October 9, 2000
Sarcopterygians and Tetrapod Ancestry
The ancestry of tetrapods (limbed vertebrates including amphibians, reptiles, birds, and mammals) lies within sarcopterygian fish. Three important preadaptations for terrestrial existence passed down to the first tetrapods by fish ancestors were lungs, choanae, and bony fin axes. (Bony fin axes are present in all sarcopterygians, and paired lungs are found in the lungfishes of Africa and South America and would have been found in some extinct groups, such as the osteolepiforms and panderichthyids, examplified by Eusthenopteron and Panderichthys, respectively, on the cladogram of chordate phylogeny.
Carnivorous, mostly freshwater, sarcopterygians of the Paleozoic (Devonian to Permian), the osteolepiforms had bones in their pectoral and pelvic fins homologous with bones seen in tetrapod forelimbs and hindlimbs, respectively. These include a humerus, radius, and ulna in the pectoral fin and a femur, tibia, and fibula in the pelvic fin. Additionally, osteolepiforms (and the closely related panderichtyids) had complex, concentric folds of tooth enamel similar to tooth enamel found in some Paleozoic tetrapods but unknown in other vertebrates.
Osteolepiforms were the first vertebrates to possess choanae (internal nares; a naris is a skeletal nostril opening). Most osteichthyans (including actinopterygians [ray-finned fish] and the coelacanths [lobe-finned sarcopterygians]) possess two pairs of nares (an anterior, incurrent pair and a posterior, excurrent pair). Olfactory (smelling) cells line the short duct between an anterior and posterior naris on either side of the skull. Note that typical fish nares are very small and cannot serve as a respiratory intake because they are not connected to the mouth. Smell and respiration are not combined in living fish the way they are in tetrapods.
In Eusthenopteron (a representative osteolepiform) and the tetrapods, each nasal cavity consists of a junction of three ducts (surrounded by bone) which mutually connect the external naris (homologous with the anterior naris of most fish), orbit (eye socket), and internal naris (choana, which opens into the mouth cavity en route to the trachea). The connection to the orbit, termed the nasolacrimal duct, conducts excessive tear fluid from the eyes into the nasal cavity (hence crying leads to a runny nose). Its presence in Eusthenopteron suggests that orbital fluid allowed the eye to be exposed to the air for extended periods without desiccation. The small size of the external naris in Eusthenopteron suggests that the primary adaptive benefit of the choana was improved smelling capability; most respiration was would still have been conducted through the mouth.
Panderichthys is a Late Devonian sarcopterygian from Latvia. It is even closer to tetrapod ancestry than Eusthenopteron, and this is reflected in several morphological features. Panderichthys, like the earliest tetrapods, possess a long caudal fin (similar to the earliest tetrapods), but lacks dorsal and anal fins. As compared to other sarcopterygians, Panderichthys had fewer, but larger skull bones, in an arrangement very similar to early tetrapods.
Among the most primitive early tetrapods is Acanthostega, from freshwater sediments deposited during the Late Devonian of Greenland (a cold and dry region today, but a warm, moist equatorial setting during the Devonian). As compared to Eusthenopteron and Panderichthys, a major development in Acanthostega is in the modification of bony fins to form a limb with digits (8 fingers were found on a preserved forelimb). Significantly, the pectoral girdle has become detached from the skull (so that crawling would not be such a "mind jarring" experience), and the pelvic girdle is attached to the vertebral column by means a specialized sacral (hip) vertebra for greater support and more efficient locomotion.
However, Acanthostega still posseses many primitive, fish-like characteristics. The vertebrae are delicate, with only weakly developed ribs and largely without well developed zygapophyses (processes that interlock adjacent vertebrae in terrestrial tetrapods). The sacral vertebra is not strongly differentiated from adjacent vertebrae, and the hip girdle is relatively small. It does not seem that Acanthostega could have supported itself on land for any great length of time, and that the limbs were primarily, perhaps exclusively, useful for locomotion in shallow water (where it is more efficient to crawl rather than swim). In this respect, it might have lived in a manner similar to the living giant salamander seen in the video (and which Acanthostega resembled in bodily proportions). The tail supported a caudal fin for propulsion in deeper water. In view of the skull, the external naris of Acanthostega is small, suggesting that, like Eusthenopteron, it was primarily a mouth breather. A preoperculum (one of the opercular series of bones found in osteichthyans) is still present. Bones on the interior of the skull and hip girdle are channeled, suggesting that in life they harbored blood vessels which would have connected to a set of internal gills that were reduced as compared to osteolepiforms, but still functional.
Ichthyostega (also from the Late Devonian of Greenland), retains a fish-like caudal fin, but exhibits morphology that is better adapted for terrestrial walking as compared to Acanthostega. The ribs of Ichthyostega are long, thick, and supportive, but their overlapping nature would have limited the range of lateral motion considerably. The limbs, hip girdle, and vertebrae are more stoutly constructed than are those of Acanthostega, and the vertebrae possess more strongly developed zygapophyses (so they could work together to produce a supporting framework). These traits indicate Ichthyostega spent considerable time out of the water in between fishing forays. The forelimbs appear to be larger than the hindlimbs, and it has been suggested that terrestrial locomotion in Ichthyostega was similar to that seen in modern sea lions (which wiggle back and forth with their powerful fore-flippers). The hindlimb bears 7 toes; Ichthyostega, Acanthostega, and a few other early tetrapods (all Devonian) are the only representatives that possess more than five digits.
It does not appear that any of the first terrestrial vertebrates were specialists for utilizing plants or insects as a potential food source, for they retain the relatively large body size and needle-like teeth of the sarcopterygians, indicating a diet of fish (which are difficult to catch, but can generally be swallowed whole). Nevertheless, coming onto land would have provided Ichthyostega and more advanced tetrapods with three primary advantages over strictly aquatic life. 1) As was the case with plants, land offered a refuge from predators. 2) Basking in the sun provides warm internal temperatures that would produce faster and more efficient digestion of food. 3) Early terrestrial tetrapods could deposit their fish-like eggs into isolated pools of freshwater (lakes and ponds). The developing young would then grow in an environment free from large predators and filled with potential food (larval insects).