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Home / Equine Research Bank / PeerJ / The petrosal and bony labyrinth of extinct horses (Perissoda...
PeerJ2026; 14; e20484; doi: 10.7717/peerj.20484

The petrosal and bony labyrinth of extinct horses (Perissodactyla, Equidae) and their implications for perissodactyl evolution.

Abstract: Perissodactyla, or odd-toed ungulates, are represented today by 16 species of rhinoceroses, tapirs, and horses. Perissodactyls were much more diverse in the past, having a rich fossil record spanning from the earliest Eocene (~56 Ma) to recent, including a myriad of extinct lineages. Despite over a century of study, the inter-relationships of some extinct perissodactyl families remain poorly resolved. New morphological characters are needed to help solve this issue. Recent studies suggest that the ear region, ., the petrosal and the bony labyrinth of the inner ear, is a valuable source of morphological characters for mammalian phylogenetic analyses. The petrosal is the bony structure protecting the inner ear, the organs of hearing and balance in mammals. However, perissodactyl petrosals are poorly documented and have not been used in such a phylogenetic framework. In this study, we describe the petrosals and inner ears of five European fossil equid taxa and perform a preliminary phylogenetic analysis. Despite its small sample size, our phylogenetic analysis recovers important groupings, which suggests the petrosal is phylogenetically informative in equids. This study supports the relevance of the ear region for phylogenetic inference and its potential to better resolve long-contentious relationships within Perissodactyla.
© 2026 Goodchild et al.
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Publication Date: 2026-01-05 PubMed ID: 41522509PubMed Central: PMC12782039DOI: 10.7717/peerj.20484Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study examines the petrosal bone and inner ear structures of extinct horse species to better understand evolutionary relationships among odd-toed ungulates (Perissodactyla).
  • The findings indicate that these ear region features provide valuable morphological data that can improve phylogenetic analysis of perissodactyls, helping clarify previously unclear family relationships.

Introduction and Background

  • Perissodactyla, commonly known as odd-toed ungulates, currently include 16 species such as rhinoceroses, tapirs, and horses.
  • Historically, perissodactyls were far more diverse, with a rich fossil record dating back to the early Eocene (about 56 million years ago).
  • Many extinct lineages within this order have uncertain evolutionary relationships despite extensive study over more than a century.
  • Better morphological characters—physical traits useful for comparing species—are needed to resolve these phylogenetic uncertainties.

The Focus on Ear Region Morphology

  • Recent research highlights the ear region, specifically the petrosal bone and the bony labyrinth of the inner ear, as a promising source of phylogenetic characters.
  • The petrosal bone serves as a protective casing for the inner ear, which consists of structures essential for hearing and balance.
  • These ear structures have proven valuable in understanding mammalian evolutionary relationships but have been underutilized for perissodactyls.
  • Prior to this study, documentation and use of perissodactyl petrosals in phylogenetic contexts were limited.

Study Objectives and Methods

  • The study focuses on describing the petrosal bone and inner ear anatomy of five European fossil species of equids (horses).
  • A preliminary phylogenetic analysis was conducted using these morphological data to test the usefulness of ear region characters.
  • The analysis aimed to evaluate whether petrosal and inner ear morphology can reveal meaningful evolutionary relationships within extinct horses.

Key Findings

  • Despite the small sample size, the phylogenetic analysis identified important clades or groupings among the studied taxa.
  • The results demonstrate that the petrosal bone morphology contains phylogenetically informative signals within equids.
  • This suggests that studying the ear region is a promising approach to resolving evolutionary questions related to extinct perissodactyl lineages.

Implications for Perissodactyl Evolution

  • Incorporating ear region characters could enhance the resolution of phylogenetic trees for perissodactyls, addressing longstanding ambiguities.
  • This approach might lead to better understanding of how different extinct and extant families of odd-toed ungulates are related.
  • The study encourages further research on petrosal and bony labyrinth morphology across a broader range of perissodactyl species.

Conclusion

  • The petrosal bone and inner ear morphology are valuable but underexplored features for studying perissodactyl evolution.
  • This study provides evidence supporting their use in phylogenetic analyses, which could improve classification and evolutionary interpretations within this important mammalian order.

Cite This Article

APA
Goodchild OA, Rosen SN, Mennecart B, Meng J, Tissier J. (2026). The petrosal and bony labyrinth of extinct horses (Perissodactyla, Equidae) and their implications for perissodactyl evolution. PeerJ, 14, e20484. https://doi.org/10.7717/peerj.20484

Publication

PeerJ
ISSN: 2167-8359
NlmUniqueID: 101603425
Country: United States
Language: English
Volume: 14
Pages: e20484
PII: e20484

Researcher Affiliations

Goodchild, Owen Axel
  • Richard Gilder Graduate School, American Museum of Natural History, New York, New York, United States.
Rosen, Sydney Nicole
  • Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee, United States.
Mennecart, Bastien
  • Naturhistorisches Museum Basel, Basel, Switzerland.
Meng, Jin
  • Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York, United States.
Tissier, Jérémy
  • Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York, United States.
  • Palaeobiosphere Evolution Unit, Royal Belgian Institute of Natural Sciences, Brussells, Belgium.

MeSH Terms

  • Animals
  • Fossils / anatomy & histology
  • Phylogeny
  • Biological Evolution
  • Horses / anatomy & histology
  • Ear, Inner / anatomy & histology
  • Perissodactyla / anatomy & histology
  • Perissodactyla / classification
  • Petrous Bone / anatomy & histology
  • Extinction, Biological

Conflict of Interest Statement

The authors declare that they have no competing interests.

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