Dicynodont Descriptive Essay

Source: Unusual tusks in a new species of dicynodont from the Permian of Brazil

Featured image above courtesy Mario Quiñones Faúndez

May has been a spectacularly busy month. And a lot of awesome research was recently published on PLOS ONE and other open access journals (check out our Fossil Friday Roundup feature for lists of the latest publications and news) while I was out traversing across the state of Utah (looking at fish fossils and dinosaur tracks – more on that later!). But now that I’ve returned from traveling (for now), and Fossil Friday Roundup is successfully off and running, I’m back and ready to share with you the latest paper, published today in PLOS ONE.

The skull of Rastodon procurvidens, holotype UNIPAMPA PV147P. Image courtesy Felipe Lima Pinheiro.

The study, by authors Alessandra Boos, Christian Kammerer, Cesar Schultz, Marina Soares, and Ana Ilha, examines a beautifully preserved dicynodont skull from the Permian of Brazil, and with this skull comes a lot of new information and implications regarding the evolutionary relationships of these herbivorous therapsids that existed globally from the Middle Permian through the end of the Triassic. The richest record with regard to abundance and diversity of these organisms is recorded from the Beaufort Group of South Africa, but the record from Brazil has been less extensive, with only one Permian-age specimen (Endothiodon) previously recorded from the Paraná Basin, and three genera from the Triassic.

This new species, dubbed Rastodon procurvidens by the authors, is now the second known taxon from the Permian of Brazil. The description is based on a single specimen, but a beautiful one at that, with an almost complete skull and lower jaws. It was collected on a private farm in Rio Grande do Sul, from the Guadalupian/Lopingian Rio de Rasto Formation. The skull is slightly crushed dorsoventrally, but is still so well preserved that it is easy to distinguish characteristics that define it as a dicynodont.

Rastodon procurvidens tusks. Photographs of the specimen in right lateral (A) and ventral (B, left side up) views, with a close-up of the caniniform tusks showing their curved morphology and tips directed anteriorly. Scale bars equal 3 cm. From Boos et al (2016).

What makes Rastodon unusual, however, is it’s unique arrangement of the tusks. Unlike other dicynodonts, the tusks of Rastodon are extremely small and curved forwards, with the tip of each tusk directed towards the front of the snout. The authors note that this is not due to pathological or taphonomical deformation because this morphology is present on both sides of the skull and fits nicely into an embayment that is present on the caniniform process of Rastodon, and each tooth displays fine striations that match the curvature of the tooth, with no cracks that would suggest deformation of the teeth.

Procurved teeth are present in one other taxon, the study notes. Abajudon, a dicynondont from Tanzania, possesses procurved postcanines, not tusks like Rastodon. In both instances, however, the function or purpose of these teeth remains unclear. Boos et al (2016) does note that these unusual tusks must have played a role when chewing because they inner (lingual) surface would have contacted the lower jaw during mastication.

Phylogenetic position of Rastodon procurvidens within Dicynodontia based on the results of the phylogenetic analysis.

The phylogenetic study of Boos et al (2016) recovered Rastodon as being a member of the dicynodont sublcade Bidentalia. The skull of Rastodon is very generalized when compared to other members of Bidentialia, but nevertheless is united with this group based on some shared derived characters outlined in the paper. More importantly what this study suggests is that Bidentalia must have split by the middle Permian, but this split must have occurred outside of Africa, as members of this group are absent in Guadalupian-age rocks of Africa. Rastodon represents the first basal bidentalian from the Middle Permian, as all other basal bidentalian dicynondonts are late Permian in age.

This beautiful specimen illustrates the need to examine more middle Permian deposits from South America, Asia, and African basins outside of South Africa. The origin of this diverse clade is still a larger mystery, and as the discovery of Rastodon shows, there is still much to learn about the origin and evolution of these unusual herbivorous tetrapods.

Read the original paper, published today in PLOS ONE:
Boos ADS, Kammerer CF, Schultz CL, Soares MB, Ilha ALR (2016) A New Dicynodont (Therapsida: Anomodontia) from the Permian of Southern Brazil and Its Implications for Bidentalian Origins. PLoS ONE 11(5): e0155000. doi:10.1371/ journal.pone.0155000

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This article is about the animal class. For other uses, see Mammal (disambiguation).

"Mammalian" redirects here. For the 2010 documentary film, see Mammalian (film).

