Mesonychia

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Mesonychia
Temporal range: Early Paleocene–Early Oligocene
Harpagolestes immanis.jpg
Harpagolestes immanis skull
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Pan-Euungulata
Mirorder: Euungulata
Clade: Paraxonia
Order:† Mesonychia
Van Valen, 1966
Families

†Hapalodectidae
†Mesonychidae
†Triisodontidae [1]

Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychians first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes , became extinct in the early Oligocene. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyles before the group became extinct. [2]

Contents

Mesonychians probably originated in China, where the most primitive mesonychian, Yantanglestes , is known from the early Paleocene. They were also most diverse in Asia, where they occur in all major Paleocene faunas. Since other predators, such as creodonts and Carnivora, were either rare or absent in these animal communities, mesonychians most likely dominated the large predator niche in the Paleocene of eastern Asia.

One genus, Dissacus , had successfully spread to Europe and North America by the early Paleocene. Dissacus was a jackal-sized predator that has been found all over the Northern Hemisphere, [3] but species of a closely related or identical genus, Ankalagon , from the early to middle Paleocene of New Mexico, were far larger, growing to the size of a bear. [4] A later genus, Pachyaena , entered North America by the earliest Eocene, where it evolved into species that were at least as large. Mesonychians in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene.

Characteristics

Mesonyx model reconstruction at the Natural History Museum, London Mesonyx model reconstruction at the Natural History Museum, London.jpg
Mesonyx model reconstruction at the Natural History Museum, London

Mesonychians have often been reconstructed as resembling wolves albeit superficially, but they would have appeared very different in life. With a short lower spine stiffened by revolute joints, they would have run with stiff backs like modern ungulates rather than bounding or loping with flexible spines like modern carnivorans. While later mesonychians evolved a suite of limb adaptations for running similar to those in both wolves and deer, their legs remained comparatively thick. [5] They would have resembled no group of living animals. Early mesonychians probably walked on the flats of their feet (plantigrade), while later ones walked on their toes (digitigrade). These later mesonychians had hooves, one on each toe, with four toes on each foot. The foot was compressed for efficient running with the axis between the third and fourth toes (paraxonic); it would have looked something like a hoofed paw. [6]

Mesonychians varied in size; some species were as small as a fox, others as large as a horse. Some members of the group are known only from skulls and jaws, or have fragmentary postcranial remains. But where skeletons are known, they indicate that mesonychians had large heads with strong jaw muscles, relatively long necks, and robust bodies with robust limbs that could run effectively but not rotate the hand or reach out to the side. An unrelated early group of mammalian predators, the creodonts, also had unusually large heads and limbs that traded flexibility for efficiency in running; large head size may be connected to inability to use the feet and claws to help catch and process food, as many modern carnivorans do. Some mesonychians are reconstructed as predatory (comparable to canids), others as scavengers or carnivore-scavengers with bone-crushing adaptations to their teeth (comparable to the large hyenas), and some as omnivorous (comparable to pigs, humans, or black bears). They may not have included hypercarnivores (comparable to felids); their teeth were not as effective at cutting meat as later groups of large mammalian predators. In some localities, multiple species or genera coexisted in different ecological niches. There is evidence to suggest that some genera were sexually dimorphic. [7] Some genera may need revision to clarify the actual number of species or remove ambiguity about genera (such as Dissacus and Ankalagon). [5]

These "wolves on hooves" were probably one of the more important predator groups in the late Paleocene and Eocene ecosystems of Europe (which was an archipelago at the time), Asia (which was an island continent), and North America. Mesonychian dentition consisted of molars modified to generate vertical shear, thin blade-like lower molars, and carnassial notches, but no true carnassials. The molars were laterally compressed and often blunt, and were probably used for shearing meat or crushing bones. The largest species are considered to have been scavengers. Many species are suspected of being fish-eaters, though some of these reconstructions may be influenced by earlier theories that the group was ancestral to cetaceans.

Phylogeny and evolutionary relationships

Cladogram showing the position of the Mesonychia Cladogram of Cetacea within Artiodactyla.png
Cladogram showing the position of the Mesonychia

Mesonychians were long considered to be creodonts, but have now been removed from that order and placed in three families (Mesonychidae, Hapalodectidae, and Triisodontidae), either within their own order, Mesonychia, or within the order Condylarthra as part of the cohort or superorder Laurasiatheria. Nearly all mesonychians are, on average, larger than most of the Paleocene and Eocene creodonts and miacoid carnivorans.

The order is sometimes referred to by its older name Acreodi.

