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Merychippus
Merychippus
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Merychippus
Temporal range: Miocene, 15.9–5.3 Ma
Skeletal reconstruction of Merychippus on display at the American Museum of Natural History
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Subfamily: Equinae
Genus: †Merychippus
Leidy, 1856
Type species
†Merychippus insignis
Leidy, 1856[1]
Species
Species
  • †M. brevidontus Bode, 1935
  • †M. calamarius Cope, 1875
  • †M. californicus Merriam, 1915
  • †M. coalingensis Clark, 1921
  • †M. coloradense Osborn, 1918
  • †M. eohipparion Osborn, 1918
  • †M. eoplacidus Osborn, 1918
  • †M. gunteri Simpson, 1930
  • †M. insignis Cope, 1874
  • †M. labrosus Cope, 1874
  • †M. missouriensis Douglass, 1908
  • †M. patrusus Osborn, 1918
  • †M. primus Osborn, 1918
  • †M. proparvulus Osborn, 1918
  • †M. quartus Stirton, 1940
  • †M. quintus Kelly and Lander, 1988
  • †M. relictus Cope, 1889
  • †M. republicanus Osborn, 1918
  • †M. secundus Abel, 1928
  • †M. sejunctus Cope, 1874
  • †M. seversus Cope, 1878
  • †M. stevensi Dougherty, 1940

Merychippus is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago.[2] It had three toes on each foot and is the first horse known to have grazed.

Discovery and naming

[edit]
Restoration of Merychippus insignis

Merychippus was named by Joseph Leidy (1856). Numerous authors assigned the type species – Merychippus insignis – to Protohippus, but this is ignored. It was assigned to the Equidae by Leidy (1856) and Carroll (1988), and to the Equinae by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006).[3][4][5] The genus name comes from Ancient Greek μηρυκασθαι (mērukasthai), meaning "to ruminate", and ἵππος (híppos), meaning "horse", but current evidence does not support Merychippus ruminating.

Description

[edit]
M. sejunctus front and back feet (right) and M. sphenodus lower jaw fragment

Merychippus lived in groups. It was about 100 cm (39 in) tall[6] and at the time it was the tallest equine to have existed. Its muzzle was longer, deeper jaw, and eyes wider apart than any other horse-like animal to date. The brain was also much larger, making it smarter and more agile. Merychippus was the first equine to have the distinctive head shape of today's horses.

The Miocene was a time of drastic change in environment, with woodlands transforming into grass plains.[7] This led to evolutionary changes in the hooves and teeth of equids. A change in surface from soft, uneven mud to hard grasslands meant there was less need for increased surface area.[7] The foot was fully supported by ligaments, and the middle toe developed into a hoof that did not have a pad on the bottom. In some Merychippus species, the side toes were larger, whereas in others, they had become smaller and only touched the ground when running. The transformation into plains also meant Merychippus began consuming more phytolith rich plants. This led to the presence of hypsodont teeth. Such teeth range from medium to intense crown height, are curved, covered in large amounts of cement, and are characteristic of grazing animals[8]

Equid size also increased, with Merychippus ranging, on average, between 71 and 100.6 kg.[9]

Classification

[edit]

By the end of the Miocene era, Merychippus was one of the first quick grazers. It gave rise to at least 19 different species of grazers, which can be categorized into three major groups. This burst of diversification, termed an adaptive radiation, is often known as the "Merychippine radiation".[citation needed]

The first was a series of three-toed grazers known as hipparions. These were very successful and split into four genera and at least 16 species, including small and large grazers and browsers with large and elaborate facial fossae. The second was a group of smaller horses, known as protohippines, which included Protohippus and Calippus. The last was a line of "true equines" in which the side toes were smaller than those of other proto-horses. In later genera, these were lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running.

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Merychippus

View on Grokipedia
from Grokipedia
Merychippus is an extinct of early that lived approximately 17 to 11 million years ago during the middle epoch in . This three-toed equid stood about 1 meter tall at the and featured a long face, long legs, and high-crowned cheek teeth adapted for grinding tough grasses, marking it as one of the first true grazing horses. Evolving from earlier genera such as Parahippus, Merychippus represented a pivotal in equid evolution by transitioning from to lifestyles amid expanding North American grasslands. Its physical adaptations included streamlined limbs for speed on hard ground, elongated feet with a larger central supported by ligaments, and specialized resembling that of ruminants—hence its name, coined by Joseph Leidy for the ruminant-like tooth crests. Fossils of various Merychippus species have been discovered at numerous middle sites across the , indicating a widespread presence in open habitats. As a hind-gut fermenter with a for breaking down fibrous vegetation, it thrived on a diet of grasses rather than softer browse, setting the stage for descendants like Pliohippus and, ultimately, modern equines. This genus diversified into multiple lineages, giving rise to all later horse groups and exemplifying the driven by environmental changes during the .

Taxonomy

Etymology and Naming

The genus was established by American paleontologist Joseph Leidy in 1857, based on fossil specimens collected from deposits in the Bijou Hills of . Leidy described the as Merychippus insignis in the Proceedings of the Academy of Natural Sciences of , noting its distinctive dental morphology suggestive of advanced mastication. This naming marked an early contribution to the systematic study of North American equid fossils, building on Leidy's broader work in vertebrate paleontology during the mid-19th century. The name Merychippus derives from the μὴρυκάσθαι (mērykásthai), meaning "to ruminate" or "to chew the cud," combined with ἵππος (híppos), meaning "horse," resulting in "ruminating horse." Leidy selected this etymology to reflect the intricately folded enamel patterns on the molars, which he interpreted as adaptations for grinding similar to those in ruminants. However, modern analyses indicate that Merychippus lacked the multi-chambered stomach characteristic of true ruminants and relied instead on teeth for processing abrasive grasses.

Species and Classification

Merychippus is a of extinct equids within the family and subfamily , encompassing early horses that lived during the Middle to epochs, approximately 17 to 11 million years ago, primarily in . The genus was established by paleontologist Joseph Leidy in 1857 based on fossils from the Miocene deposits of the , with the designated as Merychippus insignis from the Bijou Hills of ; this species is known from an immature preserving partial deciduous premolars. The taxonomic placement of M. insignis has been debated due to the fragmentary nature of the type specimen, but it remains the valid type, representing a subhypsodont form transitional between browsing and grazing adaptations. Traditionally, Merychippus has been viewed as a diverse assemblage of three-toed, incipiently equids that played a pivotal role in the of horses, but modern phylogenetic analyses regard it as a paraphyletic or polyphyletic "grade" rather than a monophyletic , comprising multiple lineages at the base of more advanced groups. This status stems from its broad morphological definition, leading to the reassignment of numerous species to distinct genera such as Acritohippus, Parapliohippus, Scaphohippus, Cormohipparion, and Astrohippus based on cranial, dental, and postcranial differences, as detailed in systematic revisions by J. MacFadden and others. For instance, species like M. isonesus have been transferred to Parapliohippus, reflecting refined understandings of evolutionary branching within . Currently accepted species within Merychippus are limited to those exhibiting core traits of subhypsodont and three-toed limbs, with key examples including M. insignis (type, early Barstovian stage), M. primus (basal to and Hipparionini clades), M. gunteri (from Florida's Hawthorn Formation, late Hemingfordian), M. californicus (Middle Miocene of ), and M. tehachapiensis (from the , ). These species illustrate the genus's role as an evolutionary hub, with M. primus and M. gunteri positioned as sister taxa to major equid lineages in cladistic analyses of dental morphology. Further taxonomic refinements continue, driven by ongoing discoveries and phylogenetic studies emphasizing hypsodonty progression and locomotor adaptations.

Physical Characteristics

Body Size and Build

Merychippus exhibited a body size comparable to that of a small modern , representing a significant increase in stature from earlier equids like Mesohippus. This size allowed for greater mobility across expansive grasslands, with estimates placing the shoulder height around 100 cm in typical specimens. The overall build was slender and agile, featuring elongated limbs that enhanced speed and endurance for evading predators and foraging over long distances. The skeletal structure emphasized adaptations, with lengthened metacarpals and metatarsals supporting a spring-like . The forelimbs and hindlimbs were proportioned for efficient , retaining tridactyly but with the central toe bearing most of the body weight, foreshadowing the monodactyl condition of later equids. The ribcage was relatively narrow, and the vertebral column was stiffened to provide stability during rapid movement. Facial morphology included a longer muzzle than in ancestral forms, housing high-crowned, cheek teeth adapted for abrasive grazing. The cranium showed an expanded braincase, while the overall proportions approached those of modern equines despite the primitive foot structure.

Cranial and Dental Morphology

The cranium of Merychippus species, such as M. insignis and M. primus, exhibits a larger overall size compared to earlier equids like Parahippus leonensis, with a retracted nasal notch positioned midway between the first cheek (C1) and the second (P2). The preorbital bar measures approximately 17 mm in width, while the dorsal preorbital fossa is distinctly developed, measuring about 64 mm in length and slightly pocketed posteriorly, with its anterior end positioned above P3. The opens above the P4/M1 boundary, and the extends dorsoventrally, reaching about 8 mm into the rear of the dorsal preorbital fossa. In some species like M. tertius, the dorsal preorbital fossa deepens to 5–10 mm, and a shallow malar fossa appears, marking early derived features within the . Dental morphology in Merychippus reflects a transitional stage toward hypsodonty, with mesodont cheek teeth featuring unworn crown heights of 24–28 mm in upper molars and premolars, covered in for enhanced durability against diets. The protocone connects to the protoloph early in wear on P2 but remains isolated longer in other teeth, often with persistent spurs; fossette borders show moderate complexity with 2–6 plications, typically two, and the hypoconal groove includes a single plication. Lower cheek teeth display deep ectoflexids penetrating the early in wear, with protostylids forming as angulate bends in p3–m2; metaconids and metastylids are subequal and rounded, separated by U-shaped linguaflexids. Across species, enamel folding increases, with features like a well-developed and pli caballin on molars in P. leonensis and stronger plications in M. tertius, facilitating prolonged grinding of grasses. These cranial and dental traits distinguish Merychippus from antecedents, showing greater hypsodonty (crowns approximately 50% taller than in Parahippus) and more complex fossette patterns than Parahippus, while differing from derived forms like Cormohipparion goorisi in protocone spur persistence and lesser isthmus penetration. The cheek tooth row length averages ~116 mm, with premolars like dP1 measuring ~13 mm and persisting as two-rooted structures. Such adaptations underscore Merychippus' role in the equid radiation, enabling efficient processing of tougher vegetation.

Evolutionary Role

Phylogenetic Relationships

Merychippus emerged during the Middle Miocene (approximately 17 to 11 million years ago) in as a transitional within the subfamily of the family , representing the first equids adapted for through the development of cheek teeth. This adaptation facilitated the exploitation of expanding grasslands, positioning Merychippus as a key node in equid evolution. However, cladistic analyses have demonstrated that the Merychippus, as traditionally defined, is paraphyletic—a grade of basal taxa rather than a monophyletic —encompassing diverse species that do not share a unique common exclusive to the group. The phylogenetic roots of Merychippus trace back to Early three-toed equids, particularly genera like Parahippus, which exhibited intermediate dental and locomotor features. For instance, "Parahippus" leonensis from the early Hemingfordian (around 17.5 million years ago) shares cranial characteristics with early Merychippus species such as "Merychippus" primus, suggesting a direct ancestral link through gradual hypsodonty and limb elongation. This transition reflects broader patterns in perissodactyl evolution, where browsing ancestors gave way to more , open-habitat forms. In terms of descendant relationships, Merychippus sensu stricto is recognized as a hipparionine equid, most closely allied with Nannippus and Cormohipparion within , forming a sister clade to the monophyletic tribes Hipparionini and . The Hipparionini, characterized by three functional toes, dispersed widely across and during the , while the lineage progressed toward monodactyly, giving rise to genera like Pliohippus and , and ultimately modern Equus in the Pliocene-Pleistocene. Cladistic studies using 39 morphological characters across 12 taxa confirm this topology, with the first divergences of these advanced groups occurring around 17 million years ago.

