Dinocephalia

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Parent taxa:
(check the following menu and phylogeny - the taxon in bold refers to the topic on this page)


THERAPSIDA
Taxonomy Phylogeny

Subphylum: Vertebrata
Class: "Reptilia"
Subclass: Synapsida
Order THERAPSIDA 

o Sphenacodontoidea
`--+?-Tetraceratops
   `==Biarmosuchia
      `--o Eutherapsida
         |--Dinocephalia
         `--o Neotherapsida
            |--o Anomodontia
            |  |==Venyukoviamorpha
            |  `--Dicynodontia
            `--o Theriodontia
               |--Gorgonopsia 
               `--o Eutheriodontia
                  |--Therocephalia
                  `==Cynodontia
                    `--MAMMALIA


Dinocephalia


The large omnivorous Gondwanan titanosuchid Jonkeria (Dinocephalia, Tapinocephalia, Titanosuchidae,) enjoying a mudbath. This animal reached upto 4 meters or more in length. Illustration © Stephen Priestley


Contents

Introduction

The Permian Dinocephalia - the name means "terrible head" and is a reference to their fierce appearance - were in many ways the most archaic of the higher therapsids, and among the earliest as well. They include carnivorous, herbivorous, and omnivorous forms, some semi-aquatic and some fully terrestrial, and were also among the largest animals of the Permian period; only the biggest Caseidae and Pareiasauridae rivalling or even exceeding them in size.


(The following menu and phylogeny refers to subtopics of this page)


DINOCEPHALIA
Taxonomy Phylogeny 
"Biarmosuchia"
 `--o Dinocephalia
    |?-Stenocybus
    |?-Estemmenosuchidae
    `--+--Anteosauria
       `--o Tapinocephalia
          |--Styracocephalus
          `--+--Titanosuchidae
             `--Tapinocephalidae


Basic description

Although these synapsids showed therapsid adaptations such as the expansion of the ilium and more erect limb posture, they retained various primitive pelycosaur characters. For example they had no secondary palate, and their dentary was of moderate size. In the more mammal-like group, the cynodonts, the dentary grew increasingly large, until it constituted the entire lower jaw, which is the mammalian condition.

All dinocephalians are distinguished by having interlocking incisors allowing a shearing contact between upper and lower teeth. In more advanced forms, the heels on the lingual sides of the incisor teeth met to form a crushing surface when the jaws were shut, allowing the grinding up of plant matter.

Most dinocephalians also developed pachyostosis, or thickening, of the bones in the skull, which seems to have been an adaptation for intra-specific behaviour (head-butting), perhaps for territory or a mate. In some types, such as Estemmenosuchus and Styracocephalus there are also horn-like structures, which evolved independently in each case.

Dinocephalians are extraordinary for their large size. The biggest herbivores (Tapinocephalus) and omnivores (Titanosuchus), may have massed up to two tonnes in weight, and were some 4.5 meters long, about the size of an adult rhinoceros. While the largest carnivores (such as Titanophoneus and Anteosaurus) were at least as long, with heavy skulls 80 cm long, and overall weights of around half a tonne.

Information

Linnaean taxonomy:

Author: Seeley, 1895

Stratigraphic range: Middle Permian (Roadian to Capitanian)

Environment: Terrestrial, Semi-aquatic

Geographic distribution: Cosmopolitan

Specialised feeding apparatus

All Dinocephalians are distinguished by the interlocking incisor (front) teeth. Correlated features are the distinctly downturned facial region, deep temporal region, and forwardly rotated suspensorium. In the anteosaur Titanophoneus and otehr species, the lower jaw closes with a simple hinge movement at the jaw articulation.

Shearing contact between upper and lower teeth (allowing food to be more easily sliced into small bits for digestion) is achieved through keeping a fixed quadrate and a hinge-like movement at the jaw articulation. The lower teeth are inclined forward, and occlusion is achieved by the interlocking of the incisors.

The later dinocephalians improved on this system by developing heels on the lingual sides of the incisor teeth which met against one another to form a crushing surface when the jaws were shut.

The dinocephalians were most certainly originally carnivorous, as represented by the anteosaurs, and basal therapsids (Bairmosuchia), but even the earliest Estemmenosuchids, and then the somewhat later titanosuchids and tapinocephalids adapted to a herbivorous life-style, replacing the big Caseid pelycosaurs as the dominant vertebrate plant eaters. In all dinocephalians the synapsid opening for attachment of jaw muscles remained relatively small, and it is assumed that the power of the bite was provided by the sheer mass of the animals' jaw and muscles. This was a less efficient system than that developed by the anomodonts and theriodonts, but it clearly worked well, because these creatures dominated the large herbivore and large carnivore niche for some millions of years.

