New frontiers and perspectives in stydying fossil and extant Equus

Morphologic and morphometric analyses are one of the most powerful tool to investigate the variety of biological processes in living and extinct species, with special remarks also to evaluate the relationships in single or multiple populations. Widespread taxa often present great phenotypic plasticity, with morphological variations related to the biotic and abiotic gradients of the heterogeneous environment present through their geographic extension. However, those ecomorphotypes not always extends to evaluate the species level. Taxonomy and systematics would be greatly benefited by a higher understanding of those morphological variations, specially through an individual trait analysis. Here, we propose the study of extinct and living Equus as a model system for trait biogeography.

Despite over a century of work, there is still no complete consensus on Equus taxonomy nor phylogeny among specialists. It is hypothesized that North American Equus can be divided into stilt and stout legged horses, but there is still a need for a better understanding of the groups as most work so far has focused on provincial horse faunas.

Frontiers in Ecology and Evolution special volume “Examining Evolutionary Trends in Equus and its Close Relatives from Five Continents” has combined the most recent and updated works and global overview of the genus Equus, and along with more recent papers it has become clear that there is still need for a more comprehensive understanding of the genus. This project, then, aims to gather Equus specialists from around the globe to discuss the most recent results and future perspectives on Equus’ evolution.

Considering the workshop time frame and the goal of having at least some preliminary results by its conclusion, we propose a comparative study of Equus from around the globe to investigate the proportion of the limb bones and how does it change among the species. Recent molecular work revealed signals of bidirectional dispersion between caballine horse populations between North America and Eurasia which brings back the question of the origin of the stout-legged Equus. Understanding the distinction of stilt and stout legged horses is an important step to recognize when did the former become replaced by the latter in North American, Eurasia and Africa, as well as the differences in molecular versus morphologically based reconstructions. It is important, then, to evaluate the variability and biogeography of the limb bones traits.

FuTRES has incorporated the Equid Bernor Global Cenozoic database, collected principally by Raymond Louis Bernor and recently by Omar Cirilli, and is on its way to incorporate specimens collected from data mining horse literature. This background should provide enough data to have preliminary results on Equus intra and interspecific variability. Helena Machado will organize new data assembly of New World Equus and Hippidion data to augment data already acquired for her PhD research project on the evolutionary relationships of North, Central and South American Equus. We invision that she will develop a database in parallel with Bernor’s database which is an objective of the FuTRES 1 NSF funded project.

This project will allow us to engage with the FuTRES ecosystems as it will either add more data or analyze pre-existing ones. Moreover, the comparative study on Equus limb bones dialogues directly with Raymond Louis Bernor FuTRES project on Equid Locomotion.

The FuTRES workshop will allow an open and important collaboration among the experts for which we propose the following topics to be covered throughout the workshop sessions:
- Paleobiogeography: to give the opportunity to exchange and develop data leading to increased knowledge of the Equus fossil record in Americas, Eurasia and Africa as well as, hopefully, integrating molecular, extant and fossil data.
- Morphometrics: to discuss the application of analytical and statistical tools for morphometrical analyses. This would provide an important opportunity to discuss the intra and interspecific variability among all the Equus species.
- Phylogeny: to discuss the most recent results on Equus phylogeny, hopefully integrating molecular data with skeletal morphological data.
- Paleoecology: to discuss and present the most recent advances on body size and paleodiet on fossil Equus species. It would be interesting to focus the discussion on how the ecological factors and the environment play a role in the adaptation both on diet and skeleton elements following the results of Saarinen et al. (2021).

The overall outcomes expected from this workshop are:
- update and extend the 2020 Frontiers Equus volume
- database development
- an application to NSF for a RCN (Research Coordinated Network) following the 1981 AMNH International Hipparion Research Initiative wherein Equid measurements were defined and used by the Equid community (Eisenmann et al., 1988; Bernor et al., 1997)

The project will be conducted throughout the workshop time frame, and we propose the following organizational structure for the Project Working Sessions:

August 20: introduction and topics (listed above) discussion

September 17: topics and project discussion

October 22 and November 19:analyzing results

December 10: reports from the working session.