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''Dimacrodon'' is an extinct
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of non-
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
from the latest
Early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...
San Angelo Formation The San Angelo Formation is a geologic formation in Texas. It preserves fossils dating back to the Permian period. Along with the Chickasha Formation is one of the two geologically youngest formations in North America to preserve fossils of case ...
of
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
. It is distinguished by toothless, possibly beaked jaw tips, large lower canines and a thin bony crest on top of its head. Previously thought to be an
anomodont Anomodontia is an extinct group of non-mammalian therapsids from the Permian and Triassic periods. By far the most speciose group are the dicynodonts, a clade of beaked, tusked herbivores. Anomodonts were very diverse during the Middle Pe ...
therapsid related to
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...
s, it was later found to lack any diagnostic features of anomodonts or even therapsids and instead appears to be a '
pelycosaur Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used, and Pelycosauria was considered an order, but this is now thoug ...
'-grade synapsid of uncertain classification.


Description

Three specimens of ''D. hottoni'' are known, although they only comprise incomplete portions of the
lower jaw In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
and
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
. The
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
specimen consists of a single partial
mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
with teeth. The jaw is long with an unusually broad and deep
mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral ha ...
, while the jaw rami are slender (although the additional material from ''D.'' sp. below suggests that the back of the jaws was also deeper). Unusually, the front of the jaw is toothless and has a rough bone texture, possibly supporting a beak like in dicynodonts, with no pre-
canine teeth In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed teeth. In the context of the upper jaw, they are also known as '' fangs''. They can appear more fl ...
. Behind the toothless region is a single pair of large caniniforms, while the teeth behind them are much smaller and uniform in size, unlike typical 'pelycosaur' dentition (although see the
edaphosaurid Edaphosauridae is a family of mostly large (up to or more) Late Carboniferous to Early Permian synapsids. Edaphosaur fossils are so far known only from North America and Europe. Characteristics They were the earliest known herbivorous amniotes ...
''
Gordodon ''Gordodon'' (meaning "fat tooth", referring to its large incisor-like front teeth) is an extinct genus of non-mammalian synapsid that lived during the Cisuralian, Early Permian of what is now Otero County, New Mexico, Otero County, New Mexico. ...
''), and are
thecodont Thecodontia (meaning 'socket-teeth'), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of early archosaurian reptiles that first appeared in the latest Permian period and flourished until the end of t ...
like the teeth of therapsids. An additional specimen referred to ''Dimacrodon'' sp. includes more extensive, but still incomplete material from the rear of skull and margins of the upper jaw. The back of the skull is deep and broad, with a large
temporal fenestra Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly (al ...
that is roughly trapezoid in shape and slopes down and back from the roof of the skull. There is a small, thin parietal crest of bone in the middle of the skull between the eyes and temporal fenestra. What is known of the
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
suggests that it had a broad, flattened plate that opposed the beak-like lower jaw and likewise was also toothless. A jaw fragment containing two large "canines" are also known from this specimen, although it is unknown if it is from the lower or upper jaw. This skull is believed to be the largest-known specimen of ''Dimacrodon'', and was estimated by Olson (1962) to be approximately in length when restored.


