reconstruction of three lobe-fin fishes Diplurus (upper left; Triassic), Eusthenopteron (upper right: Late Devonian)
and Holoptychius (bottom: Late Devonian) ©

More About Lobe-Fins: Sarcopterygii

Fleshy Fins and Us Too

Lobe-fin fishes form one of the two known lineages of bony fishes (the Osteichthyes). Ray-fin fishes (Actinopterygii), which form the other lineage, are arguably the most successful of vertebrates and certainly the most successful "fishes". On the other hand, lobe-fins (Sarcopterygii) are currently represented only by the coelacanth (Latimeria chalumnae), and six species of lungfishes: Lepidosiren paradoxa, Neoceratodus forsteri, and four species of Protopterus. However, lobe-fins experienced considerable success during the Paleozoic Era. They exhibited a greater diversity than the ray-fins during the Devonian and Carboniferous Periods, and they were typically the top predators in many of the marine and freshwater habitats they occupied. Moreover, one group of lobe-fins gave rise to the tetrapods, which have become other most successful group of vertebrates. Strictly speaking, since tetrapods evolved from lobe-fins, all tetrapods –including us— are also lobe-fins.

Lobe-fins are characterized by their fleshy pelvic and pectoral fins with well developed bones and muscles. These fins join (or articulate) to the body via a single bone (humerus to the shoulder or pectoral girdle, and femur to the pelvis). In contrast, ray-fin fishes sport fins that contain several rod-like bones that articulate directly with the pectoral and pelvic girdles. Lobe-fins also have a hinged braincase and a corresponding intracranial joint in the skull roof. This allows for a cranial kinesis (flexibility within the head) that provides additional bite strength; this feature is secondarily lost in most lungfishes, tetrapods and the tetrapod’s closest lobe-fin relatives.

Fossil lobe-fins first appear in the Lower Devonian, and diversified into several major groups by the Middle Devonian. Lobe-fin diversity remained high during the Upper Devonian, Carboniferous and Lower Permian, but declined significantly thereafter.

lobe-fin diversity over geologic time

The relationships among lobe-fins has been the subject of considerable debate. The earlier edition of Devonian Times employed a classification scheme that separated lobe-fins into two groups: Crosspterygians and lungfishes. The current edition discards this scheme and employs one that’s in general agreement with those presented in Janvier (1996) and Cloutier & Ahlberg (1996).

Most lobe-fins belong to one of three major groups: the Actinistia (coelacanths), the Dipnomorpha (lungfishes and Porolepiformes), and the Tetrapodomorpha (Rhizodontoformes, Osteolepiformes and Elpistostegalia). Less diverse groups include the Onchodontida (three Devonian genera) and two Lower Devonian genera (Powichthys and Youngolepis) that show some affinity to the Dipnomorpha.

The Actinistia first appear in the Middle Devonian and become relatively diverse during the Carboniferous, decline in the Permian and again become diverse in the Triassic. They apparently decline through the remainder of the Mesozoic, with the last fossil record occurring in the Late Cretaceous. Actinistians were thought to have become extinct until the spectacular discovery of Latimeria chalumnae in 1938. (See the reconstruction of Diplurus above for an example of an actinistian.)

The Dipnomorpha contain the Porolepiformes and the Dipnoi (lungfishes). Porolepiforms were relatively large (1-2.5 m) fishes that were prominent predators in Middle and Late Devonian nearshore and freshwater habitats. One member, Holoptychius (shown above), has been collected in several Devonian tetrapod localities. Lungfishes are among the first lobe-fins to appear in the fossil record. Their diversity peaks in the Upper Devonian, but a second diversity maxima occurs in the Triassic. Lungfishes and the Actinsitia are the only two groups of lobe-fins to have survived the Permian; both also have living representatives. (A more detailed discussion on lungfishes is presented on the web page about Red Hill lungfishes.)

Tetrapodomorpha is a clade of lobe-fins that contains rhizodonts, osteolepiforms, Elpistostegalids and tetrapods. Rhizodonts first appeared in the Middle Devonian and developed into the giants (2-7 m) of freshwater habitats from the latest Devonian through the Carboniferous and into the Lower Permian. A juvenile rhizodont (c.f. Sauripterus) is found at Red Hill.

Osteolepiformes is the group of lobe-fins that gave rise to the tetrapods. They first appeared in the late Lower Devonian or early Middle Devonian and reached a maximum diversity in the Late Devonian. The order contains a number of fishes of uncertain phylogeny and two monophyletic families, the Megalichthyidae and the Tristichopteridae. The Megalichthyids —represented at Red Hill by an unidentified species— is the only osteolepiform group to survive past the end of the Devonian. Tristichopterids apparently gave rise to the tetrapods and the Elpistostegalids. One large tristichopterid, Hyneria lindae, was the largest vertebrate at Red Hill. Thanks to the excellent work by Erik Jarvik, another tristichopterid, Eusthenopteron foordi (shown above), is one of the best known of any fossil vertebrate.

The Elpistostegelia (also known as the Panderichthyida) is a small paraphyletic group of Frasnian (early Late Devonian) lobe-fins that exhibit a number of transitional features linking tetrapods to the osteolepiforms. These include the loss of anal and dorsal fins, an enlarged and flattened head made rigid by the loss of the intracranial joint, an enlongated humerus and expanded ribs. Elpistostegelids also have features common only with the early tetrapods, including the distinctive pattern of external skull bones and elaborately infolded (labryrinthodont) teeth. This group is represented by four genera, Elpistostege, Livonia, Panderichthys, and Tiktaalik.

Five lobe-fin fishes, Hyneria lindae, a juvenile rhizodont, an unidentified lungfish, an unidentified megalichthyidid, and an unidentified adult rhizodont were found at Red Hill.

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Websites:
Australian Museum's web page on the Coelacanth:
http://www.austmus.gov.au/fishes/fishfacts/fish/coela.htm
Coelacanth: "The Fish Out of Time":
www.dinofish.com/
Jonathan Jeffery's web site on Rhizodontida (Big Dead Fish):
www.donnasaxby.com/rhizodonts/
Palaeos.com's overview on Sarcopterygii:
www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.000.html
Tree of Life's (tolweb.org) web page on Sarcopterygii:
tolweb.org/tree?group=Sarcopterygii
U.C. Museum of Paleontology's web page on Sarcopterygii:
www.ucmp.berkeley.edu/vertebrates/sarco/sarcopterygii.html
Books:
Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
Fenton, C.L. and M.A. Fenton. 1958. The Fossil Book: A Record of Prehistoric Life. Garden City, New York: Doubleday & Company, Inc.
Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Jarvik, E., 1980. Basic structure and evolution of vertebrates. Vol 1. London: Academic Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
Scientific Literature:
Cloutier, R. and P.E. Ahlberg. 1996. "Morphology, characters, and interrelationships of basal sarcopterygians" pp 445-479. In: M.L.J Stiassny, L. R. Parenti, and G.D. Johnson. (eds.) Interrelationships of Fishes. San Diego and London: Academic Press.
Jarvik, E. 1972. "Middle and Upper Devonian Porolepiformes from East Greenland with special reference to Glyptolepis groenlandica n.sp. and a discussion on the structure of the head in the Porolepiformes." Meddeleser om Grønland 187: 1-307.
Image Credits:
All images are copyrighted © 2002, Dennis C. Murphy. (See Terms of Use.) The reconstructions of Holoptychius, Diplurus and Eustenopteron were based on Fenton and Fenton (1958), Jarvik (1972), and Jarvik (1980).

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