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See detailSupplementary Information for "Global ecomorphological restructuring of dominant marine reptiles prior to the K/Pg mass extinction"
Maclaren, James ULiege; Bennion, Rebecca ULiege; Bardet, Nathalie ULiege et al

in Maclaren, James; Bennion, Rebecca; Bardet, Nathalie (Eds.) et al Global ecomorphological restructuring of dominant marine reptiles prior to the K/Pg mass extinction (2022)

Mosasaurid squamates were the dominant amniote predators in marine ecosystems during most of the Late Cretaceous. Evidence from multiple sites worldwide of a global mosasaurid community restructuring ... [more ▼]

Mosasaurid squamates were the dominant amniote predators in marine ecosystems during most of the Late Cretaceous. Evidence from multiple sites worldwide of a global mosasaurid community restructuring across the Campanian–Maastrichtian transition may have wide-ranging implications for the evolution of diversity of these top oceanic predators. In this study, we use a suite of biomechanical traits and functionally descriptive ratios to investigate how the morphofunctional disparity of mosasaurids evolved through time and space prior to the Cretaceous-Palaeogene (K/Pg) mass extinction. Our results suggest that the worldwide taxonomic turnover in mosasaurid community composition from Campanian to Maastrichtian is reflected by a notable increase in morphofunctional disparity on a global scale, but especially driven the North American record. Ecomorphospace occupation becomes more polarised during the late Maastrichtian, as the morphofunctional disparity of mosasaurids plateaus in the Southern Hemisphere and decreases in the Northern Hemisphere. We show that these changes are not associated with strong modifications in mosasaurid size, but rather with the functional capacities of their skulls, and that mosasaurid morphofunctional disparity was in decline in several provincial communities before the K-Pg mass extinction. Our study highlights region-specific patterns of disparity evolution, and the importance of assessing vertebrate extinctions both globally and regionally. Ecomorphological differentiation in mosasaurid communities, coupled with declines in other formerly abundant marine reptile groups, indicates widespread restructuring of higher trophic levels in marine food webs was well underway when the K-Pg mass extinction took place. [less ▲]

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See detailMarine tetrapod feeding guilds using automated high-density 3D geometric morphometrics
Fischer, Valentin ULiege; Maclaren, James ULiege; Bennion, Rebecca ULiege et al

Conference (2021, December)

Defining feeding guilds based on tooth morphology is an entire subfield in marine tetrapod science. However, these assessments have mostly been qualitative, relying on gross tooth shape, rarely preserved ... [more ▼]

Defining feeding guilds based on tooth morphology is an entire subfield in marine tetrapod science. However, these assessments have mostly been qualitative, relying on gross tooth shape, rarely preserved gut content, and killing behaviour. Moreover, some of the data at the foundation of these guilds have proven to be debatable and there is an ever clearer need for a testable, quantitative framework to assess feeding guilds. We develop a novel protocol that incorporates the pseudo-landmarking technique into high-density geometric morphometrics procedures, sampling 3D surface models of tooth crowns automatically and densely (e.g. 2000 surface landmarks) after placing just 5 fixed landmarks on each tooth. A crushing-to-piercing transition is evident along the first axis of the PCA-based morphospace, while the presence and shape of carinae, as well as crown curvature, is captured by the second axis. This allows an efficient visualisation of tooth shapes with just two axes. Peculiar structures such as strong crown curvature or carinae are mostly recorded on medium-sized teeth, suggesting that a scaling factor is at play. We attempt a new definition of marine reptile feeding guilds based on tooth morphology and size, using extant polarizers. [less ▲]

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See detailUncovering the biomechanical disparity of Cretaceous marine reptile feeding via finite element analysis
Fallon Gaudichon, Valentin; Boman, Romain ULiege; Chatar, Narimane ULiege et al

Conference (2021, September 07)

During the Late Cretaceous, two marine reptile groups (mosasaurs and polycotylids plesiosaurs) regulated oceanic trophic chains, diversifying in the wake of the extinction of both ichthyosaurs and ... [more ▼]