A recent study found mesonychians to be basal euungulates most closely related to the "arctocyonids" Mimotricentes , Deuterogonodon and Chriacus . "Triisodontidae" may be paraphyletic. [1]

Relationship with whales

Mesonychians possess unusual triangular molar teeth that are similar to those of Cetacea (whales and dolphins), especially those of the archaeocetes, as well as having similar skull anatomies and other morphologic traits. For this reason, scientists had long believed that mesonychians were the direct ancestor of Cetacea, but the discovery of well-preserved hind limbs of archaic cetaceans, as well as more recent phylogenetic analyses [8] [9] [10] now indicate cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychians, and this result is consistent with many molecular studies. [11] The similarity in dentition and skull may be the result of primitive ungulate structures in related groups independently evolving to meet similar needs as predators; some researchers have suggested that the absence of a first toe and a reduced metatarsal are basal features (synapomorphies) indicating that mesonychians, perissodactyls, and artiodactyls are sister groups. [5]

Most paleontologists now doubt that whales are descended from mesonychians, and instead suggest mesonychians are descended from basal ungulates, and that cetaceans are descended from advanced ungulates (Artiodactyla), either deriving from, or sharing a common ancestor with, anthracotheres (the semiaquatic ancestors of hippos). [12] However, the close grouping of whales with hippopotami in cladistic analyses only surfaces following the deletion of Andrewsarchus , which has often been included within the mesonychians. [13] [14] One possible conclusion is that Andrewsarchus has been incorrectly classified. The current uncertainty may, in part, reflect the fragmentary nature of the remains of some crucial fossil taxa, such as Andrewsarchus. [13]

Related Research Articles

<span class="mw-page-title-main">Ungulate</span> Group of animals that walk on the tips of their toes or hooves

Ungulates are members of the diverse clade Euungulata which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.

<span class="mw-page-title-main">Artiodactyl</span> Order of mammals

Artiodactyls are mammals belonging to the order Artiodactyla. Typically, they are ungulates which bear weight equally on two of their five toes: the third and fourth, often in the form of a hoof. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two is that many artiodactyls digest plant cellulose in one or more stomach chambers rather than in their intestine as perissodactyls do. The advent of molecular biology, along with new fossil discoveries, found that cetaceans fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.

<i>Andrewsarchus</i> Extinct genus of carnivorous ungulate from Eocene epoch

Andrewsarchus is an extinct genus of mammal that lived during the Middle Eocene in China. It contains two species, A. mongoliensis and A. crassum. It was formerly placed in the families Mesonychidae or Arctocyonidae, but is now the sole member of a distinct family, Andrewsarchidae. It is most notable for being estimated as the largest terrestrial, carnivorous mammal, but that status has been disputed.

<span class="mw-page-title-main">Creodonta</span> Former order of extinct flesh-eating placental mammals

Creodonta is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ancestral to the modern Carnivora, this order is now usually considered a polyphyletic assemblage of two different groups, the Oxyaenids and the Hyenodonts, not a natural group. Oxyaenids are first known from the Palaeocene of North America, while hyaenodonts hail from the Palaeocene of Africa.

<span class="mw-page-title-main">Entelodontidae</span> An extinct family of pig-like omnivores from North America and Eurasia

Entelodontidae is an extinct family of pig-like artiodactyls which inhabited the Northern Hemisphere from the late Eocene to the Middle Miocene epochs, about 38-19 million years ago. Their large heads, low snouts, narrow gait, and proposed omnivorous diet inspires comparisons to suids and tayassuids (peccaries), and historically they have been considered closely related to these families purely on a morphological basis. However, studies which combine morphological and molecular (genetic) data on artiodactyls instead suggest that entelodonts are cetancodontamorphs, more closely related to hippos and cetaceans through their resemblance to Pakicetus, than to basal pigs like Kubanochoerus and other ungulates.

<i>Pakicetus</i> Genus of ancient whales

Pakicetus is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Ypresian period, about 50 million years ago. It was a wolf-like animal, about 1 metre to 2 metres long, and lived in and around water where it ate fish and other small animals. The vast majority of paleontologists regard it as the most basal whale, representing a transitional stage between land mammals and whales. It belongs to the even-toed ungulates with the closest living non-cetacean relative being the hippopotamus.

<i>Mesonyx</i> Extinct genus of mammals

Mesonyx 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).

<i>Ichthyolestes</i> Genus of mammals

Ichthyolestes is an extinct genus of archaic cetacean that was endemic to Indo-Pakistan during the Lutetian stage. To date, this monotypic genus is only represented by Ichthyolestes pinfoldi.

<i>Sinonyx</i>

Sinonyx is a genus of extinct, superficially wolf-like mesonychid mammals from the late Paleocene of China. It is within the family Mesonychidae, and cladistic analysis of a skull of Sinonyxjiashanensis identifies its closest relative as Ankalagon. S.jiashanensis was discovered in Anhui province, China, in the Tuijinshan formation.

