. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Fig. 4. External ear structure in A, Chosmofosourus (proterosuchian thecodont) suchian crocodile); and D, Crocody/us. B, Proterochampsa; C, Pe/ogosourus (meso- joins with the quadrate. Ventrally, on the posterior face of the skull, it joins with the basioccipital. Basioccij)ital. The basioccipital forms the entire occipital condyle and the most ven- tral border of the foramen magnum. Ven- trally, it is fused completely with the basi- sphenoid, has a subrounded shape, resem- bling the thecodont rather than the eusu- ch
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Fig. 4. External ear structure in A, Chosmofosourus (proterosuchian thecodont) suchian crocodile); and D, Crocody/us. B, Proterochampsa; C, Pe/ogosourus (meso- joins with the quadrate. Ventrally, on the posterior face of the skull, it joins with the basioccipital. Basioccij)ital. The basioccipital forms the entire occipital condyle and the most ven- tral border of the foramen magnum. Ven- trally, it is fused completely with the basi- sphenoid, has a subrounded shape, resem- bling the thecodont rather than the eusu- chian condition and is \\'ithout the basioc- cipital processes common in mesosuchians. Ear structure. Frotcrochampsa shows the beginning of the acquisition of the otic notch in crocodiles, and, with the meso- suchians, provides a phylogenetic sequence for the e\'olution of the unusual crocodilian ear structure (Fig. 4). In Frotcrochampsa there is a meatal groove running from the border of the quadrate and squamosal, passing anterior to the exoccipital and into the inner ear. Haughton (1924) mentions a similar groove in Notochampsa. In Froto- suchus there is no evidence of either a groove or an otic notch. In the earliest thecodonts, there is no indication of an otic notch (later thecodonts do possess one, although it is usually formed completely within the squamosal), and the quadrate is, as a rule, completely vertical. In mesosuchians the otic notch is usuallv quite prominent, fomied by the squamosal projecting out over the quadrate (see Fig. 4C). In modern crocodiles the squamosal has extended backward and downward onto the quadrate to close the otic notch and gain a broad posterior contact \\\\h the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge
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