During the Late Cretaceous, two marine reptile groups (mosasaurs and polycotylids plesiosaurs) regulated oceanic trophic chains, diversifying in the wake of the extinction of both ichthyosaurs and pliosaurids. Whereas a series of studies have analysed tooth morphology and guild structuration among marine reptiles, biomechanical performances have rarely been assessed. Such biomechanical analyses have the potential reveal the functional disparity of the main tetrapod predators of the Late Cretaceous. This study represents the first attempt to compare biomechanical performance of multiple marine reptile clades using Finite Element Analyses. We investigated biting mechanics of mandibles through two gape angles and two different biting points. Results from mechanical efficiency, Von Mises stress along the jaw, and internal energy reveal niche partitioning among sympatric marine reptile species. One ecomorphotype is occupied by polycotylids, which have a gracile snout, narrow rostrum, and low mechanical efficiency and high internal energy. Mosasaurs occupy at least two distinct ecomorphotypes, yet with more robust mandibles, higher mechanical efficiency, and lower internal energy values. These quantitative results confirm qualitative assessments of Late Cretaceous marine reptile palaeoecology. [less ▲]

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See detailLeaving the triangle: marine tetrapod tooth guilds using automated high-density 3d geometric morphometrics
Fischer, Valentin ULiege; Maclaren, James ULiege; Bennion, Rebecca ULiege et al

Conference (2021, April 20)

Defining feeding guilds based on tooth morphology is an entire subfield in marine tetrapod science, which essentially started with a seminal paper by Judy Massare in 1987. However, these assessments have ... [more ▼]

Defining feeding guilds based on tooth morphology is an entire subfield in marine tetrapod science, which essentially started with a seminal paper by Judy Massare in 1987. However, these assessments have mostly been qualitative, subjectively placing teeth on a triangle using a series of criteria that draw both from tooth shape, rarely preserved gut content, and killing/hunting behaviour. Moreover, some of the data at the foundation of these guilds have proven to be debatable and there is an ever clearer need for a testable, quantitative framework to assess feeding guilds. We develop a novel protocol that incorporates the pseudo-landmarking technique into high-density geometric morphometrics procedures, sampling 3D surface models of tooth crowns automatically and densely (e.g. 2000 surface landmarks) after placing just 5 fixed landmarks on each tooth. This very dense sampling of shape permits full characterisation of crown shape, and we provide the first ever quantification of dental morphospace occupation among a sample of mosasaurs, ichthyosaurs, plesiosaurs, archaeocetes, and odontocetes with conical (i.e., non-multicuspid) teeth. A crushing-to-piercing transition is evident along the first axis of the PCA-based morphospace, while the presence and shape of carinae, as well as crown curvature, is captured by the second axis. This allows an efficient visualisation of tooth shape differences and definition of regions/guilds with just two axes that explain ca. 90% of the total variance. Because crown shape has a direct functional signal, our results provide important data to better understand how marine tetrapods evolved and functioned. Despite their gigantic size, the teeth of the largest marine macropredator ever, the physeteroid Livyatan melvillei, have crowns with an unremarkable shape, occupying a position close to the center of the morphospace. In fact, peculiar structures such as crown curvature or carinae are mostly recorded on medium-sized teeth, suggesting that a scaling factor is at play. Similarly, crown shapes that have rarely or never been evolved, either by certain groups or during certain time periods, also carries information. For example, the perfectly straight teeth of the Early Jurassic ichthyosaurian Temnodontosaurus platyodon with protruding flange-like carinae are unique and indicate neoichthyosaurians developed a unique pathway towards hypercarnivory, albeit only once. Further work will incorporate as many different taxa and tooth-shapes into the morphospace as possible to usher in a new, quantitative paradigm for understanding marine tetrapod feeding ecology. [less ▲]

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See detailBiogeography a key influence on distal forelimb variation in horses through the Cenozoic
Maclaren, James ULiege

in Proceedings of the Royal Society. Biological Sciences (2021)

Locomotion in terrestrial tetrapods is reliant on interactions between distal limb bones (e.g. metapodials and phalanges). The metapodial-phalangeal joint in horse (Equidae) limbs is highly specialized ... [more ▼]

Locomotion in terrestrial tetrapods is reliant on interactions between distal limb bones (e.g. metapodials and phalanges). The metapodial-phalangeal joint in horse (Equidae) limbs is highly specialized, facilitating vital functions (shock absorption; elastic recoil). While joint shape has changed throughout horse evolution, potential drivers of these modifications have not been quantitatively assessed. Here, I examine the morphology of the forelimb metacarpophalangeal (MCP) joint of horses and their extinct kin (palaeotheres) using geometric morphometrics and disparity analyses, within a phylogenetic context. I also develop a novel alignment protocol that explores the magnitude of shape change through time, correlated against body mass and diet. MCP shape was poorly correlated with mass or diet proxies, although significant temporal correlations were detected at 0-1 Myr intervals. A clear division was recovered between New and Old World hipparionin MCP morphologies. Significant changes in MCP disparity and high rates of shape divergence were observed during the Great American Biotic Interchange, with the MCP joint becoming broad and robust in two separate monodactyl lineages, possibly exhibiting novel locomotor behaviour. This large-scale study of MCP joint shape demonstrates the apparent capacity for horses to rapidly change their distal limb morphology to overcome discrete locomotor challenges in new habitats [less ▲]

