Araripesuchus rattoides, Sereno & Larsson, 2009

Sereno, Paul & Larsson, Hans, 2009, Cretaceous Crocodyliforms from the Sahara, ZooKeys 28 (28), pp. 1-143 : 57-61

Araripesuchus rattoides sp. n.

urn:lsid:zoobank.org:act:CF171699-D3FD-4B5C-909A-21C4412BCB0E

Figs. 27–30

Table 9

Etymology. Rattus (Latin) ; -oides, likeness (Latin). Named for the enlarged, procumbent first dentary tooth, which is reminiscent of the condition in many rodents.

Holotype. CMN 41893; right dentary preserving alveoli 1–14.

Referred material. UCRC PV3; anterior portion of left dentary preserving alveoli 1–8.

Type locality. Er Rachidia District (exact locality unknown), eastern Morocco (Fig. 1A, B). A referred specimen ( UCRC PV 3) was surface collected in 1990 in a small wash at Darelkarib (south of Erfoud).

Horizon. Kem Kem Beds; Upper Cretaceous (Cenomanian), ca. 95 Mya ( Sereno et al. 1996). Th e referred specimen ( UCRC PV 3) appears to have come from the lower member (pers. commun. D. Dutheil).

Diagnosis. Small-bodied metasuchian (<1 m) with an enlarged procumbent first dentary tooth that is set immediately adjacent to the midline; smaller procumbent second dentary tooth; a caniniform fourth dentary tooth that is particularly large (twice the basal dimensions of adjacent crowns); and a smooth anterior surface on the dentary symphyses with an oval fenestra opening into the first alveolus.

Dentary. The dentary of A. rattoides show a series of features that distinguishes it from the previously named species A. wegeneri and from a contemporary unnamed species from Cenomanian beds in Niger that closely resembles A. tsangatsangana ( Turner 2006) . Th e skull in A. rattoides appears to be proportionately narrower than in A wegeneri , based on the angle of divergence of the dentary tooth row from the midline. In A. wegeneri , the tooth row diverges at an angle between 20 and 25° from the midline (Fig. 18C), an angle matching the divergence of the upper tooth row (Figs. 14C, 15C). In A rattoides , by contrast, the angle of divergence is approximately 10° ( Fig. 27C ), or less than half that in A. wegeneri . Th e anterior end of the dentary in A. rattoides is proportionately deeper than in A. wegeneri and other species of Araripesuchus . This difference is visible in both anterior and lateral view (Figs. 18A, D, 27A, D).

The orientation of the alveoli for teeth d1–11 is more procumbent in A. rattoides . The first and second alveoli project more strongly anteriorly than dorsally, a difference best appreciated in anterior view (Figs. 18D, 27D). Succeeding alveoli, including the caniniform (d4) and d5–11, are visible in lateral view ( Fig. 27A ), whereas they are hidden by the dorsal edge of the alveolar margin in A. wegener i (Fig. 18A). Despite the more pronounced anterior projection of the anteriormost pair of teeth, the symphyseal region below these teeth ( Fig. 27A ) is deeper than in A. wegeneri (Fig. 18A) and in a larger contemporary of A. wegeneri (Fig. 31A). Moreover, unlike these other species, the symphyseal articular surface of the dentary is not uniformly rugose in A. rattoides as it is in A. wegeneri and its larger contemporary (Figs. 18B, 31B). Th e anterior portion is smooth and fenestrated, as seen in two specimens ( Figs. 27B , 28C ).

Tooth size is also distinctive in A. rattoides ( Table 9). The first tooth is 75% the average diameter of the caniniform tooth (d4), which is already twice the diameter of adjacent crowns. In A. wegeneri the first dentary tooth is small ( Fig. 27C ), and the caniniform is considerably less than twice as large as adjacent crowns ( Fig. 20B ).

Tooth number may have been slightly greater in A. rattoides . In A. wegeneri , the largest postcaniniform teeth are d11 and d12 (Figs. 18B, 20A). In A. rattoides the largest postcaniniform dentary teeth are d12 and d13 ( Fig. 27 , Table 9).

