On Sarcopterygii — Prosanta Chakrabarty

Introduction

Sarcopterygii, or the lobe-finned fishes, includes the coelacanths, lungfishes, fishes involved in the transition to land, and all tetrapods (mammals, amphibians, and reptiles [the birds, turtles, crocodiles, and squamates]). The lobe-finned fishes are Devonian in age and the sister group to Actinopterygii, or ray-finned fishes. Actinopterygii and Sarcopterygii are nearly of equal size (c. 30,000 spp. each). Actinopterygii is dominated by the teleost fishes, just as Sarcopterygii is dominated by tetrapods. In this essay, the focus will be the non-tetrapod members of Sarcopterygii, as I study fishes; however, it is worth noting many of the skeletal elements and organ systems of tetrapods originated in our aquatic sarcopterygian ancestors. Had actinopterygians been the group to take charge as the vertebrate class to dominate land, terrestrial vertebrates would look very different. It is likely that we would breathe through our mouths alone or through our skin, be much smaller, and be hugging the ground with soft rays holding us up against gravity rather than digits and wrist bones. It was the advent of internal nostrils, or choanae, in aquatic sarcopterygians that permitted us to breathe through our noses;


The rare ray-finned fish that can "walk" on land, the mudskipper. Image from http://www.studentsoftheworld.info/sites/animals/shadows.php

likewise, our forelimb and hindlimb bones all originated with lobe-finned fishes. As it were, the story of the water to land transition is remarkably well known given an excellent series of transitional fossils that fill the steps in the gap between “fishes” and early tetrapods. Some of these intermediate fossils, like the famous Tiktallik rosea, tell us about the evolution of everything from the “neck” to the origin of sturdy ribs and limbs. Luckily for us, there are still extant aquatic lobe-finned fishes. Although they were not directly involved in the terrestrial transition, they can tell us a great deal about how ancient lobed-fins lived. Today only lungfishes and coelacanths survive as the aquatic members of this lineage. These two forms themselves have continued to evolve from our common ancestor, and they each have an amazing array of novelties.

Evolution and Systematics

Coelacanths belong to Actinistia (or Coelacanthimorpha), which has a long fossil record (Mid-Devonian to Late Cretaceous) and that is species rich relative to the two species extant today (83 valid fossil species in nine worldwide families). Members of Actinistia are easily recognized by their tri-lobed diphycercal tail (the vertebral column enters the middle lobe). Known as fossils from both marine and freshwater deposits, they were thought to have gone extinct over 65 million years ago, until a living species was discovered in 1938 to much fanfare. (The discovery of both living species have spectacular stories behind them. See www.dinofish.com)

Lungfishes are members of Dipnoi (themselves part of the larger group Porolepimorpha, largely made up of extinct forms). This clade also evolved in Early Devonian freshwaters, and is represented in the fossil record by more than 100 species in more than 50 genera. Their great fossil record of lungfishes was likely aided by their ability to estivate. These fishes can protect themselves from drought by building a mucous-mud cocoon. They enter periods of estivation that in modern forms can last up to four years; many individuals in the past have expired waiting for that next rain. These individuals and their cocoons make for spectacular, if plaintive, fossils. From fossil forms, we see a trend toward the reduction of bone (in the skull, scales, and fins). Unique plate-like grinding toothplates easily help place extinct and extant forms as each other’s closest relatives.

Tetrapodomorphs are the intermediate forms between the first tetrapods to conquer land and their piscine ancestors. They are all limbed, extinct early Devonian forms, and air-breathers. They include Osteolepimorpha (rhipidistians), Rhizodontiformes, and Elpistostegalians. It is the tetrapodomorphs, in particular the Elpistostegalians (which includes Tiktallik) and not coelacanths or lungfishes, that are the closest relatives to tetrapods.

A modern day coelacanth, Latimeria chalumnae.

Physical Characteristics

Both lungfishes and coelacanths can reach large sizes, approaching 2 m, although lungfishes are much more slender bodied. Both groups have a number of derived features that make each group unique. Coelacanths have a special rostral electroreceptive organ, a vertebral column that is secondarily reduced, no maxilla, and an intercranial joint found in many extinct fish lineages but no other living species. Coelacanths have only external nostrils (no choanae) and a large fat-filled gas bladder (no lung). These two latter features have been used by some authors as evidence that these fishes are ancestral to lungfishes (which have both lungs and choanae), but these primitive features may have more to do with the current ecology of these animals than their biological history.

There are three extant families of lungfishes: Ceratodontidae of Australia, Lepidosirenidae of South America, and Protopteridae of Africa. Lungfishes are easily recognized by their continuous rear fins that connect their dorsal, caudal, and anal fins. The Australian Lungfish (Neoceratodus forsteri) has a number of pleisiomorphic morphological features that resemble fossil forms more so than the other extant lineages. Instead of the tiny worm-like fins of the other species, the Australian form has broad flat fins, large scales, and unpaired lungs (versus small scales and paired lungs in the other taxa). Lungfishes eat both plant and animal material, including ray-finned fishes and invertebrates.

Reproductive Biology

Coelacanths are ovoviviparous; they retain eggs in the body cavity. The young hatch and develop internally. African and South American lungfishes make nests where females lay eggs, and males guard the nests. The Australian species lays its eggs on aquatic plants. The African and South American forms have young with large external gills that often cause them to be mistaken for salamanders.

Conservation

The conservation status of most lungfishes is poorly known, but the Australian lungfish is uncommon, confined to just four rivers in Queensland.

Among coelacanths, Latimeria chalumnae is found off the eastern to southeastern coast of Africa and around the Comoros Islands and Madagascar, and L. menadoensis is only known from Sulawesi, Indonesia. Coelacanths are found at depths beyond the range of most artisanal fishermen (150 to 253m), but accidental capture occurs frequently enough that some estimate that as much as 5% of the adult population is captured annually. Coelacanths aggregate and rest in caves; they may be limited by the number of these sites that are available.

Significance to Humans

As Moyle and Cech state, “probably no single event in the history of ichthyology has received more public attention than the discovery of the coelacanth (Latimeria chalumnae) in 1938.” The discovery of this large, deep sea, limbed, fish-link-to-man made for fantastic headlines. Lungfishes, too, have a spectacular mix of features that make them popular aquarium fishes. Both sarcopterygian fish clades are important to humans for their unique position on the other side of the coin to the vertebrate transition to land.

References

Bemis, W.E, Burggren, W.W., Kemp, N.E. (1987) The biology and evolution of lungfishes. Alan R. Liss, Inc., New York.

Carroll, R.L. 1996. Vertebrate paleontology and evolution. W.H. Freeman. New York.

Helfman, G.S., Collette, B.B, Facey, D.E., Bowen, B.W. 2009. The diversity of fishes, 2^nd ed. Wiley Blackwell, West Sussex, UK.

Moyle, P.B., and Cech Jr., J.J. (2004) Fishes, an introduction to ichthyology, 5^th edition. Prentice Hall, New Jersey.

Musick, J.A., Bruton, M.N., Balon, E.K. (1991) The biology of Latimeria chalumnae and evolution of coelacanths. Environmental Biology of Fishes 32, 1-435.