Jawed vertebrates-Evolution of the jawed vertebrates: Gnathostomata- Early, extinct, jawless fishes: conodonts and "ostracoderms"Origin of gnathostome traits:-"Placoderms"-Evolutionary origins of bone, jaws, teeth and paired finsMost modern day fish, with the exception of the lamprey and hagfishes, are part of the clade called Gnathostomata, or gnathostomes, which refers to the jawed vertebrates. Of course, we and all other tetrapods are also part of the gnathostomes. The evolution of the jaw was a major evolutionary innovation associated with increased activity and feeding capabilities.Around the time that jaws appeared, we see other innovations including the appearance of paired fins and bone. Note added after posting: Lamprey and hagfishes are also "early jawless fishes", and probably (molecular data) originated in the Cambrian. Right now, I will talk about a few extinct groups that used to be abundant.Predatory “placoderms”- arthrodire (“jointed neck”) coccosteus:The next lineage and one that holds special interest for us is the "placoderms", a name that refers to their plate-like dermal armourthat covered the front 1/3 to half of their bodies. Like the ostracoderms, the assemblage of "placoderms" refers to many different lineages and is not monophyletic.Placoderms includes the earliest animals that had jaws. Until the last few years, it was not clear whether or not their jaws were homologous with other gnathstomes, including living forms. The
"conventional" placoderm fossils have a lower jaw that resembles that of more recent gnathostomes. Around 2016, a research grouppublished papers on two newly discovered placoderm species, which had bones forming an upper jaw that were clearly homologous to the bones in the gnathostome jaw. This means we can now consider the placoderms as true gnathostomes - not a group that independently developed jaw-like elements. Along witha jaw, placoderms had two nostrils (I'll say more about this later - it is more important than it may sound.) Placoderms are also the oldest group in which animals had paired pelvic fins, not only paired pectoral fins.“dunk”-an arthrodire:Dunkleosteus: 8 m long predator with powerful bite. It is most famous palcoderm, due to its size and extremely powerful bite.This image has a lot going on. But it makes a few points about theevolution of thejaw that are useful, by setting these points on an evolutionary tree.(1) Basal gnathostomes: Placoderms: Starting at the bottom, we see "conventional" placoderms such as Dunk and the Arthrodire, shown in the previous two slides. The lower jaw resembles that of gnathostomes.