RECENT DISCOVERIES of primitive tetrapod fossils continue to shed light on the transition from sea to land (for a discussion of some recent developments see our discussion of Gogonasus). The discovery last year of the tetrapod-like fish Tiktaalik provided additional information on the development of bony, articulated forelimbs and changes in the structure of the mouth, including hints at the evolution of the first neck. In later tetrapods the culmination of these changes can be seen.

Certainly the evolution of legs was an important innovation, but perhaps more complicated were the changes that had to occur in the structure of the mouth in order to allow success on land. Once again we see the domino effect of evolutionary change. The transition from the use of pharyngeal and opercular pumping of water through gills to buccal and costal pumping of air through primitive lungs required the loss of the opercula and supporting structures. But this creates another difficulty for the transitional tetrapodomorphs. The fish that preceded the tetrapods captured prey by a suction mechanism, sealing their opercula closed and opening the mouth to draw prey in on a flood of water. Obviously any tetrapod attempting to use this method of feeding on land would be sorely disappointed. So how did biting evolve as a means of prey capture?

The changes observed in Tiktaalik hint at the possibilities. The loss of the opercula made suction feeding less effective, while the increase in mobility in the neck that resulted allowed a side-to-side motion before impossible. In concept this could allow darting of the head (now equipped with enlarged cervical muscles) to capture prey animals on land or in shallow water, while in deeper water suction feeding would still be practical. But what direct evidence do we have that the later aquatic tetrapods adopted this method of feeding?

Markey and Marshall of Harvard University believe they have found this evidence. They begin by discussing the changes observed in the fish-tetrapod transition:

Stepwise morphological changes in the lower jaw, dentition, degree of ossification of the operculum, and relative size of the gill chamber in taxa that span the fish–tetrapod transition provide clues as to whether early tetrapods, including Acanthostega, captured prey using suction or biting (15). Specifically, the fishes Eusthenopteron, Panderichthys, and Tiktaalik, and the early tetrapod Ventastega, all possess large coronoid fangs, whereas these teeth are absent in the more derived Acanthostega. In addition, Eusthenopteron and Panderichthys both exhibit an ossified operculum, whereas the bony gill cover is lost in Tiktaalik, Ventastega, and Acanthostega. Finally, the glenoid fossa of the articular faces posteriordorsally in the fish taxa discussed here (Eusthenopteron, Panderichthys, and Tiktaalik) whereas, in the tetrapods Ventastega and Acanthostega, this fossa points dorsally, indicating that the lower jaw changed the nature of its articulation to the skull across the fish–tetrapod transition. (See ref. 15 for a discussion of these changes in all taxa save Tiktaalik; for Tiktaalik, see refs. 5 and 6.) These changes, along with the reduction of the gill chamber, are hypothesized to indicate a reduced reliance on suction feeding in early tetrapods compared with osteolepiform fishes (15).

To determine how these changes affected feeding method they analyzed the suture morphology of the skulls of three extinct species, the fish Eusthenopteron, the aquatic tetrapod Acanthostega, and the Permian terrestrial tetrapod Phonerpeton, and compared them to the skull of the modern fish Polypterus. They measured the physical load placed upon the sutures of this fish’s skull while capturing and eating prey and in regular swimming and opercular pumping. They then compared the suture loading of this fish with the extinct species. Eusthenopteron and Polypterus demonstrate the same loading pattern (“anterior tension, posterior compression”), indicating that both fish were suction feeders. However, with the two tetrapods the results were quite different, showing widespread compression. The authors conclude that the terrestrial and aquatic tetrapods had a similar feeding mechanism, which would necessarily be biting prey.

This indicates that capture of prey by biting evolved early in the tetrapod lineage among aquatic tetrapods. Aquatic tetrapods such as Acanthostega could have captured prey in the water by biting or caught prey on the water’s edge, lurking on the shoreline. This initial study was unfortunately limited by the samples that were well preserved morphologically and easily obtained, and the authors hope to study additional tetrapods and tetrapodomorphs to clarify the role of other skull and jaw changes in this transition.

“Terrestrial-style feeding in a very early aquatic tetrapod is supported by evidence from experimental analysis of suture morphology.” Markey, M. J.; Marshall, C. R. Proceedings of the National Academy of Sciences, USA 2007, 104, 7134.