Mourasuchus spp.

Scheyer, TM & Delfino, M, 2016, The late Miocene caimanine fauna (Crocodylia: Alligatoroidea) of the Urumaco Formation, Venezuela, Palaeontologia Electronica 176 (4), pp. 1-57 : 26-31

Mourasuchus spp. Price, 1964

In addition to the holotype and additional specimens mentioned above, several other, new specimens from the Urumaco Formation assignable to Mourasuchus were recovered in the field over the past years. These materials are presented below and a comparison of the most important specimens of the described Mourasuchus spp. is given in Table 1. 1) Incomplete skull and lower jaw material ( Figure 20 ),

associated with few postcranial remains including

10 isolated short vertebrae and three keeled osteoderms (both indicative of Mourasuchus in the fauna), a proximal fragment of a rib and one conical slightly curved and broken tooth of 7 cm in length

(AMU-CURS-073). The very slender jaw and the straight maxillae carrying more than 30 alveoli of overall uniform size, together with the postcrania, allow assignment of the specimen to Mourasuchus . In addition, there are a few more small bone fragments accessioned under the same number, which could not be identified (not shown in the figure). The skull remains include a large fragment with the basioccipital-basisphenoid-pterygoid complex to which parts of the palatines, the two ectopterygoids and a vertebral centrum are still attached ( Figure 20.1-4 ), a left quadrate, a part of a right jugal and two large maxillary fragments ( Figure 20.5-9 ), as well as a part of the left ramus and mostly complete right ramus of the lower jaw, and an isolated broken, conical tooth ( Figure 20.10-12 ). The articular facets for the exoccipitals are visible on the dorsolateral sides of the occipital condyle. The basioccipital tubera expand ventrally to the condyle. There is only little dorsoventral compaction of the pterygoids in the specimen. The two maxillary fragments derive from the right and left side of the skull. Each bone is broken into three separate pieces – showing 28 and 36 alveoli, respectively (too many if they would derive from only one side). Because both fragments show mostly a straight border, similar sized alveoli for teeth, and no sutural contacts, it cannot be elucidated with confidence which fragment derives from the right and which from the left side of the skull. The part of the left ramus is circular in cross-section and preserves four and a half alveoli. The right ramus of the lower jaw is broken into four pieces which were glued back together, but is otherwise almost complete with only part of the surangular and the coronoid missing. The ramus is 137 cm in total length and has at least 40 alveoli in the dentary; an exact number cannot be given due to preservational reasons. Based on the lower jaw length and proportions of the occipital-pterygoid complex, AMU- CURS-073 is comparable in size to Mourasuchus specimen MCNC-URU-110-72V.

2) Seven cranial fragments ( Figure 21 ) of Mourasuchus sp. (AMU-CURS-396; associated with four vertebrae not shown in the figure) including a posterior skull table with right quadrate/quadratojugal and the occipital condyle preserved, two wide maxillary fragments (with seven and ca. 26 alveoli, respectively), one left premaxillary fragment comprising four perforations for lower jaw teeth, a long and straight lower jaw fragment, one fragment belonging to the lower jaw (dentary fragment), as well as an elongated unidentified fragment. The skull fragment shows prominent squamosal eminences framing a deeply incised valley, typically found in some Mourasuchus species (see Table 1). The supraoccipital appears to be displaced somewhat posteriorly between the squamosals. The state of preservation does not allow for identifying further anatomical details. The dentary fragment housing the Meckelian canal is curved.

3) A posterior skull fragment (AMU-CURS-530) with pronounced squamosal eminences heavily encrusted with - and locally substituted by - gypsum ( Figure 22.1 ).

4) One maxillary fragment with 38 alveoli (UNEFM- CIAAP-1378). The alveoli in the specimen form a very gently curving (convex) tooth row ( Figure 22.2- 3 ).

5) Two flat and wide, heavily encrusted, maxillary fragments (UNEFM-CIAAP-1447) carrying 11 and 25 alveoli, respectively ( Figure 22.4 ). The alveoli in the larger of the two fragments form a very gently curving (convex) tooth row as well.

6) An isolated right premaxillary fragment (AMU- CURS-395) comprising four holes for lower jaw teeth and part of the rim of the external naris ( Figure 22.5-6 ).

7) A slender fragmentary straight part of a small lower jaw showing 13 alveoli ( Figure 22.7 ), belonging to a small specimen of Mourasuchus (AMU-CURS-537).

