Rhipidistia

The Rhipidistia were once divided into the Porolepiformes and the Osteolepiformes. Now is it accepted that the Porolepiformes were more related to lungfish (Dipnoi) than to the Osteolepiformes. Today rhipidistians include a diverse array of groups including the lungfish, Porolepiformes and the Osteolepiformes including the ancestors of the first verebrates that set foot on dry land as well as in theory all tetrapods including ourselves.

Dipnomorpha

The dipnomorphs were stout-bodied fish with a blunt snout and small eyes. They had elongate, leaf-shaped pectoral fins. The Porolepiformes and the modern lungfish (Dipnoi) belong to this group.

The Porolepiformes were large predators that were mostly marine during the Early Devonian but colonized brackish and freshwater environments and became much more common as a result. They were so common in the Late Devonian that their scales have been used for dating. It is possible that they used ambushing techniques to catch their prey which probably was smaller fish. Holoptychius is one of the best known genera. It had large rounded scales, long pointed pectoral fins and a short skull.

Holoptychius (Permission provided by Mike Benton)

The Dipnoi were particulary diverse in the Devonian, but dwindled and now only three living genera remain (living lungfish).

Dipterus from the Middle Devonian of Scotland had a long body, pointed fins, long central lobes supported by a symmetrical array of bones, a heterocercal (asymmetrical) tail and a skull with a complex array of bones. Its eyes were large and the mouth had no teeth on the margins of the jaws as in other bony fishes but only a pair of dentine-covered grinding plates in the middle of the palate which propably had a crushing function for feeding on tough food.

Dipterus (Permission provided by Mike Benton)

Several lineages radiated in to the Carboniferous but only two continued into the Mezosoic and Cenozoic and their derived features were fusion of the palatoquadrate (a part of the upper jaw joint) to the skull, development of a special hypermineralized dentine that increased the crushing power of the jaws and a more symmetrical body.

Rhizodontiformes

The rhizodonts appeared in the Middle Devonian and became extinct by the end of the Carboniferous. They were large hunters that reached 7 meters in length and had massive jaws with sharp, blade-like teeth. Their frontlimbs had very powerful musculature and the shoulder girdle was quite advanced for sarcopterygians. It is therefore possible that they might have been able to walk on land even if they are not considered the nearest relatives of tetrapods.

Osteolepiformes

The Osteolepiformes are probably the best known of the extinct sarcopterygians. Their heyday was in the Devonian and well known species include Osteolepis that had a long slender body with large midline fins, lobed paired fins and a heterocercal tail and Eusthenopteron which was 1 m long and had a threepointed symmetrical tail. There are hundreds of Eusthenopteron remains from the Late Devonian in Quebec, Canada. Therefore has it been possible to reconstruct its anatomy in great detail. It has famously been called the "fish with legs" as its pectoral fins are connected to a long chain of bones that are paralell to tetrapodan limb bones. These bones connect to the more anterior shoulder girdle which is attached to the back of the skull. However did they not have any "hands" as such and were therefore neither agile nor fast on land. The Osteolepiformes survived into the Permian.

Osteolepis (Permission provided by Mike Benton)

Eusthenopteron (Permission provided by Mike Benton)

The head of the Osteolepiformes is somewhat similar to actinopterygians. The skull is very kinetic and can be jointed in order to open the mouth wide. The head of Eusthenopteron has complex dermal plates covering the outer portions. The teeth are small and born on the maxilla, premaxilla and dentary bones. There are also some teeth on the palate that are heavier and more complex and are similar to the labyrinthine teeth of early tetrapods.

Head of Eusthenopteron in a) lateral view and b) dorsal view (Permission provided by Mike Benton)

Cross-section of a Eusthenopteron tooth showing labyrinthine structure ( Permission provided by Mike Benton)

Elpistostegalia

The Elpistostegalians are the nearest relatives to the Tetrapoda. The group is mostly based on one well known genus, Panderichthys. Panderichthys is from Late Devonian Latvia and is about 1 m in length. It is thin, streamlined with a long snout, flattened skull and eyes located partly on top of the head. It only has the paired pectoral and pelvic fins and the tail fin but lacks midline fins. The structure of the skull is very similar to early tetrapods such as Ichthyostega.

Panderichthys (Permission provided by Mike Benton)


Author: Snorri Sigurdsson (Email: ss4460@bristol.ac.uk)
Last updated: 14 November 2004
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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2004-5