Topic: Complex tooth occlusion in notosuchid crocodiles and tritylodonts (proto-mammals)
Two unusual Early Cretaceous crocodiles provide a shining example of convergence, as their dentition parallels that observed in a group of advanced proto-mammals called tritylodonts.
The majority of reptiles possess a fairly simple dentition composed of peg-like teeth of similar shape and size throughout the jaw (termed ‘homodont’), used to grasp prey. These teeth are continuously shed and replaced throughout the lifespan, and the upper and lower tooth rows do not contact one another; instead, the lower tooth row rests inside the upper row when the jaw is closed. However, numerous reptile groups have evolved much more specialised dentition types, and perhaps most impressive among these are reptiles whose teeth show complex tooth-to-tooth contact, or ‘occlusion’, adapted for processing food (e.g. by shearing or grinding) before swallowing. Dental occlusion in reptiles approaches the sophistication seen in the advanced occluding molars of mammals, and convergent evolution is evident not only between various reptiles and mammals, but also extends to comparisons with lungfish and unusual extinct groups such as conodonts and pycnodont fish. The following paragraphs discuss two unusual Early Cretaceous crocodiles that show complex tooth occlusion, and provide a shining example of convergence, as their dentition parallels that observed in a group of advanced proto-mammals called tritylodonts.
Notosuchid crocodile dentition
The Notosuchus-like crocodile of Africa and Chimaerosuchus paradoxus of China have both been identified as members of the group ‘Notosuchidae’. Anterior in the mouth they have 2-5 pairs of outward pointing (‘procumbent’) canine-like (‘caniniform’) teeth, presumably to aid in grasping prey, and posteriorly they possess 3-4 pairs of striking molar-like (‘molariform’) teeth. The molariform teeth have multiple cusps, arranged in anterior-posterior rows with each cusp recurved and possessing a sharp posterior ridge. The most prominent molar cusp was at the centre of the main, middle row in Chimaerosuchus and Notosuchus, and the cusp rows of the upper and lower molariforms interlock at jaw closure. Although typical crocodiles have rigid jaw movement, the skulls of Notosuchus and Chimaerosuchus allow a posterior-anterior direction (termed ‘proal’) of mandible movement, generating shear along posterior cutting edges of the recurved cusps. It is proposed that the shearing cusps of their interlocking molariform teeth indicate that these crocodiles were omnivorous, well adapted to process tough or fibrous food items. Notosuchus and Chimaerosuchus belong to a group termed the ‘Crocodylia’, in which we find crocodiles, alligators, gharials and the less familiar proterosuchids and parasuchids. Crocodylians are archosaurs, closely related to pterosaurs and dinosaurs but independent of groups such as marine reptiles (ichthyosaurs, plesiosaurs etc.), lizards, snakes and tuataras, which are lepidosaurs. Neither close nor distant reptile relatives of the Cretaceous notosuchids show exactly parallel dentition, but moving outside the reptiles we find a small group of therapsids – the fore-runners to mammals – that do.
Notosuchids and tritylodonts: convergent dentition
The therapsids evolved from ‘mammal-like reptiles’ called pelycosaurs (e.g. Dimetrodon) in the Permian, and gave rise to a diverse group called the cynodonts. The cynodonts evolved increasingly specialised, molariform teeth over time, and include an advanced group called the tritylodonts that are the closest ancestors to the true mammals. Tritylodont dentition is characterized by multi-cusped upper and lower molariforms, in which the upper molars have three rows of cusps and the lower molars two rows, spaced such that the lower cusps interlock exactly with the uppers. The resulting bite was precise, and as in Notosuchus and Chimerosuchus, jaw movement and strong mastication (chewing) muscles generated powerful shearing and grinding forces for processing plant material. Species such as Tritylodon and Bientheroides were very mammal-like herbivores, and their effective dentition was clearly critical to their competitive success even into the Paleocene (e.g. Chronoperates). Although tritylodont jaw movement was in an anterior-posterior or ‘palineal’ direction, as opposed to a posterior-anterior (‘proal’) direction as in notosuchids, their multi-cusped, interlocking dentition represents a remarkable convergent adaptation, independently evolved through evolutionary time. Furthermore, in considering dental occlusion between notosuchids and tritylodonts, a notable shared feature is their secondary palate, which enables breathing while chewing. This feeding innovation arose independently in crocodiles and advanced therapsids, evolutionarily separated as they are by hundreds of millions of years, and arguably supported the emergence of dental specializations as seen in Notosuchus, Chimaerosuchus and Tritylodon.
Notosuchid and tuatara dentition: convergent?
Notosuchid-type dentition appears to be unique among reptiles, and yet the lizard-like tuatara (Sphenodon) has an unusual form of dentition that functions in a strikingly analogous way. Tuatara have two rows of upper teeth (maxillary and palatine) and one row of lower teeth (mandibular), each tooth having only one major cusp. The lower tooth row fits into the channel between the two upper tooth rows, so that the upper rows function as if they were the inner and outer cusp rows of a single, multi-cusped ‘molariform’ tooth. This arrangement is of comparable orientation and function to the interlocking multi-cusp rows of notosuchids, and in addition, tuatara also use proal (posterior-anterior) jaw movement to generate shearing and crushing forces between tooth surfaces. In spite of the fact that the dentition of Sphenodon depends on a double tooth row, whereas notosuchids have single tooth rows, their shared overall form and function are remarkable, and as such provide another possible case of convergent evolution, this time between representatives of the two major branches of the reptiles, archosaurs (e.g. Crocodylia: Notosuchidae) and lepidosaurs (e.g. Rhynchocephalia: Sphenodontia).
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Map of Life - "Complex tooth occlusion in notosuchid crocodiles and tritylodonts (proto-mammals)"
January 20, 2020