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| Diagram of supportive tendons and ligaments of the equine foot (only selected ligaments, referred to in the text, are shown). The hoof (covering the third phalanx and the distal portion of the second phalanx) is shown in cross section. Original drawing by CMJ, final illustration rendered by James G. Napoli.
Source publication
The traditional story of horse evolution is well-known: over time, horses became larger, they attained higher-crowned teeth, and they changed from having three toes (tridactyly) to a single toe (monodactyly). Evolution is often perceived as a progression toward some optimum outcome, in this case the “Noble Steed.” However, the evolutionary advantag...
Contexts in source publication
Context 1
... these conditions of monodactyly are convergent: there is no reason to think that the retention of the metapodial shafts in Equus represents some intermediate stage in the evolution of monodactyly. The anatomy of the equid foot, and the major tendons and ligaments is shown in Figure 4. Anatomical descriptions below are taken from Nickel et al. (1986), Stashak (2006). ...
Context 2
... structures are muscular only in their most proximal portions, small superpinnate muscles that function as spring dampeners ( Wilson et al., 2001). The flexor tendons course superficial to the suspensory and sesamoidean ligaments and have "check ligaments" that aid in the passive suspensory apparatus of the foot (see Figure 4); these prevent excessive stretch of the tendons, and may act as additional parallel elastic elements ( Wilson et al., 2001). The superficial digital flexor tendon divides into two before inserting onto the second phalanx on either side of the straight sesamoidean ligament. ...
Context 3
... involves changes in digit proportions (#2b,3c,5c), the medial and lateral digits no longer contacting the ground during regular locomotion; changes in the articulation at the fetlock joint that limit lateral motion but allow for greater parasagittal rotation (#4b); changes in phalangeal proportions, with an elongated first phalanx, and a more vertically-positioned third phalanx that is now shorter than the second phalanx (#2b); and associated postural changes in the hind limb (#12c,12d,13c). There is also evidence of the development of the ligamentous suspensory apparatus with the appearance of scars for the oblique sesamoidean ligaments (#8a), and the suspensory ligament was now probably at least mostly tendinous (#10b) (see Figure 4). ...
Context 4
... changes shared by the clade of Pliohippus-Equus include the following (note that some of these are queried at this level and require further investigation): the increasing encroachment of the distal sagittal ridge on metapodials to the dorsal surface of the articulation (#4c,d); the loss of the metapodial volar gully indicating a completely tendinous suspensory ligament (#10c); enlarged proximal sesamoid bones indicative of an enhanced suspensory apparatus (#11b); the elongation and fusion of the oblique sesamoid ligaments to form a distinct V-scar (#8c); and a somewhat less-flexed knee joint (#13d). Changes occurring in more derived Equini (crownward of Pliohippus) include a more vertical position of the third phalanx within the hoof (#2c); a further less-flexed knee joint (#13,e,f); and an astragalus that limits the motion of the hind limb to the parasagittal plane to a greater extent (#12e,f). ...
Citations
... The ancestral condition for equids is four toes on the forelimbs and three toes on the hindlimbs, as exhibited by Hyracotherium Owen, 1841(MacFadden 1994. The evolution of monodactyly in equids began with a shift towards unguligrade (more upright) foot posture, involving the elongation of metapodial III, lengthening of tendons, and a proximal concentration of force-generating musculature (Clayton 2016;Janis and Bernor 2019). This is thought to have been a response to the shift from a forested habitat to grasslands; hard substrates may select for long, slim legs to increase speed for predator escape (Simpson 1951), decrease the energetic costs of movements by reducing distal limb mass (Janis and Wilhelm 1993), increase efficiency for long-distance travel to find food (Janis and Bernor 2019), and enhance stability for rapid, unidirectional movements (Shotwell 1961). ...
... The evolution of monodactyly in equids began with a shift towards unguligrade (more upright) foot posture, involving the elongation of metapodial III, lengthening of tendons, and a proximal concentration of force-generating musculature (Clayton 2016;Janis and Bernor 2019). This is thought to have been a response to the shift from a forested habitat to grasslands; hard substrates may select for long, slim legs to increase speed for predator escape (Simpson 1951), decrease the energetic costs of movements by reducing distal limb mass (Janis and Wilhelm 1993), increase efficiency for long-distance travel to find food (Janis and Bernor 2019), and enhance stability for rapid, unidirectional movements (Shotwell 1961). The evolution of large body mass among horses may also have increased the bending forces on limbs, selecting for a single toe because one digit resists bending forces better than multiple smaller digits of the same overall size (Thomason 1986;Biewener 1998;McHorse et al. 2017). ...
Unlabelled:
The metapodials of extinct horses have long been regarded as one of the most useful skeletal elements to determine taxonomic identity. However, recent research on both extant and extinct horses has revealed the possibility for plasticity in metapodial morphology, leading to notable variability within taxa. This calls into question the reliability of metapodials in species identification, particularly for species identified from fragmentary remains. Here, we use ten measurements of metapodials from 203 specimens of four Pleistocene horse species from eastern Beringia to test whether there are significant differences in metapodial morphology that support the presence of multiple species. We then reconstruct the body masses for every specimen to assess the range in body size within each species and determine whether species differ significantly from one another in mean body mass. We find that that taxonomic groups are based largely on the overall size of the metapodial, and that all metapodial measurements are highly autocorrelated. We also find that mean body mass differs significantly among most, but not all, species. We suggest that metapodial measurements are unreliable taxonomic indicators for Beringian horses given evidence for plasticity in metapodial morphology and their clear reflection of differences in body mass. We recommend future studies use more reliable indicators of taxonomy to identify Beringian horse species, particularly from localities from which fossils of several species have been recovered.
Supplementary information:
The online version contains supplementary material available at 10.1007/s10914-022-09626-4.
... The unique mode of life of Equus could potentially be defined as "ungulate mammals that are adapted to live in generally open, arid habitats and that can thrive on low-quality, high-fiber foods such as grasses and other coarse and tough vegetation" [9,19]. Potential morphological adaptations for this mode of life include modifications to the locomotory [415] and digestive systems, particularly the dentition [9]. Based on the position of the majority of purported, adaptive zone-related traits, Equus is assigned to clade 6, or possibly clade 7, in the strict consensus tree of Barrón-Ortiz et al. [9], under the uniqueness of adaptive zone paradigm. ...
Studies of horse evolution arose during the middle of the 19th century, and several hypotheses have been proposed for their taxonomy, paleobiogeography, paleoecology and evolution. The present contribution represents a collaboration of 19 multinational experts with the goal of providing an updated summary of Pliocene and Pleistocene North, Central and South American, Eurasian and African horses. At the present time, we recognize 114 valid species across these continents, plus 4 North African species in need of further investigation. Our biochronology and biogeography sections integrate Equinae taxonomic records with their chronologic and geographic ranges recognizing regional biochronologic frameworks. The paleoecology section provides insights into paleobotany and diet utilizing both the mesowear and light microscopic methods, along with calculation of body masses. We provide a temporal sequence of maps that render paleoclimatic conditions across these continents integrated with Equinae occurrences. These records reveal a succession of extinctions of primitive lineages and the rise and diversification of more modern taxa. Two recent morphological-based cladistic analyses are presented here as competing hypotheses, with reference to molecular-based phylogenies. Our contribution represents a state-of-the art understanding of Plio-Pleistocene Equus evolution, their biochronologic and biogeographic background and paleoecological and paleoclimatic contexts.
... In addition to negotiating the environment, the distal limb part mechanically affects the motion of the proximal and intermediate limb parts through an interlocking mechanism. In ungulates like horses and giraffes, the distal end of the limb skeleton is connected to the intermediate limb part by multi-articular muscles with strong tendons [73]. The distal portion of the multiarticular muscles that produce the interlocking mechanism become tendinous, thereby reducing the mass of the tip and making the body less susceptible to inertia. ...
Animals can instantly exhibit versatile behaviors in response to their circumstances to survive in harsh natural environments. For example, many animals chase prey, escape from predators, forage, utilize tools, build nests, and breed, in accordance with their niches with limited resources. To gain a deeper understanding of the source of this versatility in animals and apply the principles of their body plans to robotics, the authors, who work in the robotics and anatomy fields, jointly review and compare the essential body structure differences between quadruped robots and animals. This review provides a comprehensive discussion of the various functions of animal bodies by particularly focusing on the anatomical structure and gives examples of how these functions have been implemented in robotics. Conventional biomimetic robots have taken the approach of realizing animal functions as simple single-functional modules in order to achieve specific animal functions in the most efficient way. In contrast, animals have situation-dependent multi-functionality, in which one part of the body exhibits multiple capabilities depending on the situation. This new ‘polysemantic body’ design principle would transform the traditional processes for understanding and mimicking animals and make the interaction between biology and robotics more fruitful.
... Natural selection has rendered equine species with speed, strength, stamina, alertness and cooperation [27], enabling them to survive and thrive as prey species in the habitats for which they evolved-generally considered to be the plains of North America for horses [28] and arid lands of northern Africa for donkeys [29]. Subsequent selective breeding has capitalised on some of these traits, as well as equids' adaptability for different purposes, through breeding to enhance characteristics that are particularly desirable to humans. ...
