Abstract

The Captorhinomorpha consists of two families, the Captorhinidae and the Protorothyrididae. The distribution of morphological character states of the skeleton permits reevaluation of currently accepted theories of the relationships of captorhinomorph reptiles. Identification of characters states that are primitive for reptiles (amniotes) has been made through outgroup comparison. The Captorhinidae, Protorothyrididae, and Diapsida form a natural group that share such derived characters as reduced supratemporal, reduced tabular, narrow supraoccipital with anteriorly directed crista alaris, loss of supratemporal–postorbital contact, loss of opisthotic–tabular contact, and loss of the medial centralia pedis. These shared derived character states indicate that captorhinomorphs are not the sister taxon of all other reptiles but are advanced relative to pelycosaurs, pareiasaurs, and procolophonids. Protorothyridids share with diapsids such derived characters as short postorbital region of the skull, keeled anterior presacral pleurocentra, slender limbs, and long, slender feet. This distribution of character states indicates that protorothyridids are more closely related to diapsids than either of these taxa is to captorhinids.The morphological pattern of small, lightly built, agile insectivores, represented by protorothyridids and early diapsids, is no longer considered to be the primitive amniote condition. Available evidence indicates that the most primitive amniote adaptation was, instead, that of a small, relatively slow carnivore that probably fed on primitive, terrestrial annelids and arthropods.

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