For some considerable time now I’ve been promising that one day --- one day --- I’ll devote time and energy to coverage of that enormous, diverse, long-lived tetrapod group that we variously term the non-mammalian synapsids or stem-mammals. The most traditional term for them is ‘mammal-like reptiles’: while still in use, this term should be avoided given that the animals concerned are simply not part of the reptile lineage. The vernacular terms protomammal and paramammal have both been used for the group as well, though both have problems. Stem-mammals will be used here.

Provisional and in-prep montage (for my textbook on vertebrate history) depicting a selection of stem-mammals. I've drawn far more than the selection shown here. Credit: Darren Naish

Anyway, we’re talking about that group of tetrapods that are not mammals but are ancestral to them, and which occupy all those points on the mammal lineage outside of Mammalia. The presence of a laterotemporal fenestra (a single skull opening behind the eye socket) is a key feature distinguishing them from other amniotes. The early members of this segment of the mammal lineage have often been called pelycosaurs while the members of the more mammal-like segment of the lineage are termed therapsids. The importance of these animals concerns the fact that their comparatively excellent fossil record charts transition from an ancestral ‘reptile-like’ form to mammals via a near-perfect series of intermediates. Alas, their relative obscurity and the lack of good popular syntheses means that they are not the poster-children of evolution that they really should be… at least, not outside the palaeontological community.

Tetraceratops from the Early Permian of the USA, an early synapsid sometimes identified as one of the oldest therapsids but later re-interpreted as occupying a more root-ward position in the tree. Credit: DMITRI BOGDANOV WIKIPEDIA (CC BY 3.0)

This article is not the time and place to start a group-by-group review of the many lineages concerned… I know from experience how those projects quickly expand into gargantuan multi-part monsters that can never be finished. Rather, this is just a brief primer, a placeholder.

Cover of Kemp (1982), the best book on this group of animals so far, now out of print (but available at reasonable price online). Credit: ACADEMIC PRESS London

Before anyone asks, the one crippling, punishing problem with these animals is that – even today – there is no single, good, up-to-date, go-to volume on their diversity, history, evolution and biology. Yes, there are books on these animals, but they’re technical and mostly out of print. The best is Tom Kemp’s Mammal-Like Reptiles and the Origin of Mammals (Kemp 1982). There’s also Nick Hotton et al.’s The Ecology and Biology of Mammal-like Reptiles (Hotton et al. 1986) (a collection of papers by different authors). I have a substantial, well illustrated chapter on these animals in my giant textbook (on which go here, if you wish), but a good, dedicated, modern volume just does not exist. There are several decent review articles on the group as a whole, among the most recent being Angielczyk (2009).

Much-simplified cartoon cladogram of stem-mammals based on topologies recovered in several recent studies. Expanded versions being prepared for my in-prep textbook (more here). Credit: Darren Naish

The oldest stem-mammals date to the Moscovian part of the Carboniferous and have conventionally been depicted as very reptilian in appearance. This is probably true in broad terms but is open to some question, there being indications that their integument and so on was not ‘reptilian’ as we conventionally imagine it. These early forms belong to those lineages conventionally lumped together as ‘pelycosaurs’ – a term that clearly refers to a paraphyletic assemblage given that therapsids evolved from somewhere among them.

A somewhat dated schematic representation of synapsid evolution which I include because it does a nice job of illustrating both cranial variation within the group, and some of the main differences obvious between 'pelycosaurs', therocephalian-grade animals, and cynodonts. Credit: Palaeos, originally by Thomas Kemp

Animals from this ‘pelycosaur’ part of the tree include the long-snouted, mostly predatory varanopids and ophiacodontids, the omnivorous and herbivorous caseasaurs, and the edaphosaurids and sphenacodontids, the latter including the famous Dimetrodon. While many of these animals (especially the early members of these groups) were small (less than 50 cm long), large size (3 m or more) evolved several times independently. There are lots of other significant events here as well, including the evolution of high-fibre herbivory and the independent evolution of dorsal sails. Even in these animals there are indications of social behaviour and parental care (Botha-Brink & Modesto 2007, 2009).

Reconstruction of an assemblage (a family group?) of the varanopid Heleosaurus, pictured in the pose in which their skeletons were discovered. Credit: Botha-Brink & Modesto 2009

Dimetrodon – one of the most familiar and famous of all stem-mammals – is a fascinating creature that has recently undergone something of an image change: ideas regarding the evolution, function and anatomy of its sail have all been challenged, its ecology and lifestyle have been the source of some debate, and its life appearance and gait have undergone revision in recent years. I plan to devote an article to these issues.

You might have seen this animal before. It's Dimetrodon. Credit: D'ARCY NORMAN Wikimedia (CC BY 2.0)

Animals close to sphenacodontids gave rise to therapsids. A more erect gait and faster metabolism occurred at the time of this transition, numerous additional changes associated with dentition, palatal structure, limb posture and so on occurring as well. It’s within this vast group (Therapsida) that we find the often herbivorous, beak-jawed dicynodonts and kin, the often predatory biarmosuchians, gorgonopsians and therocephalians, and the often striking, often large dinocephalians. That last group includes both predators and herbivores, hippo-sized animals, and species with thickened skull roofs probably used in head-butting. They dominated many continental animal communities in the Permian, being best known from the fossil records of South Africa and Russia.

Tapinocephalid dinocephalians - like Tapinocephalus depicted here - had thickened skull roofs that likely had a display or combat function. The biggest of these animals were over 3 m long. Credit: DIBDG Wikimedia (CC BY SA 3.0)

Gorgonopsians and therocephalians are exciting groups that include various macropredatory, often ‘sabre-toothed’ species; both have been the subject of various recent revisions. Species within these groups have been likened to weasels, wolves and bears in approximate body form, though any resemblance would have been highly superficial. Sometime during the Late Permian, cynodonts arose from an ancestor closely related to therocephalians (both groups form the therapsid clade Eutheriodontia): Cynodontia is the group that includes mammals as well as a number of additional lineages that have their own histories and evolved their own specialisations.

And because this was meant to be a very, very brief primer, that is all I’ll say for now. There is so much more to do…

Stem-mammals have been covered on scant occasions at Tet Zoo. But see...

Refs - -

Angielczyk, K. D. 2009. Dimetrodon is not a dinosaur: using tree thinking to understand the ancient relatives of mammals and their evolution. Evolution: Education and Outreach 2, 257-271.

Botha-Brink, J. & Modesto, S. 2007. A mixed-age classed ‘pelycosaur’ aggregation from South Africa: earliest evidence of parental care in amniotes? Proceedings of the Royal Society B 274, 2829-2834.

Botha-Brink, J. & Modesto, S. 2009. Anatomy and relationships of the Middle Permian varanopid Heleosaurus scholtzi based on a social aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29, 389-400.

Hotton, N., MacLean, P. D., Roth, J. J. & Roth, E. C. 1986. The Ecology and Biology of Mammal-like Reptiles. Smithsonian Institution Press, Washington and London.

Kemp, T. S. 1982. Mammal-Like Reptiles and the Origin of Mammals. Academic Press, London.