<span class="mw-page-title-main">Mesonychidae</span> Extinct family of mammals

Mesonychidae is an extinct family of small to large-sized omnivorous-carnivorous mammals. They were endemic to North America and Eurasia during the Early Paleocene to the Early Oligocene, and were the earliest group of large carnivorous mammals in Asia. Once considered a sister-taxon to artiodactyls, recent evidence now suggests no close connection to any living mammal. Mesonychid taxonomy has long been disputed and they have captured popular imagination as "wolves on hooves," animals that combine features of both ungulates and carnivores. Skulls and teeth have similar features to early whales, and the family was long thought to be the ancestors of cetaceans. Recent fossil discoveries have overturned this idea; the consensus is that whales are highly derived artiodactyls. Some researchers now consider the family a sister group either to whales or to artiodactyls, close relatives rather than direct ancestors. Other studies define Mesonychia as basal to all ungulates, occupying a position between Perissodactyla and Ferae. In this case, the resemblances to early whales would be due to convergent evolution among ungulate-like herbivores that developed adaptations related to hunting or eating meat.

<i>Dissacus</i> Extinct family of mammals

Dissacus is a genus of extinct carnivorous jackal to coyote-sized mammals within the family Mesonychidae, an early group of hoofed mammals that evolved into hunters and omnivores. Their fossils are found in Paleocene to Early Eocene aged strata in France, Asia and southwest North America, from 66 to 50.3 mya, existing for approximately 15.7 million years.

<i>Pachyaena</i>

Pachyaena was a genus of heavily built, relatively short-legged mesonychids, early Cenozoic mammals that evolved before the origin of either modern hoofed animals or carnivores, and combined characteristics similar to both. The genus likely originated from Asia and spread to Europe, and from there to North America across a land bridge in what is now the North Atlantic ocean. Various described species of Pachyaena ranged in size from a coyote to a bear. However, recent work indicates that Pachyaena is paraphyletic, with P. ossifraga being closer to Synoplotherium, Harpagolestes and Mesonyx than to P. gigantea.

<span class="mw-page-title-main">Hapalodectidae</span> Extinct family of mammals

Hapalodectidae is an extinct family of relatively small-bodied mesonychian placental mammals from the Paleocene and Eocene of North America and Asia. Hapalodectids differ from the larger and better-known mesonychids by having teeth specialized for cutting, while the teeth of other mesonychids, such as Mesonyx or Sinonyx, are more specialized for crushing bones. Hapalodectids were once considered a subfamily of the Mesonychidae, but the discovery of a skull of Hapalodectes hetangensis showed additional differences justifying placement in a distinct family. In particular, H. hetangensis has a postorbital bar closing the back of the orbit, a feature lacking in mesonychids. The skeleton of hapalodectids is poorly known, and of the postcranial elements, only the humerus has been described. The morphology of this bone indicates less specialization for terrestrial locomotion than in mesonychids.

<span class="mw-page-title-main">Triisodontidae</span> Extinct family of mammals

Triisodontidae is an extinct, probably paraphyletic, or possibly invalid family of mesonychian placental mammals. Most triisodontid genera lived during the Paleocene in North America, but the genus Andrewsarchus is known from the middle Eocene of Asia. Triisodontids were the first relatively large predatory mammals to appear in North America following the extinction of the non-bird dinosaurs. They differ from other mesonychian families in having less highly modified teeth.

<span class="mw-page-title-main">Arctocyonidae</span> Extinct family of mammals

Arctocyonidae is as an extinct family of unspecialized, primitive mammals with more than 20 genera. Animals assigned to this family were most abundant during the Paleocene, but extant from the late Cretaceous to the early Eocene . Like most early mammals, their actual relationships are very difficult to resolve. No Paleocene fossil has been unambiguously assigned to any living order of placental mammals, and many genera resemble each other: generalized robust, not very agile animals with long tails and all-purpose chewing teeth, living in warm closed-canopy forests with many niches left vacant by the K-T extinction.

<i>Ankalagon</i> Extinct carnivorous mesonychid mammal from New Mexico, United States

Ankalagon saurognathus is an extinct carnivorous mammal of the family Mesonychidae, endemic to North America during the Paleocene epoch, existing for approximately 3.1 million years.

<i>Didymictis</i> Extinct genus of mammals

Didymictis is an extinct genus of placental mammals from extinct subfamily Didymictinae within extinct family Viverravidae, that lived in North America and Europe from the late Paleocene to middle Eocene.

<i>Triisodon</i>

Triisodon is a genus of extinct mesonychian mammal that existed during the Early Paleocene of New Mexico, North America. The genus was named by Edward Drinker Cope in 1881 as a member of the Acreodi, a now invalid taxon that encompassed both creodonts and mesonychians. The premolar teeth have three points, hence the generic name. Cope described the type specimen of T. quivirensis as "about the size of a wolf." A smaller species has also been identified from the same region. Since material from this genus is incomplete, the exact size of adults and whether they showed sexual dimorphism or regional variations in size is unknown.