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See detailA European mosasaurine (Squamata: Mosasauridae) with affinities to a clade from new zealand
Street, Hallie; Caldwell, Michael; Maclaren, James ULiege et al

Conference (2021)

The Royal Belgian Institute of Natural Sciences houses an extensive collection of Maastrichtian-aged mosasaur fossils. Among the numerous specimens of these marine reptiles is a skull and associated ... [more ▼]

The Royal Belgian Institute of Natural Sciences houses an extensive collection of Maastrichtian-aged mosasaur fossils. Among the numerous specimens of these marine reptiles is a skull and associated vertebral column, IRSNB 3211. This specimen was originally assigned to Plioplatecarpus houzeaui, but more recent studies have suggested that it represents an immature individual of Mosasaurus lemonnieri. New three-dimensional surface scans of IRSNB 3211 contribute to a detailed description of the specimen. Several characters strongly disagree with the diagnosis for Plioplatecarpus houzeaui, including the height of the coronoid process and the presence of zygosphenes and zygrantra. The proportions of the quadrate, the complex frontal/parietal suture, and the morphology of the posterior mandibular elements identify this specimen as belonging to Mosasaurinae. However, the morphology of the skull and vertebrae differs from that of Mosasaurus lemonnieri. Skeletal immaturity could account for some of these differences, such as the relatively longer rostrum or shorter quadrate of IRSNB 3211. However, ontogenetic change does not explain the differences in the morphology of the maxilla or braincase elements. Phylogenetic analyses do not find a sister-group relationship between Mosasaurus lemonnieri and IRSNB 3211. The results of these analyses instead indicate a relationship between this specimen and Moanasaurus mangahouangae within a clade of mosasaurines from the Pacific Ocean. Characters that unite IRSNB 3211 with Moanasaurus mangahouangae include dorsal excavation of the maxilla for the external naris and cervical vertebrae with laterally extended postzygapophyses. However, skull proportions and differences in dental morphology do not support assigning this specimen to Moanasaurus mangahouangae. The qualitative results of morphological comparisons and the quantitative results of phylogenetic analysis support assigning IRSNB 3211 to a new taxon. Moanasaurus mangahouangae and its closest relatives have previously been considered to have been endemic to New Zealand. The New Zealand fossils are from Campanian strata, indicating the clade originated in the Southern Pacific and migrated to the Tethys during the Maastrichtian. Other fossils in European collections are likely representatives of this new taxon, but general similarities between this moanasaur-like taxon and Mosasaurus lemonnieri masked the diversity of latest Cretaceous mosasaurines from Europe. [less ▲]

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See detailLeviathans Unleashed: Skull Ecomorphological Evolution During The Initial Aquatic Radiations Of Mosasaurs And Cetaceans
Bennion, Rebecca ULiege; Maclaren, James ULiege; Coombs, Ellen et al

Conference (2021)

The repeated return of tetrapods to aquatic environments provides many iconic examples of convergent evolution, with various groups of mammals and reptiles independently evolving streamlined body shapes ... [more ▼]

