Mourasuchus nativus ( Gasparini, 1985 )

Scheyer, TM & Delfino, M, 2016, The late Miocene caimanine fauna (Crocodylia: Alligatoroidea) of the Urumaco Formation, Venezuela, Palaeontologia Electronica 176 (4), pp. 1-57 : 18-24

Mourasuchus nativus ( Gasparini, 1985)

The type species of Mourasuchus , M. amazonensis , was described in Price (1964) based on cranial remains from the late Miocene Solimões Formation, Acre, Brazil. Mourasuchus (Nettosuchus) atopus has been described from the middle Miocene La Venta fauna, Colombia ( Langston, 1965, 1966) and is now also known from the Pebas system, Iquitos area, Peru (Salas-Gismondi, 2015), M. arendsi from the Urumaco Formation, Venezuela ( Bocquentin Villanueva, 1984), and M. nativus from late Miocene of Argentina ( Gasparini, 1985). The latter species is based on material from the late Miocene Ituzaingó Formation (“Conglomerado Osífero”) of Paraná, Argentina and was originally described as Carandaisuchus nativus ( Gasparini, 1985) , based mainly on the presence of squamosal “horns”, herein referred to as squamosal eminences. It was later synonymised with Mourasuchus based on Brazilian material from Acre (Bocquentin and Souza Filho, 1990). Although published earlier, the description of M. arendsi from Venezuela was not noted or referenced in the description of M. nativus ( Gasparini, 1985) . The most detailed anatomical study of M. nativus cranial material to date, including the holotype MLP 73-IV-15-8, was given by Bona et al. (2013a). Scheyer et al. (2013) noted and partially described skull remains (AMU-CURS-212 and - 218) of M. nativus from the Middle and Upper Member of the Urumaco Formation, mainly based on the pronounced squamosal eminences and a posterior median ridge on the parietal. Specimen UNEFM-CIAAP-1333, a right maxilla from the type locality of M. arendsi , noted by Bocquentin Villanueva (1984) could currently not be located. Additional cranial material, assigned to Mourasuchus , is known from the middle Miocene Fitzcarrald Arch, Peru ( Salas-Gismondi et al., 2007; Salas-Gismondi, 2015) and the late Miocene Yecua Formation in Bolivia ( Tineo et al., 2015). The record of Mourasuchus from the Pleistocene of Tarija, Bolivia (e.g., Bocquentin and Souza Filho, 1990; Aguilera, 2004), however, remains doubtful.

The holotype of Mourasuchus arendsi (UNEFM-CIAAP-1297) consists of a skull, associated with the right ramus and a fragment of left ramus of a lower jaw ( Figure 13 ), as well as some additional postcranial remains including the anterior portion of the cervical vertebral column in natural position up to the sixth cervical vertebra ( Bocquentin Villanueva, 1984). In the original description interpretative line drawings of the skull in dorsal, ventral, and occipital view were given, however, pictures of the actual specimen were not included. Our revision of the type material yields some important differences to the original description. Unfortunately, we were not able to study the ventral side of the skull due to the fragile status the specimen is in.

