September 15, 2016:

by Stephan Lautenschlager

The origin of mammals from non-mammalian synapsid ancestors is a key event in vertebrate evolutionary history, and a classic example of an evolutionary transition. Modern mammals possess a unique suite of morphological and physiological characters which distinguishes them from their reptilian ancestors. Mammals are warm-blooded, large brained, agile organisms. They lactate, possess differentiated jaw musculature and have differentiated (heterodont) teeth with precise occlusion and a two-phased replacement. The acquisition of these features throughout the evolutionary history from basal synapsids, to cynodonts and early mammals is remarkably well documented by a series of fossils.
Modern mammals also possess a unique middle and inner ear morphology, which evolved through the incorporation of jaw bones into the ear. ). In basal cynodonts, the quadrate and articular bones form the upper and lower components of the jaw hinge respectively. During the evolution of mammals, the bones forming the lower jaw are reduced and some of these bones (quadrate and articular) become part of the inner ear ossicles (malleus and incus). However, although we see this trend documented in detail in the fossil record a central question exists as to how the jaw hinge and masticatory system was able to remain viable and load bearing during transformation into the delicate, minute, ossicles of the ear. This is especially puzzling when all evidence points towards modifications for increased jaw muscle force and stronger skulls during mammalian evolution.
In an ongoing project in collaboration with Prof. Emily Rayfield, Dr. Pamela Gill (University of Bristol) and Prof Michael Fagan (University of Hull) and funded by the Natural Environment Research Council (NERC) were investigating the evolution of the mammalian jaw joint from basal cynodonts (mammal ancestors) to derived mammals and the biomechanical and palaeobiological implications of this transition. For this we are using a combined approach of digital restoration and reconstruction methods and a suite of bioemechanical analysis techniques.

Results of this project will be posted and updated regularly here. Digital models of different cynodonts and fossil mammals will be available for viewing, with more information on each species:

Thrinaxodon liorhinus

Thrinaxodon liorhinusis a basal, non-mammalian cynodont (the lineage including modern mammals and their fossil relatives) from the Early Triassic of Gondwana. The species is represented by several specimens from South Africa and Antarctica. It is a distant relative of today’s mammals, but represents an important stage in the evolution of mammals. The cranial skeleton of Thrinaxodon liorhinus shows a combination of basal, reptilian and more derived mammal-like characters: Unlike its predecessors, Thrinaxodon liorhinus possesses a secondary, bony palate, which separates nasal passage from the oral cavity, allowing it to breathe while keeping food in its mouth. Despite this evolutionary innovation, Thrinaxodon liorhinus still has a basal (or reptilian-style) jaw joint. The jaw joint is formed by the quadrate and the articular bones (which will eventually become parts of the inner ear bones in more derived taxa).
Thrinaxodon liorhinus ranges around 30 to 50 cm, with a skull length between 3 and 10 cm. Its dentition suggests that is was primarily carnivorous, feeding on small reptiles, insects and other invertebrates.

Diademodon tetragonus

Diademodon tetragonusis a species of derived non-mammalian cynodont. It is commonly found in Middle Triassic (ca. 235 million years) sediments of South Africa and South America. Stratigraphically, it is therefore somewhat younger than Thrinaxodon liorhinus. Diademodon tetragonus was about 1-1,5 m in size, and had a skull of about 20-30 cm in length. In contrast to Thrinaxodon, Diademodon tetragonus was most likely omnivorous or herbivorous as indicated by the flattened post-canine teeth and the large skull, which supported powerful jaw muscles.

Probelesodon sanjuanensis

Probelesodon sanjuanensis is a derived non-mammalian, chiniquodontid cynodont found in Middle to Late Triassic sediments of Argentina and Brazil. In comparison to Thrinaxodon liorhinus and Diademodon tetragonus, the species shows a number of derived, mammal-like characters, including an elongate secondary palate, a double occipital condyle and an incipient surangular-squamosal articulation.

Morganucodon oehleri

Morganucodon is a basal mammaliaform known from Late Triassic and Early Jurassic (ca. 205-200 million years) sediments of Europe, Asia and North America. The species Morganucodon oehleri is represented by nearly complete and articulated fossil skulls from the Early Jurassic of China. Further abundant, but isolated material (in particular teeth) has been found in Wales.
Morganucodon presents a further transitional stage in jaw joint evolution by possessing a ball-like dentary condyle articulating with a well-defined glenoid cavity in the squamosal. However, it also retains the ancestral articular-quadrate joint. Unlike its predecessors, Morganucodon had complex teeth, which allowed for efficient chewing. Morganucodon was less than 10 cm in size, with a skull length of around 2 cm. Like modern mammals of its size, it was probably nocturnal and fed on insects and other small animals. It most likely had fur similar to extant mammals.

Hadrocodium wui

Hadrocodium wui is a basal mammaliaform from the Lower Jurassic (ca. 195 million years) of China. The enlarged brain cavity, a jaw formed of a single teeth-bearing bone and most importantly the fully developed mammalian jaw joint between the dentary and the squamosal make Hadrocodium more derived than Morganucodon. It is therefore most likely one of the first animals to possess a mammalian middle ear, which is formed by some of the ancestral postdentary bones and fully detached from the lower jaw.
The skull of Hadrocodium is only about 1 cm in size. Although the rest of the skeleton is unknown, the whole animal was probably no larger than 3-4 cm. Similar to Morganucodon Hadrocodium was probably nocturnal and insectivore.

http://eis.bris.ac.uk/~glzsl/mesozoic_mammals.html