Occurrence of Mesoplastra in a Cryptodiran Turtle, Lepidochelys olivacea

Abstract

MESOPLASTRAL bones, a characteristic feature of most amphichelid turtles, are to-day restricted to members of the Pleurodiran family Pelomedusidae. Of the three genera of this family, two (Podocnemis and Pelomedusa) have the mesoplastra reduced to small, widely spaced bones separating the hyoplastra and hypoplastra at the outer part of their common suture. Only in Pelusios are they retained in their pristine contiguous condition, a retention perhaps connected with the necessity of the hyoplastra having a straight posterior margin to allow flexure of the hinged plastron to occur. The mesoplastra of Pelomedusa may be missing as an occasional variant1, but there appears to be no record of mesoplastra having been recorded in a cryptodire; indeed, a fossil plastron with mesoplastra would definitely not be classified as cryptodiran at all. It is therefore of interest to record the finding of a plastron of the Pacific ridley (Lepidochelys olivacea) in which the hyo- and hypoplastra appear to be separated by a pair of well-defined contiguous mesoplastra (Fig. 1). The plastron was found separate from the remainder of the animal on Shell Beach, North-West District, British Guiana. (The discovery of Lepidochelys in British Guiana is documented elsewhere2,3.) The specimen was not collected, but a colour slide was taken of it, close inspection of which leaves no doubt that the breaks are true sutures and not sulci or post-mortem fractures.