The skull roof of UNEFM-CIAAP-1297 is small, showing also small sized, sub-circular supratemporal fenestrae, the anterior (rostral) bor- der of which is formed by the parietal and the postorbital ( Figure 13.1-2 ). The postorbital-squamosal suture enters the supratemporal fenestra laterally and not posterolaterally as proposed by Bocquentin Villanueva (1984). The postorbitals thus have a rectangular shape. The supratemporal is prominently exposed on the skull roof and resides in a v-shaped valley between the squamosals, which exhibit strong squamosal eminences. These eminences ( Figure 14 ) are, however, either slightly eroded (right side) or broken off completely (left side). If reconstructed, they form a much more pronounced incised median V-shaped valley of the posterior skull margin in occipital view. The parietal carries a median crest posteriorly ( Figure 14 ). Interestingly the crest was also indicated graphically by Bocquentin Villanueva (1984, figure 1A), but it was not further described in the text. The frontal is shorter than previously reconstructed without a large anterior process (as is the case for example in UFAC-1424, see Bocquentin and Souza Filho, 1990; Bona, et al., 2013a). The frontal-prefrontal suture thus lies at mid-orbit level, just at the posterior extend of the prominent orbital knobs of the prefrontals. The prefrontal bones meet broadly medially for most of their anteroposterior extend before articulating with the nasals in a Vshaped suture. The anterior-most extent of the prefrontals, the lacrimals, and the jugals is not clear. The maxilla has a rectangular outline posteriorly. In dorsal view, the jugal does not show a clear notch as in Mourasuchus amazonensis , due to cracks in UNEFM-CIAAP-1297. There is, however, a bend of the ventral edge of the right jugal in lateral view ( Figure 15 ; for preservational reasons the left side does not show the original morphology). There is clear evidence that the posterior portion of the right jugal, quadratojugal, and quadrate has been separated by a system of fractures from the rest of the skull and was tilted downwards. If the bones are virtually relocated (moving of the actual bones was not possible and might result in a partial collapse of the specimen in its present state) into their original position, the notch becomes more pronounced in dorsal view. The jugal bars of UNEFM-CIAAP-1297 are round in cross section. The suture between the nasals, the maxillae, and the premaxillae is obscured anteriorly, so that it is questionable whether the premaxillae meet in a median posterior process as proposed by Bocquentin Villanueva (1984), or whether the premaxillae show two processes framing the nasals laterally, thus forming a W-shaped suture (as is present in M. amazonensis , see Langston, 1966). The premaxillae show a raised rim around the external naris. In addition there are four perforations anteriorly and laterally to the external naris, which accommodated the four enlarged anterior-most teeth in the lower jaws. In occipital view ( Figure 16 ), the skull shows more dorsolateral compaction than was initially indicated by Bocquentin Villanueva (1984), but the right squamosal eminence is well discernible. The quadrates show sutural scars on their dorsal surface for the posterior-most parts of the exoccipitals and the squamosals, which are partially broken off. The occipital condyle is round, and the basioccipital dorsolaterally carries prominent sutural scars for the exoccipital pillars. It cannot be discerned if the exoccipitals sent thin processes laterally to the basioccipital tubera. The lower jaw remains of M. arendsi were not figured in the original description ( Bocquentin Villanueva, 1984), but photographs of the right lower jaw ramus in lateral view were later included by Aguilera (2004) and Sánchez-Villagra and Aguilera (2006). Bocquentin Villanueva (1984) noted that the lower jaw of M. arendsi essentially resembles that of M. atopus as described by Langston (1965). Aguilera (2004) added that the large jaw of M. arendsi is thin and curved, and the dentary carries at least 32 alveoli in a straight line. In dorsal view, the line of alveoli is straight throughout the posterior part of the dentary (visible as a mineralised or sediment-filled ridge), but individual alveoli are not discernible ( Figure 13.3 ). The surangular is partially broken off, so that the angular and posterior portion of Meckel’s groove is visible. Anteriorly, the symphyseal region of the jaw is not preserved. 1) A heavily reconstructed skull and lower jaw of a large specimen (MCNC-URU-110-72V =“MCN- URU-2002-110” of Aguilera, 2004), of which Langston (2008) described associated postcranial remains (all labelled as “MCC-110-72V”). Much of the skull roof, the orbital region, and parts of the maxillae and premaxillae, as well as of the palate and dentary have been modelled in plaster ( Figure 17.1-2 ), for which we only tentatively suggest assignment of this specimen to Mourasuchus arendsi . The maxillae are slightly convergent and do not extend parallel to each other as in M. atopus . The anterior snout region and the external naris were wrongly reconstructed to resemble M. amazonensis at the time, but are here interpreted to be similar to those of the M. arendsi holotype. Forty-two plaster-cast teeth are inserted on the right ramus and 38 on the left ramus of the lower jaw. In addition, all alveoli in the upper jaw are covered by plaster-reconstructed teeth. In contrast, the specimen has still a well- preserved occipital region and quadrates, quadratojugals and jugal bones delimiting the large infratemporal fenestrae. The jugals have a pronounced notch at their lateral edges, as has been described in the holotype of M. amazonensis ( Price, 1964) and specimen UFAC-1424, assigned to M. nativus (Bocquentin and Souza Filho, 1990; Bona et al., 2013a; Tineo et al., 2015). The posterior aspect of the right ramus preserves partially the dentary, whereas the splenial and coronoid are missing so that the Meckel’s groove in the dentary is revealed ( Figure 17.3-4 ). The posterior part of the right ramus, including the articular, surangular, and retroarticular process appear to be largely remodelled.

2) A weathered posterior skull fragment (UNEFM-VF-03) with pronounced, but partially eroded squamosal eminences, small supratemporal fenestrae (their rostral border being composed of the parietal and the postorbital), and a parietal carrying a medial ridge ( Figure 18.1-4 ). The configuration of the bones and their sutures, as far as preserved, is very similar to that of the holotype skulls of Mourasuchus arendsi (UNEFM-CIAAP-1297) and M. nativus (MLP 73-IV-15-8).

