Variation of hyoid morphology in geoemydid terrapins

The hyoid apparati of 25 geoemydid species in 16 genera (including aquatic, semiterrestrial and terrestrial taxa) are compared, and for Cuora galbinifrons bourreti ontogenetic development of the hypoid apparatus is described. Generally, ossification of the hyoid apparatus increases with age. The majority of terrestrial species maintain cartilaginous or partially cartilaginous hyoid bodies and second branchial horns throughout life however. Also in tortoises (Testudinidae) hyoid body and second branchial horns remain cartilaginous, suggesting a positive correlation between mode of life and hyoid morphology (terrestrial species with partly cartilaginous hyoid apparatus vs. aquatic species with fully ossified hyoid apparatus). While skull morphology of chelonians is frequently used for taxonomic purposes (e.g. McDowell, 1964; Gaffney, 1979; Meylan, 1987; Bour, 1989), morphology of the hyoid apparatus is rarely investigated. The most comprehensive studies were published in the late 19th and early 20th centuries (Siebenrock, 1898; Fürbringer, 1922; Gräper, 1932); earlier works treating hyoid morphology were reviewed in these papers as well as in Schumacher (1973). Only Siebenrock (1898) compared hyoid morphology of more than a few taxa (40 species from 10-11 families using contemporary taxonomy) and found variation among different taxa. In the present study we investigate taxonomic variation within the hyoid apparatus of the Geoemydidae. The hyoid apparatus plays an important role in breathing (Siebenrock, 1898) and feeding of chelonians (e.g. van Damme and Aerts, 1997; Wochesländer and Weisgram, 1999; Lemell et al., 2000) and, in aquatic and terrestrial terrapins breathing (Seymour, 1982) and feeding mechanisms (Bramble and Wake, 1985; Wochesländer and Weisgram, 1999; Lemell et al., 2000) differ significantly. Therefore, differences in hyoid 1 Anna-Siemsen-Str. 17, D-07745 Jena, Germany 2 Museum of Zoology, Natural History State Collections Dresden, A.B. Meyer Building, Königsbrücker Landstr. 159, D-01109 Dresden, Germany ∗ Corresponding author; e-mail: [email protected] morphology are expected. Siebenrock (1898) studied only four aquatic (Cuora amboinensis, Mauremys caspica, Orlitia borneensis, Rhinoclemmys punctularia) and one semiterrestrial geoemydid species (Cyclemys sp.) and reported no significant differences of hyoid apparati. In this paper, the hyoid apparati of a considerably higher number of species is studied. Here we compare the hyoid apparatus of 46 skeletons representing 25 geoemydid species in 16 genera, including aquatic, semiterrestrial and terrestrial taxa (table 1), and describe ontogenetic development of the hyoid apparatus in Cuora galbinifrons bourreti. Hyoid nomenclature follows Sheil and Greenbaum (2005). Hyoid shape is similar within the Geoemydidae (figs 1-2). Differences in the extent of ossification correspond to ageand taxon-dependent variation. Generally, juveniles and subadults have mainly cartilaginous hyoid apparati, as pointed out by Siebenrock (1898). This is exemplified by a series of Cuora galbinifrons bourreti comprising specimens of different size and age (fig. 3): In juveniles with straight line carapace lengths of approximately 10 cm, the hyoid apparatus consists mostly of cartilaginous tissue and only the first branchial horn (cornu branchiale I) is ossified. In subadults with carapace length of 15-16 cm the hyoid body (corpus hyoidis) and second branchial horn (cornu branchiale II) ossify. Ossification of the hyoid body starts from four nuclei, two on the margin of each side of the body. One is located anterior to the articulation of the first branchial horn © Koninklijke Brill NV, Leiden, 2007. Amphibia-Reptilia 28 (2007): 148-153 Also available online www.brill.nl/amre