Voay, Brochu, 2007

Brochu, Christopher A., 2007, Morphology, relationships, and biogeographical significance of an extinct horned crocodile (Crocodylia, Crocodylidae) from the Quaternary of Madagascar, Zoological Journal of the Linnean Society 150 (4), pp. 835-863 : 853-855

OF VOAY

Voay lacks several derived features that characterize the crown genus Crocodylus . These are also absent from Osteolaemus . The maxilla lacks extensive development of blind recesses along the medial surface of the caviconchal recess ( Fig. 6 ). The anterior ramus of the ectopterygoid forming the posterolateral margin of the suborbital fenestra is not deeply forked ( Fig. 9 ). The posterior iliac blade is not constricted near its posterior tip ( Fig. 12 ). The pterygoid of Voay is also tall ventral to the median eustachian foramen, exposing the basisphenoid as a broad sheet ( Fig. 3); in all extant Crocodylus , the pterygoid is much shorter below the median eustachian foramen, limiting exposure of the basisphenoid. The condition seen in Voay is also found in Osteolaemus and Mecistops ( Brochu, 2000) .

The number of unambiguous synapomorphies for Osteolaeminae depends on how basal relationships within the group are resolved. If Rimasuchus is the basalmost osteolaemine or closely related to ‘ C. ’ pigotti and Euthecodon , there are two – extension of the squamosal on to the quadrate ramus and prominent preorbital crests. The former is a subtle character state, and variation in Voay suggests its secondary loss within the group (see below), but it is unique to osteolaemines. The latter is not unique to osteolaemines; most crocodyloid skulls bear nearly parasagittal ridges on the lacrimal, and they become especially prominent in Indopacific species of Crocodylus ( Brazaitis, 1973; Brochu, 2000). They are also anteroposteriorly short in extant Osteolaemus and, in both Osteolaemus and Voay , contiguous with the line along which the lateral surface of the snout is ventrolaterally reflected.

Extension of the squamosal beyond the paroccipital process is unambiguously present in Osteolaemus and the specimens used to code Euthecodon , ‘ C. ’ pigotti, and Rimasuchus . The specific shape of the descending lamina of the squamosal varies in all crocodylian species, but its extension does not vary in Osteolaemus . Nevertheless, variation in this feature in Voay calls utility of this feature into question. Unfortunately, because the other three osteolaemines are each based on single specimens, variation cannot be assessed.

A third character unambiguously diagnoses Osteolaeminae if Rimasuchus is closer to Voay and Osteolaemus – a distinct choanal neck ( Fig. 5 ). The surface of the pterygoid anterior and lateral to the choana is depressed in most crocodyloids, but this condition is elaborated in osteolaemines such that the choana appears to open at the end of a short tube or ‘neck’. Choanal necks are also seen in some alligatoroids, but these result from deeper depressions lateral and posterior to the choana. This condition is observable in Osteolaemus at all stages of posthatching ontogeny.

This feature is ambiguous because the palate is incomplete in the specimen used to code Rimasuchus lloydi in this study (BMNH r14154) and in the holotype ( Fourtau, 1920), both of which are from the same Burdigalian site in Egypt. Skulls from several sites in Africa have been referred to Rimasuchus lloydi ( Tchernov, 1986; Pickford, 1994, 1996, 2000; Leakey et al., 1996; Llinás Agrasar, 2004), including one that appears to preserve a choanal neck ( Storrs, 2003: 146). However, there is considerable variation in the overall shapes of these skulls ( Tchernov, 1986; C. A. Brochu, pers. observ.) and they derive from several horizons throughout the Miocene and into the Pleistocene. Pending a more complete review of all of this material the condition of the choana in Rimasuchus will remain unknown.

The choana is not preserved in Euthecodon arambourgii ( Ginsburg & Buffetaut, 1978; C. A. Brochu, pers. observ.). More complete specimens of other species of Euthecodon not included in this study clearly show a strongly depressed pterygoid around the choana ( Tchernov, 1986; Storrs, 2003). Given the distribution of this feature in other crocodylids, its presence in Euthecodon arambourgii is predicted.

Two character states unambiguously unite Voay and Osteolaemus – a short palatine process that does not extend far beyond the anterior margins of the suborbital fenestrae and constricted supratemporal fenestrae. Both might be related to snout shape – in both cases, the skull is dorsoventrally deep relative to snout length (see below). In addition, Voay and O. tetraspis (but not O. osborni ) have short flanges that extend from the lateral margins of the palatines into the suborbital fenestra, another feature that might reflect overall skull shape.

Figures

Figure 5. Internal choana, ventral view. A, AMNH 3101, Voay robustus. B, TMM m-1786, Crocodylus niloticus. C, AMNH 10083, Osteolaemus osborni (holotype). Scale = 1 cm.

Figure 6. Right maxillae, medial view, showing medial wall of caviconchal recess (ccr). A, BMNH r2193, Voay robustus. B, UCMP 140795, Crocodylus niloticus. C, UF 34784, Osteolaemus tetraspis. Scale = 1 cm.

Figure 9. Right suborbital fenestra, lateral oblique view, showing condition of the anterior ramus of the ectopterygoid (ear). A, BMNH uncategorized, Voay robustus. B, TMM m-1786, Crocodylus niloticus. Scale = 1 cm.

Figure 12. Right ilium, lateral view. A, Crocodylus acutus, USNM 211278. B, Voay robustus, AMNH 17008. C, Osteolaemus tetraspis, USNM 194448 (left element, image reversed). Scale = 1 cm.