The repeated return of tetrapods to aquatic environments provides many iconic examples of convergent evolution, with various groups of mammals and reptiles independently evolving streamlined body shapes and similar feeding strategies. One comparison that has received little attention is that between mosasaurs (a group of Late Cretaceous marine squamates) and early cetaceans (middle to late Eocene ancestors of modern whales and dolphins). These two groups share broad similarities in skull morphology, filling a wide range of niches and achieving global distributions. The earliest fully aquatic members of both groups had serpentine bodies and swam by axial undulation, before evolving more efficient caudal oscillatory locomotion and colonising open ocean niches. Cetaceans continued to diversify after reaching this form whereas the evolutionary history of the mosasaurs was cut short by the end-Cretaceous mass extinction. Here, we investigate possible parallel evolutionary trajectories of skull morphology that occurred during these initial aquatic radiations. A series of functionally informative ratios were calculated from 32 species of mosasaurs and early cetaceans. These were subjected to ordination techniques to reconstruct patterns of functional ecomorphospace occupation, and putative examples of convergence were tested statistically. Preliminary results show that the earliest mosasaurs had gracile skulls, specialised for smaller prey, from which they radiated in several waves across the ecomorphospace. There is considerable variation within certain genera, such as Mosasaurus. By contrast, basilosaurid cetaceans occupy a relatively constrained megapredatory niche and cetaceans only evolved new ecomorphologies after the late Eocene split into odontocetes and mysticetes. Oligocene odontocetes explore a new area of morphospace away from the basilosaurids, evolving a long, narrow snout with an increased number of small teeth. The earliest toothed mysticetes have a similar ecomorphology to the basilosaurids, with aetiocetids appearing to radiate in a similar direction to the odontocetes. The late Oligocene Janjucetus, which has a highly unusual ecomorphology, plots away from other cetaceans. Despite showing striking similarities to the mosasaur Prognathodon (e.g short robust snout and large eyes), the two taxa were not found to be statistically convergent. However, cranial convergence was found between the mosasaur Mosasaurus hoffmanni and the basilosaurid Dorudon atrox. Future work will investigate these results using 3D landmark analyses, and the evolutionary trajectories in early mysticetes will be extrapolated by including toothless species. [less ▲]

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See detailCharting new waters: changes in skull ecomorphology during the initial aquatic radiations of mosasaurs and cetaceans
Bennion, Rebecca ULiege; Maclaren, James ULiege; Coombs, Ellen et al

Conference (2020, December)

The repeated return of tetrapods to water provides many iconic examples of convergent evolution, with various groups of mammals and reptiles independently evolving streamlined body shapes and similar ... [more ▼]

The repeated return of tetrapods to water provides many iconic examples of convergent evolution, with various groups of mammals and reptiles independently evolving streamlined body shapes and similar feeding strategies. One comparison which has received little attention is that of cetaceans (whales and dolphins) and mosasaurs (a group of Late Cretaceous marine squamates). The earliest fully aquatic members of both groups had serpentine bodies and swam by axial undulation, before evolving more efficient caudal oscillatory locomotion and colonizing open ocean niches. Here we investigate possible parallel evolutionary trajectories of skull morphology that occurred during these initial aquatic radiations. A series of functionally informative ratios were calculated from 32 species of mosasaur and early cetacean. These were subjected to ordination techniques to reconstruct patterns of functional morphospace occupation. Preliminary results show that the earliest mosasaurs had gracile skulls specialized for smaller prey, from which they radiated in several waves across the morphospace. By contrast, basilosaurid cetaceans occupied a relatively constrained megapredatory niche, and only evolved new ecomorphologies after the Late Eocene split into odontocetes and mysticetes. Theresults also suggest cranial convergence between the toothed mysticete Janjucetus and the mosasaur Prognathodon. Future work will investigate these results further using 3D landmarks. [less ▲]

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See detailCharting new waters: changes in skull ecomorphology during the initial aquatic radiations of mosasaurs and cetaceans
Bennion, Rebecca ULiege; Maclaren, James ULiege; Coombs, Ellen et al

Poster (2020, June)

The earliest fully aquatic cetaceans (Middle to Late Eocene), had a serpentine body shape which was adapted for locomotion by axial undulation. This has drawn comparisons with the mosasaurs, a group of ... [more ▼]

The earliest fully aquatic cetaceans (Middle to Late Eocene), had a serpentine body shape which was adapted for locomotion by axial undulation. This has drawn comparisons with the mosasaurs, a group of Late Cretaceous marine squamates, with whom these early cetaceans also share broad similarities in skull morphology, including a triangular snout with widely spaced teeth and an unfused mandibular symphysis. Both groups filled a wide range of niches and achieved global distributions. As they became increasingly adapted to aquatic life, each clade modified their body plan to allow more efficient tail-based ‘thunniform’ locomotion, which enabled colonisation of the open ocean. Cetaceans continued to diversify after reaching this form whereas the evolutionary history of the mosasaurs was cut short by the end-Cretaceous mass extinction. We investigated the evolution of skull ecomorphology through the initial aquatic radiations of mosasaurs and cetaceans. Thirteen ecologically informative craniodental measurements and ratios were taken from 3D models of well-preserved skulls. The initial dataset consisted of ten cetaceans and eleven mosasaurs, sampling across the first twenty million years of evolution in each group. These data were subjected to ordination techniques in R to reconstruct patterns of morphospace occupation. Preliminary results show that Eocene cetaceans had a conserved skull morphology, only branching out into new morphologies in the Oligocene (early odontocetes significantly more so than early toothed mysticetes). By comparison, mosasaurs show much more ecomorphological variation. Future work will use 3D landmarks to accurately compare differences in skull shape through the evolution of each group. [less ▲]

