ABSTRACT
The rise of jawed ‘shes (gnathostomes) began about 440 million years ago (mya) with the radiation of placoderms and chondrichthyans (chimeras, sharks, and rays). However, those ‘shes were soon joined by a more advanced group collectively known as teleostomes (whole + mouth). By the Silurian period, three new and closely related classes of teleostome ‘shes had joined the existing ‘sh fauna of agnathans, placoderms, and chondrichthyans (Figure 4.1). The fates of these new ‘shes were very different. The Acanthodii (spiny + ones) suffered total extinction; the Sarcopterygii (¤esh + ‘n), most commonly called lobe-‘n ‘shes, is represented today by a few relict species (one line suffered pseudoextinction in giving rise to the tetrapods); and the Actinopterygii (ray + ‘n) expanded to develop the main line of ‘sh evolution.
