Parahippus tyleri

The type specimen of Parahippus tyleri was discovered by F. B. Loomis in 1907. It consists of a partial skull lacking the brain case. All the teeth are present except the upper canine and the first premolar of the upper and lower jaws (Loomis 1908). It was found in “the upper part of the Lower Harrison beds (~23mya), 8 miles northwest of Agate, Sioux Co., Nebraska” (Loomis 1908).

Parahippus tyleri has been found in Nebraska (Harrison Formation) and Wyoming (Colter Formation), in sediments dating to the Early Miocene (~23 mya) (Loomis 1908; Schlaikjer 1935; Love et al. 1976).

While Parahippus tyleri was a part of the Miocene radiation of horses, it still retained the ancestral brachydont dentition (Macfadden 2005; Mihlbachler et al. 2011). Brachydont teeth have low crowns and are usually found in animals that feed on softer vegetation and fruits. Mesowear patterns (sharpness of the cusps of the molars) of early parahippines that are related to P. tyleri suggest that these horses were feeding on the grasses that were now spreading in North America (Mihlbachler et al. 2011). This indicates that P. tyleri was probably a mixed feeder, feeding on grasses and leaves (Mihlbachler et al. 2011). Based on its probable descendant, P. leonensis, from Florida, it can be inferred that P. tyleri lived on average 3-4 years, with a maximum lifespan of about 9 years (Hulbert 1984)

Parahippus tyleriis an extinct species of horse that belongs to the family Equidae, a family within the order Perissodactyla (the order that also includes rhinos, tapirs, donkeys, modern horses and their extinct relatives). It is a small horse that weighed about 85 kg. It has three toes and teeth that were best suited for chewing soft leaves. This species belongs to a genus that underwent an adaptive radiation in the middle Miocene (~18mya) and is in the group of horses that later evolved into the domestic horse, Equus caballus.

Parahippus tyleri was a small horse that lived on the Great Plains region of North America in the early Miocene (~23 mya). It was about one-fourth smaller than P. nebrascensis, a closely related species, and weighed around 86kg (Osborn 1918; MacFadden 1986). Its skull was high and narrow, like that of a modern horse. Its teeth were brachydont, meaning that they did not have the high crowns (part above the gum) of modern horses (Osborn 1918). It had moderately long legs that ended in three toes (Osborn 1918). The body weight was supported on the middle toe (digit 3). Its body form, one with brachydont teeth, longer limbs, and three toes, represents an intermediate form between the modern day Equus caballus, and early hyracothere ancestors like Hyracotherium(Macfadden 2005).

In the grand theatre of equine evolution, Parahippustyleri is part of a very important scene. Equine evolution is not a linear progression towards the modern horse (orthogenesis), as many believed it to be in the 1800s and early 1900s. However, many texts still portray it as such. It is, in fact, a complex tree with numerous side branches (Macfadden 2005). Most of the changes and adaptive radiations seen in horse evolution can be attributed to genetic variation and environmental change acting as a selecting mechanism (Macfadden 2005). Parahippus tyleri was a member of one of these adaptive radiations that took place in the Miocene. The ancestors of the genus Parahippus were small horses, with shorter legs, and brachydont teeth (short crowned teeth that are best suited for chewing soft leaves and fruits). These horses lived in forested environments and likely fed on leaves and fruit (Mihlbachler et al. 2011). After the Oligocene-Miocene transition (~23 mya), C3 grasslands spread in North America (Strömberg 2006). This opening up of space likely allowed horses to increase in size and many species radiated into long limbed forms (MacFadden 1986; MacFadden and Hulbert 1988). It is estimated that about 19 genera evolved in the Miocene (Macfadden 2005). It is thought that Parahippus tyleri was descended from one of these early Miocene species of Parahippus, P. pristinus. Parahippus pristinus resembled earlier Oligocene horses more than the later P. tyleri (Schlaikjer 1937). While P. tyleri was a part of this Miocene radiation, it still retained the ancestral brachydont dentition and probably fed on a mixed diet of leaves and grass (Macfadden 2005; Mihlbachler et al. 2011). Its body form, one with brachydont teeth, longer limbs, and three toes, represents an intermediate form between the modern day Equus caballus, and early hyracothere ancestors, like Hyracotherium (Macfadden 2005). There appears to have been a radiation of species within the genus Parahippus from P. tyleri (Schlaikjer 1937). One line evolved into P. wyomingensis, and P. nebrascensis (Schlaikjer 1937). Another line evolved into P. atavus, and P. pawniensis, while a third line evolved into P. texanus, P. coloradensis, P. cognatus and P. leonensis. Parahippus leonensis is likely the ancestor of Merychippus (one of the earliest hypsodont horses) and the line of horses that evolved into Equus (Schlaikjer 1937).