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See detailComparative forelimb myology and muscular architecture of a juvenile Malayan tapir (Tapirus indicus)
Maclaren, James ULiege; McHorse, Brianna

in Journal of Anatomy (2020), 236(1), 85-97

The absence of preserved soft tissues in the fossil record is frequently a hindrance for palaeontologists wishing to investigate morphological shifts in key skeletal systems, such as the limbs ... [more ▼]

The absence of preserved soft tissues in the fossil record is frequently a hindrance for palaeontologists wishing to investigate morphological shifts in key skeletal systems, such as the limbs. Understanding the soft tissue composition of modern species can aid in understanding changes in musculoskeletal features through evolution, including those pertaining to locomotion. Establishing anatomical differences in soft tissues utilising an extant phylogenetic bracket can, in turn, assist in interpreting morphological changes in hard tissues and modelling musculoskeletal movements during evolutionary transitions (e.g. digit reduction in perissodactyls). Perissodactyls (horses, rhinoceroses, tapirs and their relatives) are known to have originated with a four‐toed (tetradactyl) forelimb condition. Equids proceeded to reduce all but their central digit, resulting in monodactyly, whereas tapirs retained the ancestral tetradactyl state. The modern Malayan tapir (Tapirus indicus) has been shown to exhibit fully functional tetradactyly in its forelimb, more so than any other tapir, and represents an ideal case‐study for muscular arrangement and architectural comparison with the highly derived monodactyl Equus. Here, we present the first quantification of muscular architecture of a tetradactyl perissodactyl (T. indicus), and compare it to measurements from modern monodactyl caballine horse (Equus ferus caballus). Each muscle of the tapir forelimb was dissected out from a cadaver and measured for architectural properties: muscle‐tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and resting fibre length. Comparative parameters [physiological cross‐sectional area (PCSA), muscle volume, and % muscle mass] were then calculated from the raw measurements. In the shoulder region, the infraspinatus of T. indicus exhibits dual origination sites on either side of the deflected scapular spine. Within ungulates, this condition has only been previously reported in suids. Differences in relative contribution to limb muscle mass between T. indicus and Equus highlight forelimb muscles that affect mobility in the lateral and medial digits (e.g. extensor digitorum lateralis). These muscles were likely reduced in equids during their evolutionary transition from tetradactyl forest‐dwellers to monodactyl, open‐habitat specialists. Patterns of PCSA across the forelimb were similar between T. indicus and Equus, with the notable exceptions of the biceps brachii and flexor carpi ulnaris, which were much larger in Equus. The differences observed in PCSA between the tapir and horse forelimb muscles highlight muscles that are essential for maintaining stability in the monodactyl limb while moving at high speeds. This quantitative dataset of muscle architecture in a functionally tetradactyl perissodactyl is a pivotal first step towards reconstructing the locomotor capabilities of extinct, four‐toed ancestors of modern perissodactyls, and providing further insights into the equid locomotor transition. [less ▲]

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See detailThe macroevolutionary landscape of short-necked plesiosaurians
Fischer, Valentin ULiege; Maclaren, James ULiege; Soul, Laura C. et al

in Scientific Reports (2020), 10(16434), 1--12

Throughout their evolution, tetrapods have repeatedly colonised a series of ecological niches in marine ecosystems, producing textbook examples of convergent evolution. However, this evolutionary ... [more ▼]