Equestrian sports, including racing (e.g., flat, steeple-chasing, harness or donkey derby); show-jumping; cross-country; dressage; polo; polocrosse; endurance; carriage driving; vaulting and hunting; are hugely popular in the UK, and they involve a significant number of people, both as participants and spectators, and tens of thousands of equids. In this paper, we discuss animal welfare as a complex and disputed issue, clarifying what the term means and how it can be measured. We review many aspects of welfare risk to equids used for sport, addressing issues encountered throughout their lives, including housing, feeding, veterinary intervention, shoeing, handling, training, breeding and equipment. This is followed by a unique exploration of the institutions and social processes influencing equine welfare. The institutional components comprise the rules of competition, the equids, attributes of the stakeholders and the space where participants strive to achieve a common purpose. We endeavour to untangle the most significant elements that create barriers or provide opportunities for equine welfare improvement. We expose the challenges faced by a broad range of stakeholders with differing ethics, attitudes and values. Evidently, there are many welfare risks to which equids used in sports continue to be exposed. It is also evident that significant improvements have occurred in recent times, but there remains a barrier to reducing the risks to an acceptable level. We conclude with recommendations regarding a process for change, involvement of stakeholders and management of knowledge to improve equine welfare that involves identifying and prioritising the risk factors and ultimately leading to interventions, further research and/or education.
... Macropus, but also a distal extent of these scars, possibly representing the oblique sesamoid ligaments seen in Equus, where they form a distinctive "V-scar" extending to around 70% the length of the bone (see Janis & Bernor, 2019). While M. ferragus and S. stirlingi have similarly sized proximal paired sesamoid bones ( Figure S5), there is indeed a greater distal extent of the sesamoid ligament scars of S. stirlingi and other sthenurines, extending to around 50% of the length of the phalanx, and bearing some similarity to the equid V-scar. ...
The extinct sthenurine (giant, short-faced) kangaroos have been proposed to have a different type of locomotor behavior to extant (macropodine) kangaroos, based both on physical limitations (the size of many exceeds the proposed limit for hopping) and anatomical features (features of the hind limb anatomy suggestive of weight-bearing on one leg at a time). Here, we use micro-CT scans of the pedal bones of six kangaroos, three sthenurine and three macropodine, ranging from ~50-150 kg, to investigate possible differences in bone resistances to bending and cortical bone distribution that might relate to differences in locomotion. Using second moment of area analysis we show differences in resistance to bending between the two subfamilies. Distribution of cortical bone shows that sthenurines had less resistant calcaneal tubers, implying a different foot posture during locomotion, and the long foot bones were more resistant to the medial bending stresses. These differences were the most pronounced between Pleistocene monodactyl sthenurines (Sthenurus stirlingi and Procoptodon browneorum) and the two species of Macropus (the extant M. giganteus and the extinct M. cf. M. titan) and support the hypothesis that these derived sthenurines employed bipedal striding. The Miocene sthenurine Hadronomas retains some more macropodine-like features of bone resistance to bending, perhaps reflecting its retention of the fifth pedal digit. The Pleistocene macropodine Protemnodon has a number of unique features, possibly indicative of a type of locomotion unlike the other kangaroos.
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... Nevertheless, there are similarities in other aspects of their limb anatomy. In proterotheriid litopterns, the proximal third phalanx has proportions more similar to those of extinct equids with a "spring foot"-an important adaptation in the evolution of equine unguligrady [105]-than equids that retained three functional digits. The length/width ratio of this element in early Miocene Thoatherium and Diadiaphorus is 3.05 and 2.95, respectively [78]; these values are much higher than those of the early stem equine anchitheres analyzed by O'Sullivan [values � 2.41; 106], indicating relatively longer, thinner proximal phalanges in these proterotheriids. ...
During the Neogene, many North American ungulates evolved longer limbs. Presumably, this allowed them to move more efficiently or quickly in open habitats, which became more common during this interval. Evidence suggests that open habitats appeared even earlier in South America, but no study to date has investigated whether the ungulate-like mammals of South America (South American native ungulates or SANUs) evolved similar limb adaptations. We analyzed limb elongation in the two predominant SANU groups, notoungulates and litopterns, by compiling genus-level occurrences from the late Oligocene to the Pleistocene and calculating metatarsal/femur ratio (Mt:F). None of the groups or subgroups we analyzed show a pronounced increase in Mt:F across this interval, with the possible exception of proterotheriid litopterns. Proterotheriids are thought to have inhabited forested environments rather than open ones, which raises questions about the selective forces responsible for limb elongation in ungulates. Conversely, notoungulates, which are traditionally thought to have lived in open habitats, show no strong trend of increasing Mt:F across this interval. Our study suggests that the macroevolutionary trend of limb elongation in ungulate-like mammals is not universal and is highly influenced by the evolutionary affinities of the groups being analyzed.
... Yet, tooth damage marks typical of mixed-grazing diets are present millions of years before hypsodonty emerged (16,17). Likewise, monodactyly does not appear immediately optimal to withstand heavier body sizes and likely required additional conformational and locomotory changes (18). That extinct equid species often exhibited similar anatomies, with a range of intraspecific plasticity largely overlapping the magnitude of between-species divergence, further contradicts the theoretical outcome of adaptive radiations. ...
The equid family contains only one single extant genus, Equus, including seven living species grouped into horses on the one hand and zebras and asses on the other. In contrast, the equine fossil record shows that an extraordinarily richer diversity existed in the past and provides multiple examples of a highly dynamic evolution punctuated by several waves of explosive radiations and extinctions, cross-continental migrations, and local adaptations. In recent years, genomic technologies have provided new analytical solutions that have enhanced our understanding of equine evolution, including the species radiation within Equus; the extinction dynamics of several lineages; and the domestication history of two individual species, the horse and the donkey. Here, we provide an overview of these recent developments and suggest areas for further research.
Expected final online publication date for the Annual Review of Animal Biosciences, Volume 9 is February 16, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... All present-day Perissodactyla have either one or three hoofed toes. The hoof itself contains wide and flattened bones that bear circular edges and are surrounded by a keratinous layer (comparable to the nails of claws) (Janis and Bernor 2019). The perissodactyls use unguligrade foot postures (walking on the toes) that serve to lengthen the legs, allowing for faster and more energy-efficient running (Janis and Bernor 2019). ...
... The hoof itself contains wide and flattened bones that bear circular edges and are surrounded by a keratinous layer (comparable to the nails of claws) (Janis and Bernor 2019). The perissodactyls use unguligrade foot postures (walking on the toes) that serve to lengthen the legs, allowing for faster and more energy-efficient running (Janis and Bernor 2019). The tapir is unique in that it retains the structure of the ancestral tetradactyl perissodactyls and presents with four digits on its foreleg (MacLaren and McHorse 2020). ...
... As recent authors have hypothesized (e.g. [35][36][37]), the interplay between intrinsic biological traits (in addition to body mass) and extrinsic ecological factors is most likely associated with selection pressures at local or species-level scales, with multiple micro-evolutionary changes leading to the patterns of morphological expression observed in the equid locomotor and feeding apparatuses. This study demonstrates that derived and highly hypsodont NW hipparionins (such as Nannippus) drive MCP joint morphological variation into new regions of morphospace (figure 1b; positive PC1 region), quite separate from ancestral basal equids (figure 1), and also separate from derived equinins and OW hipparionins (figure 1b). ...
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.
... These energetically efficient hunting opportunities effectively balance out the energy expended during exhaustive predation episodes. In another digitally reduced mammalian lineage, Equidae, the vestigial second and fourth metacarpals serve an important function in supporting the articulation of the carpus and manus (Janis & Bernor, 2019). Another possible function for the reduced pollical muscles in L. pictus is proprioception. ...
Background
The African wild dog ( Lycaon pictus ), an endangered canid native to southern and eastern Africa, is distinct among canids in being described as entirely tetradactyl and in its nomadic lifestyle and use of exhaustive predation to capture its prey instead of speed, strength, or stealth. These behavioral and morphological traits suggest a potentially unique set of adaptations.
Methods
Here, we dissected the forelimbs of an adult male L. pictus specimen and performed detailed descriptions and quantitative analyses of the musculoskeletal anatomy.
Results
Statistical comparisons of muscle masses and volumes revealed that L. pictus has relatively smaller wrist rotators (mm. pronator teres, pronator quadratus, supinator) than any other included carnivoran taxon, suggesting adaptive pressures for antebrachial stability over rotatory movement in the carpus of L. pictus . While a complete digit I is absent in L. pictus , a vestigial first metacarpal was discovered, resulting in changes to insertions of mm. extensor digiti I et II, abductor (et opponens) digiti I and flexor digiti I brevis. Mm. anconeus, brachialis and flexor carpi ulnaris caput ulnare all have more extensive origins in L. pictus than other canids suggesting an emphasis on posture and elbow stability. M. triceps brachii caput laterale has a larger origin in L. pictus and m. triceps brachii caput longum has an additional accessory head. Electromyographic studies have shown this muscle is active during the stance phase of trotting and galloping and is important for storing elastic energy during locomotion. We interpret these differences in size and attachments of muscles in L. pictus as adaptations for long distance running in this highly cursorial species, likely important for exhaustive predation. Absence of a full digit I in L. pictus may increase speed and stride length; however, the retention of a vestigial digit permits the attachment of reduced pollical muscles which may provide additional stability and proprioception to the carpus.