Adaptive Radiation

Merychippus represents a pivotal grade in the of the horse subfamily , marking the transition from earlier, less specialized forms to diverse grazing-adapted lineages during the middle Miocene. This radiation began in the late Hemingfordian, approximately 17.5 million years ago, with the emergence of incipiently hypsodont species that exhibited morphological innovations suited to , grass-based diets, such as increased crown heights in cheek teeth and refined postcranial features for locomotion on open plains. The , traditionally viewed as a broad assemblage of North American equids, underwent rapid , driven by climatic shifts toward cooler and drier conditions and the expansion of grasslands, which opened ecological opportunities for diversification. Phylogenetic analyses reveal Merychippus to be paraphyletic, serving as a stem group from which major clades like the Hipparionini and arose, with species such as Merychippus primus acting as the sister taxon to these advanced groups. This early phase of the radiation, spanning about 3 million years, featured high rates (0.5–1.4 per million years) and low , leading to a proliferation of forms that varied in body size, dental wear patterns, and habitat preferences across , including southern extensions into . Key innovations included the development of deeper preorbital fossae and more efficient mastication, enabling exploitation of tougher vegetation and contributing to the subfamily's peak diversity of 14–20 species by the Clarendonian. The Merychippine radiation exemplifies an explosive evolutionary event within , with diversification extending into the Barstovian (around 15–13.6 Ma) and influencing subsequent patterns, though later phases saw balanced speciation and extinction until a decline in the . This burst not only filled niches in expanding prairie ecosystems but also set the stage for global dispersal of equines, underscoring the interplay between and morphological in mammalian .

Paleobiology

Habitat and Diet

Merychippus inhabited during the Middle to epoch, approximately 18 to 10 million years ago, in transitional paleoecosystems characterized by expanding open grasslands interspersed with forested patches. This environment reflected the broader climatic shift toward warmer, drier conditions that favored the spread of prairie-like habitats across the continent. sites, such as those in the region, indicate that Merychippus coexisted with diverse communities in these mixed woodland-grassland settings, where vegetation included both C3-dominated grasses and woody plants. The diet of Merychippus consisted primarily of a mixed regime of grasses and browse, adapted to the abrasive qualities of . Dental reveals fine scratch textures consistent with consumption of low-abrasion C3 grasses, marking an early shift from pure to or mixed feeding among equids. Carbon studies of further confirm a predominance of C3 in their diet, including cool-season grasses and dicotyledonous browse rather than abrasive C4 grasses. Phytolith evidence from dental calculus provides direct support for this dietary reconstruction, showing that grass silica short-cell phytoliths from C3-pathway subfamilies (BOP clade) comprised 45–57% of samples, alongside 18–29% hardwood microfossils indicating browse inclusion. The hypsodont dentition of Merychippus, with taller crowns and complex occlusal patterns, facilitated efficient processing of tough, silica-rich vegetation in these open habitats, enabling the genus to exploit emerging grassland resources effectively.

Locomotion and Social Behavior

Merychippus exhibited adaptations in limb morphology that facilitated efficient cursorial locomotion suited to open environments. Its long, slender limbs and three-toed feet, with the central bearing most of the weight, supported rapid movement for predator evasion and over expansive ranges. The and carpal joints were hinge-like, promoting parasagittal limb motion that minimized lateral deviation and enhanced stride efficiency during trotting or galloping. Biomechanical analyses of the third metacarpal reveal that Merychippus experienced higher locomotory stresses compared to earlier equids like Mesohippus and even modern Equus, with peak compressive and bending forces concentrated during weight-bearing phases of the . These elevated stresses, estimated from geometric models of bone cross-sections and strain data from extant mammals, suggest a posture approaching full unguligrady, where the animal's weight was supported primarily on the tips rather than the full foot sole. This configuration likely allowed for greater speed and , though the reduced side toes still provided stability on uneven terrain. Canine teeth in males evolved a more pronounced, knife-like form during this period, likely used in intra-sexual combats to establish dominance hierarchies within groups, ensuring reproductive access and group stability.