Throughout their evolution, tetrapods have repeatedly colonised a series of ecological niches in marine ecosystems, producing textbook examples of convergent evolution. However, this evolutionary phenomenon has typically been assessed qualitatively and in broad-brush frameworks that imply simplistic macroevolutionary landscapes. We establish a protocol to visualize the density of trait space occupancy and thoroughly test for the existence of macroevolutionary landscapes. We apply this protocol to a new phenotypic dataset describing the morphology of short-necked plesiosaurians, a major component of the Mesozoic marine food webs (ca. 201 to 66 Mya). Plesiosaurians evolved this body plan multiple times during their 135-million-year history, making them an ideal test case for the existence of macroevolutionary landscapes. We find ample evidence for a bimodal craniodental macroevolutionary landscape separating latirostrines from longirostrine taxa, providing the first phylogenetically-explicit quantitative assessment of trophic diversity in extinct marine reptiles. This bimodal pattern was established as early as the Middle Jurassic and was maintained in evolutionary patterns of short-necked plesiosaurians until a Late Cretaceous (Turonian) collapse to a unimodal landscape comprising longirostrine forms with novel morphologies. This study highlights the potential of severe environmental perturbations to profoundly alter the macroevolutionary dynamics of animals occupying the top of food chains. [less ▲]

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See detailShoulder-blade Runners: Utility of the Scapula Fossa Ratio to Investigate Locomotor Evolution in Equids (Mammalia: Perissodactyla)
Van Houtven, Karianne; Maclaren, James ULiege

in Journal of Morphology (2019, June), 280(S1), 232

The scapula fossa ratio (SFR) has been used in recent years to demonstrate functional locomotor differences in perissodactyls. The SFR describes the relative attachment areas of the lateral shoulder ... [more ▼]

The scapula fossa ratio (SFR) has been used in recent years to demonstrate functional locomotor differences in perissodactyls. The SFR describes the relative attachment areas of the lateral shoulder muscles: supraspinatus (extensor/stabilizer) and infraspinatus (lateral collateral stabilizer). Here, we use the SFR to investigate deep scapula muscle attachment sites of equids (Perissodactyla: Equidae), and whether any variation observed between extinct and extant species can be explained by habitat variation. 3D-surface models of scapulae were obtained via laser scanning and photogrammetry. Areas of the supraspinous and infraspinous fossae were calculated for 14 species of equids. We also included 9 cervid (Artiodactyla: Cervidae) and 3 alcelaphine (Artiodactyla: Bovidae) genera to test whether extinct equid SFRs are comparable to modern ungulates living in forested habitats (cervids) or open-grassland habitats (monodactyl equids; alcelaphines). Our results demonstrate that extinct tridactyl (three-toed) equid SFRs exhibit significant differences to modern monodactyl (one-toed) species. The infraspinous fossa is relatively larger in cervids and tridactyl equids compared to monodactyl equids; cervid and tridactyl equid SFRs could not be statistically separated. We interpret that the infraspinatus muscle is relatively more important for shoulder stability in tridactyl equids (and cervids) than in monodactyl equids. As monodactyl equids possess an advanced passive stay apparatus (muscle-tendon mechanism locking the shoulder to enable standing in open terrain with minimal energetic input), it is possible that the infraspinatus has become less integral for shoulder support during stance. Without such a mechanism, tridactyl equids still required shoulder support from the infraspinatus to exploit open habitats. This study demonstrates the utility of the SFR as a tool for investigating links between the musculoskeletal system and habitat use through equid evolution. [less ▲]

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See detailFetlock Morphology in Equoids (Mammalia: Perissodactyla) and its Relationship to Mass, Diet and Extrinsic Evolutionary Drivers
Maclaren, James ULiege; Hanegraef, Hester; Indekeu, Paul et al

in Journal of Morphology (2019, June), 280(S1), 170

The fetlock joint of equoids (Perissodactyla: Equoidea) is a highly specialized joint with multiple functions, including shock absorption, distal limb stability, and facilitating efficient elastic recoil ... [more ▼]

The fetlock joint of equoids (Perissodactyla: Equoidea) is a highly specialized joint with multiple functions, including shock absorption, distal limb stability, and facilitating efficient elastic recoil during the swing phase of locomotion to enable sustained, rapid running. Here, we investigate how this joint has changed in shape (and potential function) from extinct tetradactyl equoids (equids + palaeotheres) to modern horses in a quantitative framework, comparing patterns of shape change with internal (mass / diet) and external (geological / climatic) ecological drivers. We used landmark based geometric morphometrics to quantify fetlock shape in Equoidea using landmarks applied to the distal metacarpal. 268 metacarpals were laser scanned and landmarked. A hypothetical ancestor was calculated from basal-most metacarpal shapes using a time-calibrated phylogeny. Divergence from the ancestral shape was calculated using ordinary sum of square distances (OSS) from geometric morphometric analysis, averaged per species (n=57). First time derivatives for body mass, hypsodonty and OSS were taken at 1 Ma intervals to study trait shifts. When examined through time, positive correlations between shape divergence and bodymass / hypsodonty occurred at < 1 Ma, with negative correlations at -5Ma. Small-bodied palaeotheres drive a divergence in fetlock shape in the late Eocene, with North American anchitheres also inducing a morphological shift away from the ancestral morphology in the late Oligocene. Increases in divergence were observed at the 'Grande Coupure' and Mid Miocene Climatic Optimum, with decreases at the Vallesian turnover and Quaternary Glaciation. Overall, we demonstrate that fetlock morphology in equoids is linked with fluctuations in body mass, diet regime, and with key turnover events. Localized morphological evolution also plays a key role, with morphological peaks and troughs correlating with origination and extinction of specialized endemics (e.g., Plagiolophus, Nannippus). [less ▲]

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See detailModern Tapirs as Morphofunctional Analogues for Locomotion in Endemic Eocene European Perissodactyls
Maclaren, James ULiege; Nauwelaerts, Sandra

in Journal of Mammalian Evolution (2019)

Tapirs have historically been considered as ecologically analogous to several groups of extinct perissodactyls based on dental and locomotor morphology. Here, we investigate comparative functional ... [more ▼]

Tapirs have historically been considered as ecologically analogous to several groups of extinct perissodactyls based on dental and locomotor morphology. Here, we investigate comparative functional morphology between living tapirs and endemic Eocene European perissodactyls to ascertain whether tapirs represent viable analogues for locomotion in palaeotheres and lophiodontids. Forelimb bones from 20 species of Eocene European perissodactyls were laser scanned and compared to a forelimb dataset of extant Tapirus. Bone shape was quantified using 3D geometric morphometrics; coordinates were Procrustes aligned and compared using Principal Component Analysis and neighbor-joining trees. Functional traits included lever-arm ratios (LARs; proxy for joint angular velocity), long-bone proportions (speed proxy), and estimated body mass. Results suggest that Paralophiodon and Palaeotherium magnum resemble Neotropical tapirs in humeral morphology and LARs. Palaeotheres demonstrate extensive forelimb shape disparity. Despite previous assessments, metacarpal shape analyzes do not support a strong morphological similarity between palaeotheres and tapirs, with Tapirus pinchaque representing the closest analogue for Eocene European equoid manus morphology. Our analyses suggest lophiodontids were not capable of moving as swiftly as tapirs due to greater loading over the manus. We conclude that the variation within modern tapir forelimb morphology confounds the assignment of one living analogue within Tapirus for extinct European equoids, whereas tapirs adapted for greater loading over the manus (e.g., T. bairdii, T. indicus) represent viable locomotor analogues for lophiodontids. This study represents a valuable first step toward locomotor simulation and behavioral inference for both hippomorph and tapiromorph perissodactyls in Eocene faunal communities. [less ▲]

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See detailA morphometric analysis of the forelimb in the genus Tapirus (Perissodactyla: Tapiridae) reveals influences of habitat, phylogeny and size through time and across geographical space
Maclaren, James ULiege; Hulbert Jr., Richard; Wallace, Steven et al

in Zoological Journal of the Linnean Society (2018), 184(2), 499-515

The limb skeleton of tapirs (Perissodactyla: Tapirus spp.) was traditionally thought to exhibit morphological variation only as a result of changes in body size. Here, we test whether forelimb variation ... [more ▼]

The limb skeleton of tapirs (Perissodactyla: Tapirus spp.) was traditionally thought to exhibit morphological variation only as a result of changes in body size. Here, we test whether forelimb variation exhibited by Tapirus is solely an artefact of size fluctuations through the tapir fossil record or whether it is influenced by habitat differences. We investigated the forelimb osteology of 12 species of Tapirus using three-dimensional geometric morphometrics on laser surface scans. Aligned shape coordinates were regressed against intrinsic bone size to account for allometry. Taxa of equivalent body mass exhibited significant differences in size-corrected bone shape. Stable carbon isotope values were averaged per species as a proxy for habitat density. Multivariate regressions of the humerus, pisiform, cuneiform, unciform, third and fourth metacarpals revealed no significant influence of size on shape. The lateral car-pals (pisiform, cuneiform, unciform) demonstrated variation across the habitat density gradient. Observed variation is likely driven by species in the extinct subgenus Helicotapirus tapirs, which inhabited drier, more open woodland than modern taxa. We conclude that tapir forelimb variation is not exclusively an artefact of body size, with lateral wrist bones displaying notable differences across a habitat density gradient, beyond that resulting from size and phylogenetic effects. [less ▲]