3) A posterior skull part of a large specimen (AMU- CURS-768) from El Vijiadero locality in the Lower

Member of the Urumaco Formation, presenting the skull table, occipital region and posterior portion of the rostrum ( Figure 18.4-5 ). The preserved portion of the skull reaches about 65 cm in length and a maximum width of 40 cm. Dorsally the squamosals show prominent eminences, the jugal has a jugal notch, and there are ornamental knobs present on the prefrontals. Ventrally the palatines separate the suborbital fenestrae and the left and right maxillary portions present 30 and 17 alveoli, respectively. In addition, there is a small maxillary fragment with three alveoli present, but it does not preserve a direct contact with the other parts of the skull. 4) A distorted posterior skull part and associated orbitofrontal region (AMU-CURS-218). Of this specimen, the posterior skull part was originally identified as belonging to Mourasuchus nativus based on: a) prominent squamosal eminences forming a transverse ridge; b) raised skull table with V-shaped outline in occipital view; and c) a midline crest in the posterior part of parietal ( Scheyer et al., 2013). The posterior portion was recently complemented with another skull fragment, which had been stored previously in a separate storage section of the collections. This second part preserves mainly the frontal and prefrontal region of the skull ( Figure 19 ). In combination these fossils indicate that this individual of Mourasuchus had both bony knobs at the orbits and strong squamosal eminences.

Figures

FIGURE 13. Photographs (1, 3, 4-6) and interpretative drawing (2) of the holotype skull and associated lower jaw remains (UNEFM-CIAAP-1297) (3) of Mourasuchus arendsi. 1, 2, skull in dorsal view. 3, right mandibular ramus in dorsal view. 4-6, Fragment of left mandibular ramus in dorsal (4), medial (5) and lateral (6) view. Abbreviations: f, frontal; j, jugal; la, lacrimal; mx, maxilla; n, nasal; p, parietal; pmx, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal.

FIGURE 14. Photograph without (1) and with superimposed interpretative drawing (2) of the skull roof of holotype skull (UNEFM-CIAAP-1297) of Mourasuchus arendsi in left dorsolateral view. Abbreviations: f, frontal; mc, medial crest on parietal; p, parietal; po, postorbital; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra.

FIGURE 15. Photograph without (1) and with superimposed interpretative drawing (2) of the right side of the posterior portion of holotype skull (UNEFM-CIAAP-1297) of Mourasuchus arendsi in angled lateral view. Note clear notch of jugal and system of cracks (indicated by dotted white lines and arrows) leading to displacement of right posterolateral portion of skull. Abbreviations: j, jugal; po, postorbital; q, quadrate; qj, quadratojugal; sq, squamosal.

FIGURE 16. Photograph (1) and interpretative drawing (2) of the holotype skull (UNEFM-CIAAP-1297) of Mourasuchus arendsi in occipital view. Note dorsoventral compaction of specimen. Abbreviations: bo, basioccipital; eo, exoccipital; oc, occipital condyle; pt, pterygoid; q, quadrate; so, supraoccipital; sq, squamosal.

FIGURE 17. Photographs of skull and lower jaw (MCNC-URU-110-72V) assignable to Mourasuchus arendsi. 1, dorsal view. 2, ventral view. 3, dorsal view. 4, medial view. Note that large parts of the specimen have been heavily reconstructed and all tooth alveoli are covered by plaster-cast teeth.

FIGURE 18. Photographs and interpretative drawing of weathered posterior skull portion (UNEFM-VF-03) and large skull part (AMU-CURS-768) of Mourasuchus arendsi. 1, 2, incomplete skull in dorsal view. 3, 4, incomplete skull in occipital view. 5, 6, large porterior skull part in dorsal and ventral view. Note squamosal eminences and posterior median ridge on parietal in UNEFM-VF-03 and both, the squamosal eminences and the knobs in front of the orbits in AMU-CURS-768. Abbreviations: eo, exoccipital f, frontal; mx, maxilla; oc, occipital condyle; ok, ornamental knob on prefrontal; pa, parietal; po, postorbital; pt, pterygoid; q, quadrate; se, squamosal eminences; so, supraoccipital; sof, suborbital fenestra; sq, squamosal; stf, supratemporal fenestra.

FIGURE 19. Photographs and interpretative drawings of the posterior skull portion and orbitofrontal region as well as additional fragments (possibly jugal and lower jaw fragments) of Mourasuchus arendsi (AMU-CURS-218). 1, 2, skull fragment in dorsal view. Note squamosal eminences and orbital bony excrescences. 3, 4, skull fragment in occipital view. 5-9, skull fragments (partially from jugals?) found associated with the larger skull portions. Abbreviations: bo, basioccipital; eo, exoccipital; f, frontal; j, jugal; la, lacrimal; oc, occipital condyle; o, orbit, ok, orbital knob; p, parietal; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra.