Sarcopterygian Fishes, the “Lobe-Fins”

Abstract

Sarcopterygian fishes, the “lobe-fins”, are today represented by two species of coelacanth and six lungfishes. Lungfish are the closest living group to the tetrapods, the first four-footed terrestrial vertebrates and all of their descendants. However, when extinct groups are taken into account, the evolutionary history and relationships of the Sarcopterygii become much more complex. In addition to lungfishes and the coelacanth, this chapter will introduce you to groups now known exclusively from fossils such as “dagger-toothed” onychodonts and porolepiforms named so for the special pores in their scales. Furthermore, the fish-tetrapod transition occurred gradually with many stem-tetrapods (finned tetrapods) progressively acquiring characters that would later become fixed features of terrestrial vertebrates. Primitive sarcopterygians have skulls that are divided in two halves, a feature still observable in the coelacanth today but lost in lungfishes and tetrapods. Lobe-finned fishes have teeth made of dentine and enamel; early members also possessed a tissue called cosmine covering their skulls and scales. A large variety of dentitions are visible across the Sarcopterygii, such as the fearsome tooth whorls in onychodonts and porolepiforms, crushing tooth plates in lungfishes or dagger-like fangs in tetrapodomorphs. The first appearance of an inner ear bone (stapes), an internal nostril (choana) and separation of the skull and shoulder girdle to form a neck occurred within this group. The earliest tetrapodomorphs retained fins and were still fully aquatic and are known from the Early Devonian onwards.