Phalangeal formulae and ontogenetic variation of carpal morphology in Testudo horsfieldii and T. hermanni

Abstract

AbstractWe compared variation in phalangeal and carpal morphology of the Central Asian Testudo horsfieldii, a burrow-digging tortoise species, with its sister taxon T. hermanni, a Mediterranean species without extensive digging behaviour. Thirty-two Testudo horsfieldii kazachstanica shared the same phalangeal formula (0-2-2-1-1). The distal carpal 1 and metacarpal I as well as the pisiform were consistently lacking, sometimes also the medial centrale. Phalangeal morphology was more variable in Testudo hermanni hermanni. In 29 specimens four phalangeal formulae were found that differed with respect to the reduction of digit 1 (1-2-2-2-1, M-2-2-2-1, D-2-2-2-1, 0-2-2-2-1). The pisiform develops late in ontogeny and is lacking in very most juveniles and subadults. In contrast to T. h. kazachstanica, the medial centrale is always present. In both T. h. kazachstanica and T. h. hermanni carpalia increasingly fuse with age and size. The underlying morphological patterns differ however. The extreme character state in aged T. h. kazachstanica is one large solid bone element, formed by the fused intermedium, ulnare and both centralia. Aged T. h. hermanni have, in contrast, two separate larger carpal elements, one formed by the fused lateral and medial centralia and the other by the fused distal carpalia 1 and 2; the intermedium and ulnare never fuse with one another or with other carpalia. While a partial or complete loss of digit 1 seems to be characteristic for all Testudo species, we propose that the extensive fusion of carpal elements in T. horsfieldii is correlated with its natural history because a rigid manus could be advantageous for burrow-digging. Also the reduction of digit 4 (one phalanx present), a rare character among testudinids and not occurring in other Testudo species, could be linked with its mode of life.