Other features in A. rattoides confirm its status as a species of Araripesuchus . Both A. rattoides and A. wegeneri have an unusual anterior extension of the articular scar for the splenial located dorsal to the symphysis on the subhorizontal palatal surface. This articular extension of the splenial, which is located medial to the alveoli for d4–6 ( Fig. 27C ), is continuous posteriorly with the more typical vertical splenial attachment scar dorsal to Meckel’s canal. A. wegeneri shows a similar articular extension of the splenial (Fig. 18C). The alveoli posterior to d11, in addition, are open medially with alveolar septa poorly developed as low rounded ridges ( Fig. 27B, C ). A similar condition is present in A. wegeneri (Fig. 18B) and some other species (Fig. 31C) ( Pol and Apesteguia 2005).

Figures

Figure 20. Unworn dentition of the crocodyliform Araripesuchus wegeneri. Detailed views of the anterior and middle portions of the tooth rows (MNN GAD20). A Left tooth rows in medial view. B Anterior portionof left tooth rows in medial view. C Middle portion of left tooth rows in medial view. Scale bar equals 1 cm in A and 5 mm in B and C. Abbreviations: ca, carina; d2, 4, 5, 7, 9, 11, 14, 16, dentary tooth 2, 4, 5, 7, 9, 11, 14, 16; m1, 3, 5, 7, 9, 13, maxillary tooth 1, 3, 5, 7, 9, 13; ne, neck; pm2–5, premaxillary tooth 2–5; rt, root.

Figure 27. Right dentary of the crocodyliform Araripesuchus rattoides sp. n. Isolated right dentary lacking teeth (CMN 41893). A Lateral view (reversed). B Medial view. C Dorsal view. D Anterior view. Scale bars equal 2 cm in A-C and 1 cm in D. Abbreviations: ad1, 3, 4, 8, 11, 13, 14, alveolus for dentary tooth 1, 3, 4, 8, 11, 13, 14; asp, articular surface for the splenial; dsym, dentary symphysis; fen, fenestra; fo, foramen; Mc, Meckel’s canal; rdsym, rough dentary symphysis; sdsym, smooth dentary symphysis.

Figure 28. Left dentary of the crocodyliform Araripesuchus rattoides sp. n. Pencil drawing of isolated left dentary ramus lacking teeth (UCRC PV3). A Dorsal view. B Ventral view. C Medial view. Scale bar equals 1 cm. Parallel lines indicate broken bone surface. Abbreviations: ad1, 4, 5, 8, alveolus for dentary tooth 1, 4, 5, 8; asp, articular surface for the splenial; fen, fenestra; fo, foramen; sym, symphysis.

Figure 29. Computed-tomographic scan of the crocodyliform Araripesuchus rattoides sp. n. Isolated left dentary ramus lacking teeth (UCRC PV3). A Drawing in dorsal view showing the location of cross-sections (B-D). B Parasagittal section showing the size and orientation of the alveolus for dentary tooth 1. C Cross-section through the alveolus of the fourth dentary tooth. D Cross-section through the alveolus of the sixth dentary tooth. Scale bar for B-D equals 1 cm. Abbreviations: ad1, 4, 6, alveolus for dentary tooth 1, 4, 6; sym, symphysis; vc, vascular cavity.

Figure 30. Reconstruction of the dentition of the crocodyliform Araripesuchus rattoides sp. n. Anterior dentition restored based on the size and orientation of the alveoli in CMN 41893 and UCRC PV3. A Dorsal view. B Ventral view. C Anterolateral view with premaxillary and anterior maxillary dentition restored to match those in the dentary. Given the presence of large, adjacent first dentary teeth, there may have been a median diastema between the premaxillary teeth and one or two fewer teeth in each premaxillary tooth row. Scale bar equals 2 cm in A and B. Abbreviations: d1, 4, 8, dentary tooth 1, 4, 8; d, dentary; di, diastema; m1, maxillary tooth 1; pm1, 2, premaxillary tooth 1, 2.