Mesonyx

{{Automatic taxobox

| fossil_range = {{Fossil range|Eocene}}

| image = Mesonyx.jpg

| image_caption = Life restoration by Charles R. Knight

| image_upright = 1.1

| taxon = Mesonyx

| authority = Cope, 1872

| subdivision_ranks = Species

| subdivision =

Mesonyx nuhetingensis
Mesonyx obtusidens
Mesonyx uqbulakensis

}}

Mesonyx ("middle claw") is a genus of extinct mesonychid mesonychian mammal. Fossils of the various species are found in Early to Late Eocene-age strata in the United States and Early Eocene-aged strata in China, 51.8—51.7 Ma (AEO).[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=42923 Mesonyx], Paleobiology Database, collection #16240, Huerfano II site, Huerfano County, Colorado, authorized by John Alroy

Description

File:Andrewsarchusscull2.jpg

Mesonyx species have been estimated as 1.25-1.5m (4.5–5 ft.) long in life, not including the tail. Weight estimates vary, from 20 to 55 kg (about 45-120 lbs).{{Cite book|last=Radinsky|first=Leonard B.|url=https://www.biodiversitylibrary.org/bibliography/5164|title=The brain of Mesonyx, a Middle Eocene mesonychid condylarth /|date=1976|publisher=Field Museum of Natural History|location=Chicago|doi=10.5962/bhl.title.5164}} Like other mesonychids, the toes ended in small hooves.{{Cite journal|last=Radinsky|first=Leonard|date=1977|title=Brains of Early Carnivores|journal=Paleobiology|volume=3|issue=4|pages=333–349|doi=10.1017/S0094837300005509|jstor=2400308|bibcode=1977Pbio....3..333R |s2cid=88286017 |issn=0094-8373}}

Like other mesonychids, they had large heads and long necks in proportion to their body size, and cranial material is most often preserved. Mesonyx uintensis from the Upper Eocene of Wyoming is described as having a total cranium length of 429 mm. (17 inches) and a facial length of 206 mm (8 inches).{{cite journal | last = Osborn | first = Henry Fairfield | author-link = Henry Fairfield Osborn | title = Andrewsarchus, giant mesonychid of Mongolia | journal = American Museum Novitates | issue = 146 | publisher = The American Museum of Natural History | date = November 11, 1924 | url = https://digitallibrary.amnh.org/items/011b33cb-d709-457c-bd6b-064b42beb19e | access-date = December 15, 2024}}{{PD-notice}} The long skull had a relatively large sagittal crest above the braincase to anchor large jaw muscles and give it a powerful bite. Brain casts show that M. obtusidens had an unusually well-developed neocortex for an Eocene mammal. Although modern Carnivora have more complex brains, their ancestors did not; Mesonyx species would have been intelligent animals for their time.

Size estimates of Mesonyx were used to generate the proposal that Andrewsarchus was the largest predatory land mammal that ever lived. Since Andrewsarchus is known only from a single isolated skull, the estimate of its size was based on scaling up material from Mesonyx. However, it is now known that the two genera are not closely related,{{Cite journal|last1=Spaulding|first1=Michelle|last2=O'Leary|first2=Maureen A.|last3=Gatesy|first3=John|date=2009-09-23|title=Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution|journal=PLOS ONE|volume=4|issue=9|pages=e7062|doi=10.1371/journal.pone.0007062|issn=1932-6203|pmc=2740860|pmid=19774069|bibcode=2009PLoSO...4.7062S|doi-access=free}} their skulls indicate different lifestyles, and they likely had very different proportions in life.

Paleoecology

While this was the earliest genus of mesonychid to be named (by Edward Drinker Cope in 1872) and the group is named after it, Mesonyx was one of the most derived genera of mesonychids, evolving features for active running.{{Cite web|url=https://scienceblogs.com/tetrapodzoology/2009/08/15/mesonychians-part-iv|title=Mesonyx and the other mesonychid mesonychians (mesonychians part IV)|date=August 15, 2009|website=scienceblogs.com|language=en|access-date=2020-02-15}} These animals had a reduced sense of smell and likely relied on sight and hearing to find food.{{Cite book|last=Radinsky|first=Leonard B.|url=https://www.biodiversitylibrary.org/bibliography/5164|title=The brain of Mesonyx, a Middle Eocene mesonychid condylarth /|date=1976|publisher=Field Museum of Natural History|location=Chicago|doi=10.5962/bhl.title.5164}} They were probably active hunters.{{Cite web|url=https://prehistoric-fauna.com/Mesonyx|title=Mesonyx|website=prehistoric-fauna.com|access-date=2020-02-15}}

Distribution and habitat

Mesonyx specimens have been unearthed in Colorado, Wyoming, Utah and China. An additional two species – Mesonyx uqbulakensis and M. nuhetingensis – have been described from the early Eocene Arshanto Formation in China.{{cite journal |author=Xun Jin |year=2012 |title=New mesonychid (Mammalia) material from the Lower Paleogene of the Erlian Basin, Nei Mongol, China |url=http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/201208/P020120803322394521641.pdf |journal=Vertebrata PalAsiatica |volume=50 |issue=3 |pages=245–257 }}

Phylogeny

In the generally accepted cladogram by Spaulding et al. (2009), Mesonyx is classified, together with other mesonychians, outside of ungulates.{{Cite journal|last1=Spaulding|first1=Michelle|last2=O'Leary|first2=Maureen A.|last3=Gatesy|first3=John|date=2009-09-23|title=Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution|journal=PLOS ONE|language=en|volume=4|issue=9|pages=e7062|doi=10.1371/journal.pone.0007062|issn=1932-6203|pmc=2740860|pmid=19774069|bibcode=2009PLoSO...4.7062S|doi-access=free}}

{{clade

|1={{clade

|grouplabel1=Mesonychia

|1={{clade

|1={{extinct}}Hapalodectes|barbegin1=blue

|2={{clade

|1={{extinct}}Sinonyx

|2={{extinct}}Mesonyx

|3={{extinct}}Harpagolestes uintensis

|4={{extinct}}Dissacus zanabazari

|5={{clade

|1={{extinct}}Dissacus praenuntius