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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Symposium: Functional (secondary) adaptation to an aquatic life in vertebrates 1

Room: Salon A   9:30 am–11:00 am

Moderator(s): A. Houssaye & F. Fish
AQU1-1  9:30 am  Primary and secondary adaptations to aquatic feeding in salamanders. Heiss Egon*, Institute of Systematic Zoology and Evolutionary Biology   
Abstract: Salamanders exhibit a primary bimodal lifestyle with an aquatic larva that metamorphoses to a more or less terrestrial postmetamorphic stage. All free swimming larval salamanders bear gills and are primary adapted to aquatic suction feeding where a unidirectional water flow enters the mouth and leaves the pharyngeal area through gill slits—a system strikingly similar to fish-like vertebrates. During metamorphosis, gill slits are closed and the larval "gill system" is transformed into a "tongue system". The tongue is a key innovation that allows food uptake on land. The general mechanics of the tongue and its function in capturing prey is roughly similar amongst all metamorphosed salamanders, though some salamander groups forego metamorphosis and become adult with no or only little changes from the larval somatic condition and might be regarded as primary aquatic forms. Consequently, the feeding apparatus of metamorphosed salamanders can be viewed as primary adaptation to terrestrial feeding but the ability to employ a suction feeding mechanism, slightly modified from the larval condition, was maintained in some salamander groups and is found in many salamandrids and ambystomatids. Other salamanders, such as most plethodontids, have changed their life-history to direct development and as a consequence, have lost their free larval stage and have freed themselves from an obligatory aquatic phase. The specialized tongues in plethodontids are amongst the most efficient tools for terrestrial prey capture in salamanders but prevent suction feeding in water, though some plethodontids show secondary adaptations to aquatic feeding by using their jaws or their tongue to capture prey.

AQU1-2  10:00 am  Feeding modes in Sirenia (Mammalia): more of them than you probably thought! Domning D*, Howard University
Abstract: From their first appearance in the Eocene, sirenians have been feeding on aquatic plants, especially seagrasses. Although this may not seem complicated, they have in fact tried and refined several, surprisingly diverse approaches to this task: 1) selective browsing, with narrow rostra and mandibular symphyses, as seen in prorastomids; 2) progressively less selective grazing, with broader symphyses and loss of incisors, canines, and premolars, as in protosirenids and dugongids; 3) rhizivory, with progressively enlarged, bladelike I1 tusks, as in dugongines (paralleled in a protosirenid and one or two halitheriines); 4) degenerate rhizivory, with loss of enamel, in Dugong dugon; 5) algivory, with total loss of teeth, in Hydrodamalis; 6) grazing on freshwater true grasses, with polydonty and unlimited horizontal tooth replacement, in trichechines; 7) durophagy (possibly on shellfish and/or calcareous algae) in miosirenines. Mode 1 was the primitive condition; modes 2 and 3 comprise the majority of known fossil species. Mode 3 characterized the striking Oligocene-Pliocene adaptive radiation of dugongines, with several lineages independently achieving the most extreme morphology; this appears to have been a risky specialization that consistently ended in extinction. The three genera that survived into Recent times each displayed divergent and unique specializations: modes 4 in Dugong, 5 in Hydrodamalis, and 6 in Trichechus. Finally, the Miosireninae, a short-lived clade in northwestern Europe, evolved massively reinforced palates, evidently for crushing some sort of hard food. Mode 2, the most conservative adaptation, was the safest over the long haul, characterizing ecologically-generalist seagrass eaters from Eocene to Pliocene—yet it was not represented in the Quaternary fauna. Apparently, the radical environmental changes of the last 3 million years exceeded the tolerances of that adaptation, which was optimized for the most stable sirenian niches of the Tertiary.

AQU1-3  10:30 am  Evolutionary innovation and ecology in mysticete cetaceans: transition from teeth to baleen and raptorial to bulk filter feeding. Berta A, San Diego State University; Lanzetti A*, San Diego State University; Ekdale EG, San Diego State University; Deméré TA, San Diego Natural History Museum   
Abstract: The origin of baleen and filter feeding in mysticete cetaceans occurred approximately 28-24 million years ago and represents a major macroevolutionary transition in cetacean morphology (teeth to baleen) and ecology (raptorial to filter feeding). We explore this dramatic change in feeding strategy by employing a diversity of tools and approaches: morphology, molecules, and isotopes. Adaptations for raptorial feeding in extinct toothed mysticetes provide the phylogenetic context for evaluating morphological apomorphies preserved in the skeletons of stem and crown edentulous mysticetes. In this light, the presence of novel vascular structures on the palates of Oligocene toothed mysticetes is interpreted as the earliest evidence of baleen and points to an intermediate condition between an ancestral condition with teeth only and a derived condition with baleen only. Supporting this step-wise evolutionary transition, isotopic evidence shows how changes in dental chemistry in early toothed mysticetes tracked the modification in diet and environment. Recent discoveries also demonstrate how this transition was made possible by radical changes in cranial ontogeny. In addition, genetic mutations and the possession of dental pseudogenes in extant baleen whales support the fact of a toothed ancestry for mysticetes. Based on genetic and morphologic data, we provide a hypothesis that reconstructs the dramatic shifts that take place in extant baleen whales before birth, in skull development, resorption of a fetal dentition and growth of baleen. Comparisons are also made with filter feeding Mesozoic bony fishes and marine reptiles to define common themes in this striking example of convergent evolution. The mechanisms involved in this complex evolutionary transition that entails multiple, integrated aspects of anatomy and ecology are only beginning to be understood, and future work will further clarify the processes and the development underlying this macroevolutionary pattern.

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