FEEDING SHIFTS ACROSS THE FISH-AMPHIBIAN TRANSITION ARE REVEALED BY CHANGES IN CRANIAL SUTURAL MORPHOLOGY

Several features of the skull – including tooth morphology and the degree of skull flattening – have been used to make inferences about the mode of feeding in early sarcopterygian fish and amphibians. However, taxa that span the fish-amphibian transition, which probably lived in different environments and captured prey in different ways, exhibit relatively minor differences in these features. New lines of evidence for inferring feeding in fossil fish and amphibians may help resolve this problem. It has been suggested that changes in cranial suture morphology across the fish-amphibian transition may capture changes in feeding associated with the transition. To explore this question, we: (1) Quantified suture morphologies in taxa that span the fish-amphibian transition and the invasion of land by tetrapods: i.e., in the osteolepiform fish Eusthenopteron, the early amphibian Acanthostega, and the Permian terrestrial amphibian Phonerpeton; (2) Inferred the strain types these fossil sutures experienced using correlations between suture deformation and morphology previously determined experimentally in the extant fish Polypterus; and, (3) Connected inferred skull deformation patterns with aquatic (suction) feeding or terrestrial (biting) feeding in these fossil taxa. In Polypterus, suction feeding loads the interfrontal (IF) suture in tension and compresses the interparietal (IP) suture. The IF and IP sutures of Eusthenopteron, also a suction feeder, are morphologically similar to those of Polypterus. Therefore, we hypothesize that the pattern of sutures reflecting “tension anteriorly, compression posteriorly” strains in the skull indicates suction feeding. In contrast, the IF and IP suture morphologies of Phonerpeton indicate of compression only, suggesting that widespread compression in the skull roof is correlated with terrestrial (biting) feeding. Interestingly, the sutures of Acanthostega are not consistent with prey capture solely via suction, but suggest that Acanthostega may have bitten directly on prey items in the water or near the water's edge. Analysis of suture morphologies in other transitional forms, such as Tiktaalik and Ichthyostega, in combination with further experiments on living taxa, will aid in interpreting these differences across the fish-amphibian transition.