8) Posterior portion of skull ( Figure 22.8 ) with strong squamosal eminences (AMU-CURS-695) from Tío Gregorio locality in the Urumaco Formation.

9) Twelve isolated fragments ( Figure 22.9 ) of a lower jaw (AMU-CURS-430).

10) Associated lower jaw fragments and isolated left prefrontal of Mourasuchus sp. (AMU-CURS-748;

anterior portions and there is a gap in between the bones. The left part preserves the angular and surangular up to the posterior portion of the external mandibular fenestra, as well as the articular. The right part preserves the articular and a large portion of the surangular, whereas the angular is largely missing. This part, together with the anterior portion of the right ramus experienced some distortion. The splenial is missing in both rami, but according to the attachment scar on the dentary it would have reached the thirteenth alveolus from anterior (tenth alveolus in M. atopus according to Langston, 1965).

The anterior portions of the rami are 71 (right) and 74 cm (left) in length, the posterior ones 35 cm (right) and 21.5 cm (left) respectively. Total length of the lower jaw might have been similar or slightly smaller than specimen AMU-CURS-073. The isolated left prefrontal ( Figure 23.7-10) has a length of 21 cm and a maximum width of 9 cm. It shows a long medial articulation with the right prefrontal of about 14 cm in length. The large but strongly eroded and anteromedially situated bony knobs in front of the orbits continue into a descending ridge onto the prefrontal shelf. The sutural contacts with the nasal, frontal, and the lacrimal are not well traceable due to the poor preservation of the specimen. A stout descending process is preserved ventrally, below the medial portion of the orbital rim, which is the dorsal portion of the prefrontal pillar. There are two low ventral keels or laminae protruding from the anterior portion of the prefrontal.

Figures

FIGURE 20. Photographs (1, 2, 5-12) and interpretative drawings (3, 4) of skull and lower jaw remains (AMU-CURS- 073) of Mourasuchus sp. 1, 2, images of the largest fragment of the skull including the basioccipital-basisphenoidpterygoid complex in dorsal (1) and occipital (2) view. 3, 4, interpretative drawings of the skull bones shown in 1 and 2. 5, 6, left quadrate in dorsal (5) and ventral (6) view. 7, fragment tentatively assigned to the postorbital bar of a right jugal in dorsal (?) view. 8, 9, large fragments of the maxillae in ventral view. 10, right lower jaw ramus in medial view. 11, small fragment of the left lower jaw ramus in dorsal view. 12, conical, slightly curved isolated tooth (the tip is set off by a break). Note different scale bar in 10 and 12. Abbreviations: bo, basioccipital; bs, basisphenoid; cf, carotid foramen; ec, ectopterygoid; eof, facet for exoccipital on basioccipital; ffm, floor of foramen magnum on basioccipital; oc, occipital condyle; pa, palatine; pt, pterygoid; vc, vertebral centra.

FIGURE 21. Photographs of weathered skull and lower jaw remains (AMU-CURS-396) of Mourasuchus sp. 1, posterior portion of skull and braincase in occipital view. Note squamosal eminences. 2, right premaxilla in dorsal view with perforations for anterior-most teeth of lower jaw. 3, fragment of maxilla with seven alveoli in ventral view. 4, portion of dentary in medial view. 5, unidentified bone fragment. 6, fragment of maxilla with about 26 alveoli in ventral view. 7, straight portion of lower jaw in dorsal view. The exact number of alveoli could not be elucidated.

FIGURE 22. Photographs of skull and lower jaw remains of several individuals of Mourasuchus sp. 1, heavily encrusted posterior portion of skull (AMU-CURS-530) in occipital view. Note squamosal eminences. 2, 3, maxillary fragment (UNEFM-CIAPP-1378) in dorsal (2) and ventral (3) view. 4, two maxillary fragments (UNEFM-CIAPP-1447) preserved in overlapping position in ventral view. 5, 6, right premaxilla (AMU-CURS-395) in dorsal (5) and ventral (6) view. 7, small fragment of lower jaw in dorsal view with 13 alveoli (AMU-CURS-537). 8, posterior portion of skull (AMU-CURS-695) in dorsal view. Note massive squamosal eminences. 9, twelve fragments of a lower jaw (AMU- CURS-430), with at least the larger fragments belonging to the right ramus. The largest pieces are in dorsal view, showing alveoli.