Classification

Olson and Beerbower tentatively assigned ''Dimacrodon'' to Therapsida in 1953 (along with other unusual synapsids from the San Angelo Formation) due to its therapsid-like teeth and general resemblance to various therapsids. The jaw structure was similar to the
dinocephalia Dinocephalians (terrible heads) are a clade of large-bodied early therapsids that flourished in the Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinction event. ...
n ''
Titanophoneus ''Titanophoneus'' ("titanic murderer") is an extinct genus of carnivorous dinocephalian therapsid from the Middle Permian. It is classified within the family Anteosauridae. The type species is ''Titanophoneus potens''. Remains of ''Titanophone ...
'', the toothless beak-like jaw tips similar to dicynodonts, and they also noted general similarities to
gorgonopsia Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of Saber-toothed predator, sabre-toothed therapsids from the Middle Permian, Middle to the Upper Permian, roughly between 270 and 252 million years ago. ...
ns as well. However, it could not be attributed to any one of these groups, and so they did not classify it beyond being a probable therapsid. They suggested that it represented a synapsid that had independently reached the therapsid "level of organisation" from the better-known Russian and South African therapsids, reflecting the pre-
cladistics Cladistics ( ; from Ancient Greek 'branch') is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesiz ...
method of classification employed at the time. Olson later provided a more definitive classification in 1962 when he assigned it to Anomodontia, specifically to the
infraorder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between Family_(biology), family and Class_(biology), class. In biological classification, the order is a taxonomic rank used in the classific ...
Venyukovioidea Venyukovioidea is an infraorder of anomodont therapsids related to dicynodonts from the Permian of Russia. They have also been known as Venjukovioidea, as well as by the similar names Venyukoviamorpha or Venjukoviamorpha in literature. This in pa ...
, and erected its own
monogeneric In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
, the Dimacrodontidae. Olson also described an additional specimen in 1962 which he attributed to a possible second species as ''D.'' sp., based on possible presence of two caniniform teeth in the tooth row, as well as variations in the spacing and shape of the postcanine teeth and the overall size difference. ''D. hittoni'' was characterised as having only a single lower "canine" tooth in each jaw; however, the number of upper "canines" is unknown. Two lower "canines" could distinguish ''D.'' sp. from ''D. hittoni'', although it is possible that ''D. hittoni'' had two upper "canines" and the jaw fragment is simply from the upper jaw of ''D. hittoni''. It is unclear then if ''D.'' sp. truly represents a distinct species or if it is simply a large specimen of ''D. hittoni''. ''Dimacrodon'' was mistakenly assigned to Anomodontia primarily because of its broad, toothless mandibular symphysis, which resembled the beaks of dicynodonts. Olson also interpreted the deep jaw tips and cheek teeth as resembling those of the Russian anomodont '' Venyukovia'', leading him to group them together in the infraorder Venyukovioidea (although unlike ''Dimacrodon'', venyukoviids possess front teeth). It was also briefly regarded as a dinocephalian in
Tapinocephalia The Tapinocephalia are one of the major groups of dinocephalian therapsids and the major herbivorous group. Tapinocephalia has been found to consist of three clades: Styracocephalidae, Titanosuchidae, and the very successful Tapinocephalidae. ...
by
Robert L. Carroll Robert "Bob" Lynn Carroll (May 5, 1938 – April 7, 2020) was an American–Canadian vertebrate paleontologist who specialised in Paleozoic and Mesozoic amphibians and reptiles. Biography Carroll was an only child and grew up on a farm nea ...
in 1988, but this was never elaborated upon. In 1995, Sidor and Hopson revisited the San Angelo therapsids and reported that all of them were likely to be based on 'pelycosaur' material and that none of them were therapsids, including ''Dimacrodon''. This re-identification has been accepted and followed since then; however, features of its skull such as the differentiated teeth and large temporal fenestra have been described as therapsid-like. No further studies on ''Dimacrodon'' have been carried out since then, and it remains unclear what its true
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
relationships are amongst early-derived therapsids.


Palaeoecology

In the upper San Angelo Formation, ''Dimacrodon'' co-existed with and was found alongside the abundant giant
herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
caseid Caseidae are an extinct family of basal synapsids that lived from the Late Carboniferous to Middle Permian between about 300 and 265 million years ago. Fossils of these animals come from the south-central part of the United States (Texas, Oklaho ...
''
Cotylorhynchus ''Cotylorhynchus'' is an Extinction, extinct genus of herbivorous Caseidae, caseid synapsids that lived during the late Cisuralian, Lower Permian (Kungurian) and possibly the early Guadalupian, Middle Permian (Roadian) in what is now Texas and Ok ...
'', along with the
captorhinid Captorhinidae is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Captorhinids are a clade of ...
'' Rothianiscus'' and similarly enigmatic synapsid '' Tappenosaurus''. The upper San Angelo Formation has been interpreted as a near-shore terrestrial environment, cut by numerous channels and experiencing periods of flooding, during which the bones of ''Dimacrodon'' were likely deposited. Olson and Beerbower regarded ''Dimacrodon'' as a
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...
, although the beak-like jaws were noted as being more similar to the herbivorous dicynodonts.


See also

*
List of pelycosaurs This list of pelycosaurs is an attempt to create a comprehensive listing of all Genus, genera that have ever been included in the synapsida excluding therapsida and purely vernacular terms. The list includes all commonly accepted genera, but also g ...


References

Bibliography: *


External links


''Dimacrodon''
in the
Paleobiology Database The Paleobiology Database (PBDB) is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms. History The Paleobiology Database originated in the NCEAS-funded Phanerozoic Marine Pale ...
. {{Taxonbar, from=Q5277143 Cisuralian synapsids of North America Taxa named by Everett C. Olson Fossil taxa described in 1953 Cisuralian genus first appearances Cisuralian genus extinctions Prehistoric synapsid genera