Mammals are the vertebrates within the classMammalia ( from Latinmamma "breast"), a clade of endothermicamniotes distinguished from reptiles (including birds) by the possession of a neocortex (a region of the brain), hair, three middle ear bones, and mammary glands. Females of all mammal species nurse their young with milk, secreted from the mammary glands.

Mammals include the largest animal on the planet, the blue whale. The basic body type is a terrestrial quadruped, but some mammals are adapted for life at sea, in the air, in trees, underground or on two legs. The largest group of mammals, the placentals, have a placenta, which enables the feeding of the fetus during gestation. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30-meter (98 ft) blue whale. With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents, bats and Soricomorpha (shrews and allies). The next three biggest orders, depending on the biological classification scheme used, are the Primates (apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).

Living mammals are divided into the Yinotheria (platypus and echidnas) and Theriiformes (all other mammals). There are around 5450 species of mammal, depending on which authority is cited. In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria. While mammal classification at the family level has been relatively stable, several contending classifications regarding the higher levels—subclass, infraclass and order, especially of the marsupials—appear in contemporaneous literature. Much of the changes reflect the advances of cladistic analysis and molecular genetics. Findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora.

The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade. The early synapsid mammalian ancestors were sphenacodontpelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been among the dominant terrestrial animal groups from 66 million years ago to the present.

Some mammals are intelligent, with some possessing large brains, self-awareness and tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.

In human culture, domesticated mammals played a major role in the Neolithic revolution, causing farming to replace hunting and gathering, and leading to a major restructuring of human societies with the first civilizations. They provided, and continue to provide, power for transport and agriculture, as well as various commodities such as meat, dairy products, wool, and leather. Mammals are hunted or raced for sport, and are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, and appear in literature, film, mythology, and religion. Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, though there are efforts to combat this.

Classification[edit]

Main article: Mammal classification

See also: List of placental mammals, List of monotremes and marsupials, and List of mammal genera

Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.[1]George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals.[2]

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemurs; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies.[3] According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders.[3] In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species.[4] According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 recently extinct.[5]

Definitions [edit]

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latinmamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.[6] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.[7] If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.[8]

T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.[9] The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.[10][11]

McKenna/Bell classification[edit]

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,[12] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.[2]

Extinct groups are represented by a dagger (†).

Class Mammalia

  • Subclass Prototheria: monotremes: echidnas and the platypus
  • Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
    • Infraclass †Allotheria: multituberculates
    • Infraclass †Eutriconodonta: eutriconodonts
    • Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
      • Superlegion †Kuehneotheria
      • Supercohort Theria: live-bearing mammals
        • Cohort Marsupialia: marsupials
        • Cohort Placentalia: placentals

Molecular classification of placentals[edit]

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposonpresence/absence data.[14] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible different hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria).[15] Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial)[16] and varying interpretations of paleogeographic data.[15]

The cladogram above is based on Tarver et al. (2016)[17]

Group I: Superorder Afrotheria[18]

Group II: Superorder Xenarthra[18]

  • Order Pilosa: sloths and anteaters (neotropical)
  • Order Cingulata: armadillos and extinct relatives (Americas)

Group III: Magnaorder Boreoeutheria[18]

  • Superorder: Euarchontoglires (Supraprimates)
  • Superorder: Laurasiatheria
    • Order Eulipotyphla: shrews, hedgehogs, moles, solenodons
    • CladeScrotifera
      • Order Chiroptera: bats (cosmopolitan)
      • CladeFereuungulata
        • CladeFerae
          • Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
          • Order Carnivora: carnivores (cosmopolitan), including cats and dogs
        • CladeEuungulata

Evolution[edit]

Main article: Evolution of mammals

Origins[edit]

Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from reptilian and avian lineages. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.[19]

Evolution from amniotes[edit]

The first fully terrestrial vertebrates were amniotes. Like their amphibious tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous. They descended from earlier reptiliomorph amphibious tetrapods,[20] which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians, dinosaurs and birds.[21] Synapsids have a single hole (temporal fenestra) low on each side of the skull. One synapsid group, the pelycosaurs, included the largest and fiercest animals of the early Permian.[22] Nonmammalian synapsids are sometimes called "mammal-like reptiles".[23][24]

Therapsids, a group of synapsids, descended from pelycosaurs in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates.[23] They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs.[23] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their pelycosaur ancestors and ending with probainognathiancynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:[25]

  • The gradual development of a bony secondary palate.
  • Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
  • The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

First mammals[edit]

The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.

The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.
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