Classification

A number of writers have included the dinocephalians in the suborder Anomodontia, but it is now acknowledged they constitute a separate group. The dinocephalians are divided into three main groups, the mostly carnivorous Anteosauria (which included giants up to 5 meters in length), and the medium to very large herbivorous Estemmenosuchidae and Tapinocephalia. The tapinocephalians are again divided into two branches, the titanosuchids, which were large ponderous herbivores or omnivores, and the tapinocephalians, which were equally large and ponderous but more specialized as purely herbivores, both of which reached the size of an ox or even a rhinoceros in the larger species. The tapinocephalians also have the dubious distinction to be -- almost without dispute -- the ugliest vertebrates that nature has ever produced. The anteosaurs clearly preyed largely on their estemmenosuchid and tapinocephalian cousins.

Evolutionary History

The Dinocephalians are an ancient group and their ancestry is not clear. It is assumed that they must have evolved during the earlier part of the Ufimian/Roadian, or even Kungurian epoch, from pelycosaur-like therapsids, that lived in the latest Early Permian epoch, but no trace has been found; the fauna described by Olson of this age has turned out, on reinspection, to be not therapsid but misinterpretation of caseid remains [ref. pers. communication Christian Kammerer]. Even the earliest members, the estemmenosuchids and early brithopodids of the Russian Ocher fauna, (Roadian to early Wordian age) were already a diverse group of herbivores and carnivores.

In any case, these animals radiated into ecological niches vacated by the pelycosaurs, who had dominated terrestrial ecosytems during the early part of the Permian.

The Wordian and Capitanian epochs, during which the dinocephalians flourished, is generally said to have lasted only a million or two years each. Such evolutionary change over such a short period (say 3 million years in all) is difficult to accept, especially if one looks at comparable evolutionary rates among Mesozoic dinosaurs and Cenozoic mammals, which are at least several times slower. To be sure, evolution can proceed very fast, perhaps over periods of only hundreds or thousands of years in small, geographically isolated island populations. However, in general and in the large picture, the average lifetime of a terrestrial animal species is on the order of some two to three million years. For this reason it may be that the conventional dating is probably slightly in error. Generally, generally there is an uncertainty of some five or ten million years either way with radiometric dating). Thus, the total Dinocephalian span (perhaps beginning in the Ufimian/Roadian or even the Kungurian and going through to the middle or late Capitanian age) may have been on the order of some twelve million years or more.

During the early Capitanian, advanced dinocephalia radiated into a large number of herbivorous forms; representing a diverse megafauna. This is well known from the Tapinocephalus Assemblage Zone of the Southern African Karoo.

At the end of mid-Permian time (at the end of the Capitanian age), at the height of their diversity, the dinocephalians suddenly died out, leaving no descendants. The reason for their extinction is not clear. The conventional explanation -- that they were out-competed by the more efficient herbivorous anomodonts and carnivorous theriodonts -- is about as unpersuasive as the old idea that the mammals out-competed the dinosaurs. Possibly disease, sudden climatic change, or other factors of environmental stress brought about their end. With their passing some of the most interesting prehistoric creatures this Earth has seen disappeared. They were replaced by much smaller Therapsids: herbivorous Dicynodontia and carnivorous biarmosuchians, Gorgonopsians and Therocephalians.

Technical description

Characters

Skull usually massive, $ nares not terminal [RS01]; bone horns on maxillae or skull table; orbits small; temporal opening fairly large; some saggital crest; some with conspicuously thickened crania (head-butting?); highly integrated; no secondary palate; $ premaxilla vomerine process absent (vomer contacts body of premaxilla directly) [RS01]; $ pterygoid transverse process anterior to orbit [RS01]; $ reflected lamina of angular smooth, without ridges or fossae [RS01]; $ foramen on medial jaw, between angular and prearticular [RS01]; incisors interdigitate; canines sometimes long; cheek teeth small; tail very long; hindlimb posture somewhat upright, but forelimbs sprawl; metapodial V at least as long & robust as Mp IV [RS01]; phalangeal formula 23345 / 23333; carnivores (early) or herbivores (later).

Reference

  • Rubidge, BS & CA Sidor (2001), Evolutionary patterns among Permo-Triassic therapsids. Ann. Rev. Ecol. Syst. 32: 449-480. [RS01]

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