African hinged terrapins
The tropical African freshwater pelomedusid turtles with hinged plastra belong to the genus Pelusios. The carapace is elongated, oval, and somewhat flattened in most species. A middorsal keel represented by raised protuberances on the posterior parts of the vertebrals may be present, and the posterior carapacial rim is serrated in one species. Neural bones vary in number from five to eight (rarely four) and they may be in contact with or separated from the nuchal and single suprapygal bones, or both, depending on the species. A hinge is present on the large plastron, which also contains a pair of mesoplastral bones that extend to the midline. The hinge lies between the hypoplastra and mesoplastra, permitting the anterior lobe to close in adults (but rather rigid in P. broadleyi). Bramble and Hutchison (1981) studied the hinge kinesis and found that a modified axillary buttress serves as a lever for attachment of a large closing muscle, the M. levator plastralis. This muscle is apparently derived from the exhalent respiratory muscle M. diaphragmaticus. A supratemporal roof is lacking on the skull, and the quadratojugal is separated from the parietal by the postorbital and from the squamosal by the quadrate. The jugal is also prevented from touching the parietal by the postorbital. Posterior emargination of the skull prevents contact between the parietal and the squamosal bones. The prefrontals are well-developed and in contact, but nasals are absent. The palatines are in wide contact in the roof of the mouth, but a vomer is usually absent. Only an indistinct medial ridge occurs on the triturating surface of the upper jaw. Between the orbits is a pair of supraorbital scales separated by a longitudinal seam; behind these a large frontal scale is bordered by temporal scales. All toes are webbed, and the middle toe contains three phalanges.
Williams (1954b) and Auffenberg (1981a) proposed that the living species of Pelusios can be divided into two apparently valid groups. The P. adansonii group, which also includes P. broadleyi, P. gabonensis, and P. nanus, is characterized by having the anterior lobe of the plastron relatively long and the abdominal scutes relatively short, so that the anterior lobe is twice as long or longer than the length of the interabdominal seam, and a short bridge. Also, the mesoplastra are more or less tapered toward the plastral midseam. The P. subniger group includes the remaining species. In these turtles the plastral anterior lobe is shorter and, correspondingly, the abdominal scutes are longer, so that the anterior lobe is less than twice as long as the length of the interabdominal seam (usually it is about 1.5 times as long), and the bridge of moderate length; their mesoplastra are not tapered medially, having instead parallel transverse contacts with the hyo- and hypoplastra.
This genus is in dire need of revision. There is disagreement on the status of several taxa between Broadley (1981a) and Bour (1983, 1984a), who have most studied this group, and Obst (1986) and Iverson (1992).
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