Sarcopterygians and actinopterygians form the two known lineages of bony fishes.(88 kb)
The former are distinguished from the latter by several characteristics, the most notable of which is their lobed fins. In contrast to actinopterygian fins, which are supported by thin rod-like bones known as rays, sarcopterygians sport thick and fleshy pectoral fins that join to the trunk by a single bone (the humerus) and are lined with muscles that help adjust the body position. The name sarcopterygian means fleshy fin, from the Greek words sarx
The earliest sarcopterygians had additional distinguishing features. Their jaws were lined with strong muscles and their skull was divided into two halves (front and rear) held together by an intracranial joint. They had two dorsal fins, an epicercal tail, and a body covered with a film of cosmine, a substance similar to dentine. Another innovation was the hard enamel coating on their teeth.
Sarcopterygians appeared in the Upper Silurian at the same time as many groups of bony fish. They diversified during the Devonian when they became more abundant and varied than the actinopterygians.
There are six main groups of sarcopterygians: onychodontiforms, rhizodontiforms, actinistians, osteolepiforms, elpistostegalians and dipnomorphs (this latter group comprising the dipnoi and porolepiforms). All but the first two groups are represented among the Miguasha fossils.
The sarcopterygians were relatively large fish that prospered in fresh, salty or brackish water. Many became extinct during the mass extinction event at the end of the Devonian Period. The only ocean dwellers to survive were the coelacanths, an actinistian subgroup. Those that lived in fresh or brackish water, like the rhizodontiforms and dipnoi, were not overly affected by the event.
During the Mesozoic Era, coelacanths became increasingly abundant in the sea, whereas only a few dipnoi survived in continental environments. Today, the once diverse sarcopterygians have been reduced to a mere eight species.
This apparently dramatic demise is somewhat deceptive. After all, the sarcopterygians left behind many successful descendants in the form of tetrapods. Land-dwelling vertebrates, including humans, are all sarcopterygian offspring, descendants of the elpistostegalians whose highly specialized adaptations for aquatic life proved very useful for conquering land about 365 million years ago. For example, the type and relative positions of the elpistostegalian fin bones, which include the humerus, ulna and radius, are directly comparable to those of tetrapod limbs.
With the discovery of at least nine sarcopterygian species, but only one actinopterygian, it seems that the waters of the Miguasha estuary were representative of the biodiversity during this part of the Devonian Period.