Fossil Record

Discovery History

The genus Merychippus was established by American paleontologist Joseph Leidy in 1856, with the M. insignis described from an incomplete, immature bearing broken premolars (dp2 and dp3). The type specimen, housed at the Academy of Natural Sciences of (ANSP 10660), was collected by geologist V. Hayden from deposits in the Bijou Hills of southeastern (likely Charles Mix or Brule County) during a U.S. expedition to the country, led by G. K. Warren in 1855. These early finds represented one of the first documented horse fossils from the , highlighting the transition from browsing to grazing equids in North American . The name Merychippus derives from Greek roots meaning "ruminating " (meryx for and hippos for ), chosen by Leidy due to the complex, folded enamel crests on the teeth resembling those of ruminants, despite Merychippus lacking true rumination. Following Leidy's description, the genus became central to 19th-century debates on amid the "" rivalry between and , who together named dozens of additional Merychippus from sites across the western United States, including , , and . By the early , over 20 had been proposed, though many were later synonymized as research refined stratigraphic correlations and morphological distinctions.

Key Fossil Sites

Fossils of Merychippus have been recovered from numerous localities across , reflecting the genus's widespread distribution during the Middle to Late . These sites, primarily from the Barstovian to Clarendonian North American Land Mammal Ages (approximately 16 to 12 million years ago), include formations in the western and as well as , providing insights into the animal's diversification and adaptation to environments. In the region, the Sheep Creek Formation of Sioux County, Nebraska, stands out as a prolific source of Merychippus remains. This early Barstovian (about 15.9 to 14.8 million years ago) unit has yielded specimens of species such as M. primus and M. isonesus, including postcranial elements and teeth that illustrate the transition to more for . The formation's beds and volcanic tuffs have facilitated precise , making it a benchmark for correlating other faunas. Western North America hosts important Merychippus assemblages in , particularly the Barstow Formation in the . This thick sequence (over 1,000 meters) spans the early to middle Barstovian (about 15 to 14 million years ago) and contains species like M. insignis and M. carrizoensis, with well-preserved crania, dentition, and limbs from localities such as the Mud Hills and Alvord Mountain. The site's diverse , including associated grazers and predators, highlights Merychippus's role in early horse radiation amid expanding prairies. Further south in , the Kinnick Formation of the , Kern County, has produced the type specimen of M. tehachapiensis, a primitive middle (Hemingfordian-Barstovian transition, around 16 million years ago) form from Phillips Ranch locality. Recovered from gray shales and sandstones, these fossils include showing early hypsodonty, underscoring regional variations in horse evolution. In the , Florida's deposits yield Merychippus fossils from sites like the Quincy area in Gadsden County, part of the Alum Bluff Group (late Hemingfordian to early Barstovian, about 18 to 16 million years ago). Here, M. gunteri was described from fullers earth pits, with cheek teeth exhibiting moderate crown heights adapted to mixed browsing-grazing diets; this represents one of the easternmost records, indicating dispersal across subtropical environments. Colorado's Troublesome Formation in Middle Park (Clarendonian, approximately 12.7 million years ago) has provided skulls and jaws of M. sejunctus, showcasing advanced dental wear consistent with abrasive foraging. This high-elevation site offers evidence of Merychippus's adaptability to montane habitats. Extending into southern , the Matatlán and El Camarón Formations in , (late early Barstovian, about 15 million years ago), contain the southernmost Merychippus records. Localities in the Valles Centrales and Nejapa areas have yielded maxillary fragments, mandibular pieces, and isolated teeth referable to primitive Merychippus or close relatives, dated via K-Ar methods and indicating faunal exchange with northern temperate zones despite tropical conditions.

References

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