Condylarthra

Arctocyon, a plantigrade ungulate, was once classified as a condylarth.

Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs.<ref name=McK&B>Template:Harvnb</ref> They are considered early, primitive ungulates and is now largely considered to be a wastebasket taxon, having served as a dumping ground for classifying ungulates which had not been clearly established as part of either Perissodactyla or Artiodactyla, being composed thus of several unrelated lineages.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Cooper2014">Template:Cite journal</ref><ref name=Janis93>Template:Harvnb</ref>

Taxonomic historyEdit

Condylarthra always was a problematic group. When first described by Template:Harvnb, Phenacodontidae was the type and only family therein. Template:Harvnb, however, raised Condylarthra to an order and included a wide range of diverse placentals with generalized dentitions and postcranial skeletons. More recent researchers (i.e. post-WW2) have been more restrictive; either including only a limited number of taxa, or proposing that the term should be abandoned altogether.<ref name="Thewissen-1990-20">Template:Harvnb</ref> Due to their primitive characteristics condylarths have been considered ancestral to several ungulate orders, including the living Artiodactyla, Cetacea, Perissodactyla, Hyracoidea, Sirenia, and Proboscidea, as well as the extinct Desmostylia, Embrithopoda, Litopterna, Notoungulata, and Astrapotheria.<ref name=RoseArchaicUng>Template:Cite book</ref>

Template:Harvnb delimited condylarths as those having the following characters, but lacking the specializations present in more derived orders:<ref name="Thewissen-1990-20" />

superior ramus of stapedial artery shifted to petrosal or lost
mastoid foramen lost
bulla if present composed of ectotympanic
relatively bunodont teeth with low cusp relief
trigonids of lower molars shortened anteroposteriorly
large, posteriorly projecting hypoconulid on M₃ (lower third molar)
head of astragalus is short and robust

Evolutionary historyEdit

File:Ectocion Ralstonensis.jpg

Ectocion, a small plant-eating phenacodontid found in Wyoming

The disappearance of the non-avian dinosaurs opened up an ecological niche for large mammalian herbivores. Some condylarths evolved to fill the niche, while others remained insectivorous. This may explain, in part, the tremendous evolutionary radiation of the condylarths that we can observe throughout the Paleocene, resulting in the different groups of ungulates (or "hoofed mammals") that form the dominant herbivores in most Cenozoic animal communities on land, except on the island continent of Australia.

Among recent mammals, Paenungulata (hyraxes, elephants, and sea cows), Perissodactyla (horses, rhinoceroses, and tapirs), Artiodactyla (pigs, deer, antelope, cows, camels, hippos, and their relatives), Cetacea (whales), and Tubulidentata (aardvarks) are traditionally regarded as members of the Euungulata.<ref name=McK&B/><ref name=Novacek86>Template:Harvnb</ref> Besides these, several extinct animals also belong to this group, especially the endemic South American orders of ungulates, (Meridiungulata). Although many ungulates have hooves, this feature does not define the Euungulata. Indeed, some condylarths had small hooves on their feet, but the most primitive forms are clawed.

Recent molecular and DNA research has reorganised the picture of mammalian evolution. Paenungulates and tubulidentates are seen as afrotherians, and no longer seen as closely related to the laurasiatherian perissodactyls, artiodactyls, and cetaceans,<ref name=Madsen01>Template:Harvnb</ref><ref name=Murphy01>Template:Harvnb</ref> implying that hooves were acquired independently (i.e. were analogous) by at least two different mammalian lineages, once in the Afrotheria and once in the Laurasiatheria. Condylarthra itself, therefore, is polyphyletic: the several condylarth groups are not closely related to each other at all. Indeed, Condylarthra is sometimes regarded as a 'wastebasket' taxon.<ref name=Janis93>Template:Harvnb</ref> True relationships remain in many cases unresolved.

In addition to meridiungulates and living ungulates, a condylarthran ancestry has been proposed for several other extinct groups of mammals, including Mesonychia<ref name=VanValen66>Template:Harvnb</ref> and Dinocerata.<ref name=VanValen88>Template:Harvnb</ref>