Howenegg Hippotherium primigenium: geological context, cranial and postcranial morphology, palaeoecological and biogeographic importance

Howenegg is an early Vallesian (MN9, 10.3 Ma) vertebrate locality in Hegau, Southwest Germany renowned for its preservation of complete mammalian skeletons, diverse invertebrate and plant fossils. We provide the first to be published photographic images of the Howenegg Hippotherium primigenium skulls, mandibles and dentitions for describing critical character states used to define hipparion species. We compare these states to those for North American Cormohipparion occidentale, Turkish Cormohipparion sinapensis Algerian 'Cormohipparion' africanum, Austrian Pannonian C Hippotherium sp., Austrian locality Inzersdorf Hippotherium primigenium, the China type specimens of Hippotherium weihoense and 'Hipparion' chiai, and Moldovan Cremohipparion moldavicum. Also provided are univariate statistical comparisons of cranial-dental characters and Log10 ratio analyses of third metapodials dimensions to better evaluate taxonomic comparisons and define the genetic pool from which Old World hipparions are derived. We concur with previous authors that North American Cormohipparion is the likely source of first occurring Old World hipparions offering alternatives of Cormohipparion occidentale or Cormohipparion quinni as the most closely related species for the Old World Cormohipparion Datum. We find that the best evidence for the chronology of the Cormohipparion Datum suggests and age of 11.4-11.0 Ma, or 11.2-11.1 Ma rather than the 11.5 Ma datum recently alleged for China.

Automated summary

Howenegg in Southwest Germany is known for its well-preserved mammalian skeletons, invertebrate and plant fossils. In this study, photographic images of Hippotherium primigenium skulls, mandibles, and dentitions from Howenegg are provided to describe important character states used to define hipparion species. Comparisons with other specimens from North America, Turkey, Austria, China, and Moldova are made, and statistical analyses are conducted to improve taxonomic comparisons and understand the origins of Old World hipparions. The results suggest that the first occurring Old World hipparions may have originated from North American Cormohipparion, and provide new evidence for the chronology of hipparion evolution in the Old World.