The Evolution of Some Morphological Characters of the Upper Cheek Teeth of the Fossil Horses.

This research explores the evolution of specific morphological factors in the upper cheek teeth of fossil horses, focusing on the connection between the metaloph and ectoloph, and the development and connection of the crochet to the protoloph.

The Importance of Metaloph and Crochet in Fossil Horses

• Characters in upper cheek teeth such as the connection of the metaloph to the ectoloph, as well as the development and connection of the crochet to the protoloph, are phylogenetically significant in Equidae.
• Observations show a gradual increase in the frequency of these characteristics, from complete absence in early Equidae forms to almost complete development in later species.
• The distribution of the crochet and connected metaloph, along with the probable genetic mechanisms for their development, were analyzed using the data gathered.

Metaloph and Crochet Analysis in Different Genera

• Genera in the Hyracotheriinae sub-family, including Hyracotherium, Orohippus, Epihippus, and Haplohippus, show no connection between the metaloph and ectoloph, nor any indications of a crochet.
• In early Anchitheriinae representatives, the metaloph is still disconnected but there are specimens which have a crochet, although this crochet does not connect to the protoloph.
• The first appearance of the crochet in Equidae happened slightly earlier than the connection of the metaloph with Chadronian Mesohippus bairdi exhibiting a crochet but Mesohippus texanus lacking one.

Increased Frequency of Metaloph and Crochet in Equidae Evolution

• The Miohippus antiquus from the Anchitheriinae sub-family showed fluctuations in the frequency of the crochet, while the frequency of a connected metaloph increased swiftly to 100% in P3-M2 and M3.
• Interestingly, while Anchitherium had reached 100% frequency of connected metaloph in P3-M2 and M3, Parahippus teeth contemporary samples still had a common disconnection of the metaloph.
• This connected metaloph distinguished contemporaneous horse groups and was almost universal in Anchitherium, long before it became universal in the Parahippine predecessors of the more advanced equines.

Hypohippus-Megahippus Frequency Increase

• In Hypohippus-Megahippus, the frequency of the crochet showed some increase over that of Anchitherium.
• This could be due to the crochet becoming distinct in this line of equid evolution, although deriving from an ancestral form with higher frequencies of the crochet than Anchitherium cannot be excluded.