Unidentified Lungfish (Dipnoi)

Lungfish toothplate (15mm longl). Photo courtesy of Ted Daeschler, ANS.

A lungfish toothplate has been recovered from Red Hill. It hasn't been identified —and will probably remain so until more complete material is found— but it clearly differs from those of another lungfish, Soederberghia sp., from another central Pennsylvanian locality that belongs to the same geologic formation (Catskill Formation).

Red Hill has also yielded several trace fossils that have been attributed by some investigators (e.g., Woodrow et. al., 1995) to be lungfish burrows. These trace fossils are similar to the burrows made by aestivating modern lungfishes from Africa and South America. The oldest undisputed reports of aestivation burrows comes from the Early Permian; some of these burrows actually contained the remains of the lungfish Gnathorhiza. Suspected, but empty, burrows have also been reported from several Carboniferous localities and Late Devonian burrows have been reported from elsewhere in the Catskill Formation (e.g., Driese and Mora, 2000), but their attribution to lungfishes is not universally accepted.

The earliest lungfishes are known from the Early Devonian. They quickly became diverse and worldwide in distribution. In fact, family-level diversity of lungfishes during the Devonian was second only to that of the placoderms. Diversity decreased considerable following the Devonian and only three genera belonging to two families survive today. The Lepidoseridae (Permian-present) are represented by several African species of Proptopterus and one South American species of Lepidosiren. Neoceratodus fosteri, which lives in Australia, is the sole representative of the Ceratodontidae (Triassic-present). It’s known from Early Cretaceous fossils dating back about 100 million years, making it the oldest known vertebrate species.

Lungfishes are highly specialized lobe-fins. They exhibit a set of features that helped produce a powerful bite. These include the loss of the intercranial joint, a heavily ossified braincase, the fusion of the palate to the braincase, strengthened gill arches, a reinforced symphysis (where the lower jaws meet), and specialized tooth plates adapted to either crush or shred food. Ossification of the skull and axial skeleton tended to diminish in the later forms that first appeared in the Late Carboniferous and Early Permian. Later forms also have a consolidation of the two dorsal fins, tail fin and single anal fin into one continuous fin.

One notable lungfish feature, lungs, has led to considerable speculation and debate on their relationship to tetrapods. Although largely discredited today, several 20th Century authorities championed the idea that tetrapods descended from lungfish-like ancestors. One problem in resolving the dispute is that lungs don’t readily fossilize. Instead, one has to look for other, more readily fossilized, structures. Unfortunately, the most widely accepted such structure, the choana (external nostrils that open into the mouth), are found in both lungfishes and osteolepiform lobe-fins (currently considered the most credible ancestral group to tetrapods). One resolution to this problem came with the discovery that the early and presumably ancestral dipnomorph Diabolepis had the primitive type of external nostrils that did not open into the mouth. This strongly suggests that choanae evolved independently in both groups.

Most Devonian forms are marine fishes with large gill chambers covered by a large, round opercular bone (e.g., Dipterus, Griphognathus, Scamenacia, and Uranolophus). The majority of these were probably bottom-dwelling fishes that depended exclusively on gill respiration. The evolution of air-breathing by lungfishes has traditionally been associated with their entry into freshwaters. Some Late Devonian genera (e.g., Barwickia and Howidipterus in Australia and Soederberghia in Euamerica) have been recovered from freshwater environments, as have several Carboniferous and Permian genera (e.g., Megapleuron). However, many of these have also been recovered from estuarine and shallow marine localities. Even the Early Permian genus Gnathorhiza, which presumably engaged in air-breathing while aestivating in its burrows, has been found in nearshore and estuarine deposits.

Modern lungfishes have an orobranchial pump that enables them to efficiently gulp air. The pump is associated with the presence of cranial ribs and certain features of the skull (i.e., a long stalk on the parasphenoid bone and the shape of the ceratohyal bones). These same features have been used as morphological evidence for air-gulping in some Late Devonian and all post-Devonian genera. Nonetheless, many late Paleozoic forms (e.g., Gnathorhiza) still retained large gill chambers and probably depended significantly on gill-breating.

In any case, the lungs in lungfishes and tetrapods evolved independently from the swim-bladders that were a primitive feature of all bony fishes (ray-fin and lobe-fin fishes). A number of modern ray-fin fishes living in low-oxygen environments will swallow air and obtain oxygen from their swim-bladders. It’s probable that some lungfishes and other lobe-fins also swallowed air in oxygen-poor habitats such as anoxic freshwaters, estuaries and nearshore marine habitats. Indeed, this ability may have been crucial to their survival during the widespread marine anoxia prevalent in the Late Devonian.

Other lobefins, including Holoptychius sp., a juvenile rhizodont (c.f., Sauripterus), Red Hill rhizodont, Red Hill megalichthyidid, and Hyneria lindae were also found at Red Hill. You can also learn more about lobe-fin fishes.

Top of Page .

Web:: Australia Museum Online's web page on Neocerodus:
www.amonline.net.au/fishes/fishfacts/fish/nforsteri.htm; J.W. Foltz's web page on lungfishes:
people.clemson.edu/~jwfoltz/WFB300/subjects/Dipnoi/dipnoi.htm; Palaeos.com's information on Dipnoi:
www.palaeos.com/Vertebrates/Units/140Sarcopterygii/
140.500.html#Dipnoi; A Reconstruction of Scaumenacia:
cuirsfins.com/francois/fran/scaumenacia.html; U.C. Museum Paleontology web page on lungfishes:
www.ucmp.berkeley.edu/vertebrates/sarco/dipnoi.html
Books:: Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.; Janvier, P. 1996. Early Vertebrates. Claredon Press. Oxford.; 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 Papers:: Campbell, K.S.W. and R.E. Barwick, 1988. "Geological and paleontological information and phylogenetic hypotheses." Geological Magazine 125: 207-227.; Woodrow, D.L., R.A.J. Robinson, Anthony R. Prave, A. Traverse, E.B. Daeschler, N.D. Rowe and N.A. DeLaney. 1995. "Stratigraphic, sedimentologic, and temporal framework of Red Hill (Upper Devonian Catskill Formation) near Hyner, Clinton County, Pennsylvania: site of the oldest amphibian known from North America." Field Trip Guide. 60th Annual Field Conference of Pennsylvanian Geologist: 1-8.
Image Credits:: The reconstruction of Eustenopteron is copyrighted © 2002, Dennis C. Murphy. (See Terms of Use.) It's based on Campbell and Barwick (1988).

Top of Page .