 4. Interpretative drawings of skull bone configuration in the rostro-orbital region of three small sized caimanines from the Urumaco Formation, in comparison to three living caimanines (anterior is to the top). 1, Melanosuchus fisheri based on holotype skull MCNC-243. 2, Globidentosuchus brachyrostris based on holotype skull AMU- CURS-222. 3, Caiman brevirostris based on MCNC-1829 from the late Miocene Urumaco Formation, Urumaco, Venezuela. 4, Caiman brevirostris from the Pleistocene of Acre, Brazil (UFAC-196) originally described by Souza Filho (1987, also figured in Fortier et al., 2014). Note that the anterior projection of the frontal appears much broader in the Venezuelan specimen than in the Pleistocene specimen. 5, extant Melanosuchus niger based on ZSM 76/1911 (picture courtesy: Julia Desojo; see also Foth et al., 2015). 6, extant Caiman crocodilus based on Iordansky (1973, figure 7A). 7, extant Caiman latirostris based on Bona and Desojo (2011, figure 2). 8, extant Paleosuchus trigonatus and 9, extant Paleosuchus palpebrosus based on Medem (1958, figure 36). The bridge of the ‘spectacles’ between the orbits is indicated in 1, 5, 6, and 7. Drawings not to scale. Abbreviations: f, frontal; n, nasal; o, orbit; prf, prefrontal.

FIGURE 5. Photograph (1) and interpretative drawing (2) of Caiman brevirostris (MCNC-1829) in occipital view. Abbreviations: bo, basioccipital; eo, exoccipital; fm, foramen magnum; j, jugal; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra; oc, occipital condyle; pt, pterygoid; q, quadrate; qj, quadratojugal.

FIGURE 6. Photographs (1, 3, 5) and interpretative drawings (2, 4, 6) of Caiman wannlangstoni (AMU-CURS-49). 1, 2, dorsal view. 3, 4, ventral view. 5, 6, right lateral view. Occlusion pits are marked in grey. The partially preserved teeth in the third premaxillary alveolus and the third and tenth maxillary alveoli are marked in dark grey. Note that the two occlusion pits anteromedial and posteromedial to the fourth premaxillary alveolus are preserved only as very faint impressions on the bone surface. Abbreviations: mx, maxilla; n, nasal; op, occlusion pit; pmx, premaxilla.

FIGURE 7. Photographs (1, 3) and interpretative drawings (2, 4) of Caiman latirostris (MCNC-URU-145-72V). 1, 2, dorsal view. 3, 4, ventral view. Alveoli are marked with numbers, occlusion pits in dark grey. The internal trabecular space within the maxilla is marked in light grey. Abbreviations: en, external naris; mx, maxilla; n, nasal; op, occlusion pit; pmx, premaxilla.