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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Symposium: New insights into the functional relationship between anatomy and physiology of extinct and extant vertebrates 2

Room: Salon F   4:30 pm–6:00 pm

Moderator(s): W. R. Porter & G. Tattersall
PHY2-1  4:30 pm  The function of the carotid rete - the unique "wonderful net" of the Cetartiodactyla. Strauss WM*, University of the Witwatersrand (WITS); Hetem RS, University of the Witwatersrand (WITS); Mitchell D, University of the Witwatersrand (WITS); Maloney SK, University of Western Australia; Meyer LCR, University of the Witwatersrand (WITS); Fuller A, University of the Witwatersrand (WITS)
Abstract: Retia mirabilia have been described in all vertebrate groups, and are generally involved in counter-current exchange, be it the exchange of gas (e.g., oxygen in teleost fish and birds), or heat (e.g., tuna, mammals). A well-developed carotid rete is common only among members of the Order Cetartiodactyla - the artiodactyls (even-toed ungulates like sheep, goats, and antelope) and also cetaceans - and members of the Felidae (Order Carnivora). In the carotid rete of artiodactyls, warm arterial blood destined for the brain loses heat to cooler venous blood draining into a cavernous sinus, mainly from the nasal mucosa. The arterial blood leaving the rete en route to the brain is therefore cooler than when it entered the rete, resulting in selective brain cooling, defined as a hypothalamic temperature lower than arterial blood temperature. Using Dorper sheep dosed with Deuterium, we show that selective brain cooling serves a water conservation function. Sheep that used selective brain cooling more frequently, and with greater magnitude, lost less body water than did conspecifics using selective brain cooling less. We show that a 50 kg sheep can save 2.6 L of water per day (ca. 60% of daily water intake) when it employs selective brain cooling for 50% of the day during heat exposure. We then investigate whether selective brain cooling differs between free-living artiodactyls with varying water dependencies. Using generalised linear mixed-effect models we show that carotid arterial blood temperature and brain temperature affect selective brain cooling attributes. Neither heat load, nor species, had an effect on selective brain cooling attributes. Indeed, variability in selective brain cooling was greater within a species than between species. The carotid rete, which may have helped facilitate the evolutionary success of artiodactyls, may play an important part in individual adaptability to a changing climate.

PHY2-2  5:00 pm  Complicated noses keep cool heads: the thermoregulatory effects of nasal passage shape in extant birds and reptiles, with implications for dinosaurs. Bourke JM*, North Carolina Museum of Natural Sciences; Porter WR, Ohio University; Witmer LM, Ohio University
Abstract: Sauropsids (reptiles and birds) evolved a diversity of nasal anatomies, but how this diversity affects nasal airflow and thermal physiology is largely unknown. In general, bird noses are thought to offer superior heat and water savings due to the presence of nasal turbinates. To test this hypothesis, we simulated nasal airflow in various birds, lizards, and crocodylians using computational fluid dynamics (CFD) software. Bird noses are relatively short and compact, disrupted by multiple nasal convolutions (turbinates) that increase surface area within the confined space. Turbinates split the air field into multiple, parallel air channels. In contrast, lizard airways are relatively long due to elongation and convolution of the nasal vestibule. However, with only minor invasions by turbinates, air travels through lizard noses in a single channel, or serially. Crocodylian noses are intermediate, being long like lizards but having more turbinate invasions as in birds. Our CFD analyses found that serial and parallel nasal arrangements provided similar benefits for reducing heat and respiratory evaporative water loss. Air-cooled nasal mucosa reduces the temperature of the underlying blood vessels, and many of these veins pass from the nose to the brain, offering the potential for substantial selective brain cooling. Drawing on our analyses of extant sauropsids, we extended our analysis into the fossil record by simulating airflow through the noses of the dinosaurs Stegoceras, Panoplosaurus, Euoplocephalus, and Majungasaurus. Most reconstructed dinosaur noses provided thermoregulatory results on par with extant animals. Majungasaurus proved to be an outlier with a fairly inefficient nasal passage. We suggest that theropods emphasized their paranasal sinuses for brain cooling, rather than the nasal passage. Enhancing the nose appears to have been necessary for large-bodied dinosaurs, likely as a means of maintaining brain temperatures within safe limits.

PHY2-3  5:30 pm  Macroevolutionary impact of selective brain cooling on artiodactyl diversity patterns throughout the Cenozoic. O'Brien HD*, Ohio University
Abstract: Artiodactyls are known for having the highest diversity of extant large-bodied mammals. They are also known to have a number of unique physiologies, particularly foregut-fermentation-based digestion. This digestive physiology is considered key to artiodactyl success in modern environments. While the link between digestion and herbivore evolution is undisputed, there is an additional feature that may also bolster artiodactyl diversity over Cenozoic environmental shifts: the carotid rete [CR]. The CR is an intracranial arterial meshwork that replaces the internal carotid artery. Sitting in a sinus of turbinate-cooled venous blood, the CR drives brain cooling and delays water-costly responses to heat stress (panting, sweating). Thus, the CR may be selectively advantageous across Cenozoic periods of warming or aridification. Much is known about CR physiology, but its macroevolutionary impact is heretofore unknown. This study uses both soft- and hard-tissue morphological surveys to identify presence/absence of the CR across 10 extant and 16 extinct artiodactyl families. Evolutionary modeling was then used to test whether hypothesized innovations (digestion and the CR) have a demonstrable effect in generating modern diversity. When modern lineages are examined in a phylogenetic comparative framework, results show that diversification is not significantly different per trait, but that extinction probability is lower for artiodactyls with a CR. These physiologies are significantly correlated in living animals (87%), but when fossil data is added, only 8 of 26 families exhibit trait overlap. When evolutionary models of speciation and extinction take extinct groups into account, artiodactyls with a CR diversify earlier, speciate faster, and are more insulated from extinction. The results of both suites of models suggest that brain cooling may be a prerequisite for ruminant digestion, protecting the brain from the higher core temperatures that are needed to support fermentation.

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