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Session Schedule & Abstracts

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Friday 1st July, 2016

Paleontology 4

Room: Salon F   11:30 am–1:00 pm

Moderator(s): Marcot JD, McCurry MR
PAL4-1  11:30 am  Extreme longirostry in Miocene odontocetes: the ecomorphology and biomechanics underlying the repeated evolution of a superlative snout. McCurry M.R.*, National Museum of Natural History, Smithsonian Institution; Pyenson N.D., National Museum of Natural History, Smithsonian Institution
Abstract: Different odontocete lineages evolved cranial elongation during the Miocene, resulting in skulls that are proportionally more extreme than any living or extinct aquatic tetrapod. In some taxa, the rostrum comprises as much as 80% of total skull length; others have as many as 350 tooth sockets in the cranium and mandible combined. In this study, we CT scanned a range of long-snouted fossil odontocetes from the middle and late Miocene, including Parapontoporia sternbergi, Xiphiacetus bossi, Pomatodelphis inaequalis, Zarhinocetus errabundus and Zarhachis flagellator, to characterise and quantify their extreme morphology, which provides a basis for inferring performance and behaviour. We used beam theory to predict the biomechanical limitations of this morphological specialisation compared to extant pelagic longirostral taxa, including odontocetes and billfishes. Predicted stress in the rostrum of some fossil taxa is higher than those found in extant odontocetes, indicating a more limited dietary niche. Second moment of area of rostral cross sections from the CT data also show that there is substantial variation in the shape of the rostrum between fossil species. Dorsally flattened taxa such as Zarhachis flagellator and Pomatodelphis inaequalis likely fed using lateral rostral sweeps. In contrast, Xiphiacetus bossi appears to be convergent on the cranial morphology of swordfish, using an elongate rostrum to sweep in a wide range of directions. The repeated and phylogenetically disparate origin of longirostry among odontocetes raises questions about how it evolved, and why such extreme longirostry went extinct. Given our results, we suggest that these traits likely originated in association with prey capture, potentially fast-start swimming fish. For several coeval middle Miocene taxa, eustatic sea level maxima and high ocean temperatures might have provided ideal environments the evolution of this cranial morphology in odontocetes.

PAL4-2  11:45 am  Morphological consequences of tooth loss: A comparison of the course of the mandibular canal in mysticete cetaceans using 3D models. Peredo C. M.*, George Mason University; Pyenson N. D., Smithsonian National Museum of Natural History; Uhen M. D., George Mason University
Abstract: Mysticete cetaceans (baleen whales) are a diverse and successful clade of mammals that have evolved to include the largest animals in the history of the earth. The success of this clade has been, in large part, to a shift in their feeding mechanism. Over their evolutionary history mysticete cetaceans lost functional teeth and instead evolved baleen plates. These keratinous structures hang down from the palate and allow mysticetes to bulk filter feed on small to medium sized prey. While baleen plates replace the dentition of the palate, no secondary structure is developed in the mandible whatsoever. Though tooth buds are known to form in embryonic mysticetes, they are completely resorbed prior to birth. The result is an entirely edentulous mandible in mysticete cetaceans. Despite the lack of teeth, a shallow alveolar groove persists on the dorsomedial surface of the mandible. The internal anatomy of the mandible, specifically the course of the mandibular canal and any connections made with the alveolar groove, have never been studied and described. Here, we use computed tomography scans to create 3D models of the internal anatomy of the mandibles of a typical artiodactyl (Sus scrofa), an archaeocete cetacean (Zygorhiza kochii), and an extant mysticete (Balaenoptera acutorostrata). In doing so, we compare the internal anatomy of the mandible by charting the course of the mandibular canal and all of its distributaries. Our results confirm a highly unique morphology in the mandibles of mysticete cetaceans compared to their artiodactyl relatives and archaeocete ancestors. Our results verify the persistence of dorsomedial branches of the mandibular canal to feed the alveolar groove.

PAL4-3  12:00 pm  The diversity and evolution of supraorbital crests in Platanistoidea (Cetacea: Odontoceti), and their implications for echolocation. Boersma A. T.*, Smithsonian National Museum of Natural History; Pyenson N. D. , Smithsonian National Museum of Natural History
Abstract: Cetacean skulls are dramatically different in construction from their closest mammalian relatives. Many skull elements are posteriorly "telescoped" with overlapping of facial bones that were once adjacent in ancestral configurations. In odontocetes, telescoping has layered the maxilla over the frontals in the antorbital notch region. This coincides in some groups with supraorbital crests surrounding the melon, an organ involved in sound generation. The most elaborate crests occur in the Ganges and Indus River dolphin (Platanista spp.), where they rise above the level of the nuchal crest as heavily pneumatized, extremely thin wings. Their position wrapping around the melon suggests that they may be involved in the generation or propagation of sound in the head (though this hypothesis is difficult to test). Supraorbital crests also appear in other lineages (e.g., Ziphiidae, Inioidea) but are composed of different skull elements and are less extreme than in Platanista. To better understand the origin of the crests in this lineage, we examined the supraorbital crests in a group phylogenetically allied with Platanista. Termed Platanistoidea, this group of cetaceans spans from the Oligocene to the present, including almost exclusively marine forms. We measured the physical extent of their crests and categorized the crests’ position relative to key skull elements. In addition, we used computed tomography to examine the internal morphology and relative bone density of the crests. Preliminary results reveal that marine fossil platanistoids, such as Pomatodelphis and Zarhachis, have elevated, robust crests that do not exhibit pneumatization as Platanista does. Oligocene platanistoids, older and with smaller crests, also show no patent pneumatization. We propose that the elaboration and pneumatization of the crests in Platanista is a relatively recent innovation, plausibly linked with the reinvasion of freshwater river systems.