The Willwood Formation is a sedimentary sequence deposited during the late Paleocene to early Eocene, or Clarkforkian, Wasatchian and Bridgerian in the NALMA classification.

<span class="mw-page-title-main">Hyaenodonta</span> Extinct order of mammals

Hyaenodonta is an extinct order of hypercarnivorous placental mammals of clade Pan-Carnivora from mirorder Ferae. Hyaenodonts were important mammalian predators that arose during the early Paleocene in Europe and persisted well into the late Miocene.

References

  1. 1 2 Sarah L. Shelley, Thomas E. Williamson, Stephen L. Brusatte, Resolving the higher-level phylogenetic relationships of “Triisodontidae” (‘Condylarthra’) within Placentalia, October 2015, Society of Vertebrate Paleontology (abstract)
  2. ↑ "New Mesonychid mammals found from lower Paleogene of Erlian Basin, Nei Mongol". phys.org. Retrieved 2019-05-16.
  3. ↑ Jehle, Martin (2006). "Carnivores, creodonts and carnivorous ungulates: Mammals become predators". Paleocene Mammals of the World (Online).
  4. ↑ O'Leary, Maureen A.; Lucas, Spencer G.; Williamson, Thomas E. (2000). "A new specimen of Ankalagon (Mammalia, Mesonychia) and evidence of sexual dimorphism in mesonychians". Journal of Vertebrate Paleontology. 20 (2): 387–93. doi:10.1671/0272-4634(2000)020[0387:ANSOAM]2.0.CO;2. JSTOR   4524103.
  5. 1 2 3 "Mesonyx and the other mesonychid mesonychians (mesonychians part IV) | ScienceBlogs". scienceblogs.com. Retrieved 2019-05-16.
  6. ↑ O'Leary, Maureen A.; Rose, Kenneth D. (1995). "Postcranial Skeleton of the Early Eocene Mesonychid Pachyaena (Mammalia: Mesonychia)". Journal of Vertebrate Paleontology. 15 (2): 401–430. doi:10.1080/02724634.1995.10011238. ISSN   0272-4634. JSTOR   4523639.
  7. ↑ O'Leary, Maureen A.; Lucas, Spencer G.; Williamson, Thomas E. (2000-06-27). "A new specimen of Ankalagon (Mammalia, Mesonychia) and evidence of sexual dimorphism in mesonychians". Journal of Vertebrate Paleontology. 20 (2): 387–393. doi:10.1671/0272-4634(2000)020[0387:ANSOAM]2.0.CO;2. ISSN   0272-4634.
  8. ↑ Geisler, Jonathan H.; Uhen, Mark D. (2003). "Morphological support for a close relationship between hippos and whales". Journal of Vertebrate Paleontology. 23 (4): 991–6. doi:10.1671/32. JSTOR   4524409. S2CID   59143599.
  9. ↑ Geisler, Jonathan H.; Uhen, Mark D. (2005). "Phylogenetic Relationships of Extinct Cetartiodactyls: Results of Simultaneous Analyses of Molecular, Morphological, and Stratigraphic Data". Journal of Mammalian Evolution. 12 (1–2): 145–60. doi:10.1007/s10914-005-4963-8. S2CID   34683201.
  10. ↑ Boisserie, J.-R.; Lihoreau, F.; Brunet, M. (2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences. 102 (5): 1537–41. Bibcode:2005PNAS..102.1537B. doi: 10.1073/pnas.0409518102 . JSTOR   3374466. PMC   547867 . PMID   15677331.
  11. ↑ Gatesy, J.; Hayashi, C.; Cronin, M. A.; Arctander, P. (1996). "Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls". Molecular Biology and Evolution. 13 (7): 954–63. doi: 10.1093/oxfordjournals.molbev.a025663 . PMID   8752004.
  12. ↑ Geisler, Jonathan H.; Theodor, Jessica M. (2009). "Hippopotamus and whale phylogeny". Nature. 458 (7236): E1–4, discussion E5. Bibcode:2009Natur.458....1G. doi:10.1038/nature07776. PMID   19295550. S2CID   4320261.
  13. 1 2 Thewissen, J. G. M.; Cooper, Lisa Noelle; Clementz, Mark T.; Bajpai, Sunil; Tiwari, B. N. (2009). "Thewissen et al. Reply". Nature. 458 (7236): E5. Bibcode:2009Natur.458....5T. doi: 10.1038/nature07775 . S2CID   4431497.
  14. ↑ O'Leary, Maureen A.; Gatesy, John (2008). "Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): Combined analysis including fossils". Cladistics. 24 (4): 397–442. doi: 10.1111/j.1096-0031.2007.00187.x . PMID   34879630. S2CID   85141801.
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