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See detailInterspecific variation in the tetradactyl manus of modern tapirs (Perissodactyla: Tapirus) exposed using geometric morphometrics
Maclaren, James ULiege; Nauwelaerts, Sandra

in Journal of Morphology (2017), 278(1), 1517-1535

The distal forelimb (autopodium) of quadrupedal mammals is a key morphological unit involved in locomotion, body support, and interaction with the substrate. The manus of the tapir (Perissodactyla ... [more ▼]

The distal forelimb (autopodium) of quadrupedal mammals is a key morphological unit involved in locomotion, body support, and interaction with the substrate. The manus of the tapir (Perissodactyla: Tapirus) is unique within modern perissodactyls, as it retains the plesiomorphic tetradactyl (four-toed) condition also exhibited by basal equids and rhinoceroses. Tapirs are known to exhibit anatomical mesaxonic symmetry in the manus, although interspecific differences and biomechanical mesaxony have yet to be rigorously tested. Here, we investigate variation in the manus morphology of four modern tapir species (Tapirus indicus, Tapirus bairdii, Tapirus pinchaque, and Tapirus terrestris) using a geometric morphometric approach. Autopodial bones were laser scanned to capture surface shape and morphology was quantified using 3D-landmark analysis. Landmarks were aligned using Generalised Procrustes Analysis, with discriminant function and partial least square analyses performed on aligned coordinate data to identify features that significantly separate tapir species. Overall, our results support the previously held hypothesis that T. indicus is morphologically separate from neotropical tapirs; however, previous conclusions regarding function from morphological differences are shown to require reassessment. We find evidence indicating that T. bairdii exhibits reduced reliance on the lateral fifth digit compared to other tapirs. Morphometric assessment of the metacarpophalangeal joint and the morphology of the distal facets of the lunate lend evidence toward high loading on the lateral digits of both the large T. indicus (large body mass) and the small, long limbed T. pinchaque (ground impact). Our results support other recent studies on T. pinchaque, suggesting subtle but important adaptations to a compliant but inclined habitat. In conclusion, we demonstrate further evidence that the modern tapir forelimb is a variable locomotor unit with a range of interspecific features tailored to habitual and biomechanical needs of each species. [less ▲]

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See detailA three-dimensional morphometric analysis of upper forelimb morphology in the enigmatic tapir (Perissodactyla: Tapirus ) hints at subtle variations in locomotor ecology
Maclaren, James ULiege; Nauwelaerts, Sandra

in Journal of Morphology (2016), 277(11), 1469-1485

Forelimb morphology is an indicator for terrestrial locomotor ecology. The limb morphology of the enigmatic tapir (Perissodactyla: Tapirus) has often been compared to that of basal perissodactyls, despite ... [more ▼]

Forelimb morphology is an indicator for terrestrial locomotor ecology. The limb morphology of the enigmatic tapir (Perissodactyla: Tapirus) has often been compared to that of basal perissodactyls, despite the lack of quantitative studies comparing forelimb variation in modern tapirs. Here, we present a quantitative assessment of tapir upper forelimb osteology using three-dimensional geometric morphometrics to test whether the four modern tapir species are monomorphic in their forelimb skeleton. The shape of the upper forelimb bones across four species (T. indicus; T. bairdii; T. terrestris; T. pinchaque) was investigated. Bones were laser scanned to capture surface morphology and 3D landmark analysis was used to quantify shape. Discriminant function analyses were performed to reveal features which could be used for interspecific discrimination. Overall our results show that the appendicular skeleton contains notable interspecific differences. We demonstrate that upper forelimb bones can be used to discriminate between species (>91% accuracy), with the scapula proving the most diagnostic bone (100% accuracy). Features that most successfully discriminate between the four species include the placement of the cranial angle of the scapula, depth of the humeral condyle, and the caudal deflection of the olecranon. Previous studies comparing the limbs of T. indicus and T. terrestris are corroborated by our quantitative findings. Moreover, the mountain tapir T. pinchaque consistently exhibited the greatest divergence in morphology from the other three species. Despite previous studies describing tapirs as functionally mediportal in their locomotor style, we find osteological evidence suggesting a spectrum of locomotor adaptations in the tapirs. We conclude that modern tapir forelimbs are neither monomorphic nor are tapirs as conserved in their locomotor habits as previously described. [less ▲]

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