PAL4-4  12:15 pm  Digital reduction patterns in terrestrial artiodactyls: how many mechanisms? Theodor JM*, University of Calgary
Abstract: Artiodactyls show strong reduction of lateral digits around the paraxonic axis to varying degrees. This pattern had been used early on as a phylogenetic character, with the general assumption that digit reduction and loss should be irreversible, but more recent phylogenetic work and additional fossil material has shown this to be improbable. Digit reduction seems to have followed a complex evolutionary pathway among lineages. The first digits of both fore and hindfeet have been lost at least three times, while reduction of the side toes shows a minimum of four disparate morphologies: digits 2 and 5 reduced in diameter to less than half of the median digits, but similar in length to digits 3-4; digits 2 and 5 reduced in length, but similar in diameter to digits 3-4; digits 2 and 5 reduced to metapodial splints or nodules, lacking phalanges; and, rarely, asymmetrical reduction of digit 5 to a nodule with a full-sized digit 2. Developmental work on digit reduction in pigs, cows and camelids shows that more than one developmental trajectory can produce reduction of lateral toes (Cooper, 2014; Lopez-Rios et al. 2014). In both pigs and cows, derived changes in early patterning established by Shh have resulted in the reduction of lateral toes without extensive apoptosis in the lateral limb bud. In camelids, the developing limb bud shows the ancestral patterning of Shh, with extensive apoptotic activity in the lateral digits. Because these living clades of terrestrial artiodactyls show different mechanisms of digital reduction, this suggests a basis for convergence in digit reduction. The complex patterns of digit reduction found among extinct taxa highlights the possibility that additional potential developmental mechanisms for digit reduction may have existed.

PAL4-5  12:30 pm  Beam mechanics of digit reduction in fossil horses. McHorse B.K.*, Harvard University; Pierce S.E., Harvard University; Biewener A.A., Harvard University
Abstract: The evolutionary history of equids is a classic story—initially told as a straightforward, orthogenetic progression from small to large, many- to single-toed, low-crowned teeth to hypsodont teeth, and browsing to grazing. While the relationship between hypsodonty and the spread of grasslands has since been shown to be less than linear, few studies have quantitatively approached related hypotheses about digit reduction. Proposed drivers for the transition to a single toe include locomotor economy, speed and straight-line locomotion, and increased body mass. To investigate this last hypothesis, we modeled the beam mechanics of metapodials through the evolutionary history of horses using micro-CT scans. Taxa include Hyracotherium (tetra- and tridactyl), Equus (monodactyl), and six other fossil equids at varying stages of digit reduction. We analyzed stresses in compression and bending under simulated body-weight loads, both in forward locomotion and lateral dodging. Without accounting for the lateral toes offloading some force, stresses at midshaft in tridactyl horses surpass the tensile strength of bone, with the exception of later horses with more reduced digits, such as Neohipparion. If the force is scaled according to “toedness index,” a measure of the relative proportions of the digits, all equid metapodials experience similar low levels of stress when loaded. Finally, we make forelimb-hindlimb comparisons, with the hypothesis that if body weight is a mechanical driver of digit reduction, evolutionary shifts should appear first in the forelimb because it carries more weight. We conclude that selection for reduced distal limb mass, combined with the mechanical demands of increasing body mass, is a plausible driver for digit reduction in equids. Future work incorporating gait data from extant tapir (a perissodactyl with the same digit configuration as Hyracotherium) and wild equids will provide more nuanced force and digit loading data.

PAL4-6  12:45 pm  Limb evolution of North American Equidae. Marcot J. D.*, University of Illinois; Maier J. A., University of Illinois; Kozak K., University of Illinois
Abstract: The pattern of environmental change throughout the Cenozoic is characterized by a trend toward increasing ecological dominance of open habitats (e.g., grasslands) at the expense of closed habitats (e.g., forests). Numerous lines of evidence constrain the spatial and temporal pattern of this transition. Traditionally, the radiation of hypsodont horses in North America has been assumed to signal the spread of open, grass-dominated environments. However, grass phytoliths show a rise in the numerical abundance of open-habitat grasses at least 4My earlier. A possible reason for this discrepancy is that dental evolution was slow to respond to the spread of grass-dominated environments. Limb evolution might show a more immediate response to the spread of open environments. Many studies of extant taxa demonstrate a link between morphology of the limb skeleton and habitat, which are corroborated by biomechanical studies of locomotion. Studies of fossil taxa have leveraged these relationships to establish patterns of limb evolution within clades of mammals in the context of Cenozoic environmental changes. In this study, we analyze the evolution of limb evolution in the family Equidae. We use both linear measurements and geometric morphometrics to quantify the morphology of six limb elements. We use a dated estimate of North American equid phylogeny to estimate rates between 55 and 5 Ma, spanning most of horse evolution in North America. Our results show a generally low rate of evolution between 35 and 25Ma, followed by a rapid increase in evolutionary rate that persists across the Oligocene/Miocene boundary. This rapid increase is coincident with estimates of grassland expansion based on phytoliths, and precedes the radiation of hypsodont horses. These results also corroborate previous studies of ungulate limb evolution and suggest that limb evolution might be more sensitive to environmental change than dental evolution.

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