Preface added the day after posting: M. Mortimer gratefully informed me that some authors consider all taxa closer to mammals than to other living taxa as ‘stem’ mammals. Perhaps that is how ‘stem’ taxa are defined. That came as news to me because I understood the term ‘stem’ to refer to immediate outgroups only based on the terms usage in other works. So you learn as you go. The broad definition quickly loses relevance and adds to confusion. In M. Mortimer’s example, Diplodocus is a ‘stem’ bird. Please read the following with these caveats in mind.
Preface added 10/26/2016: Just found out there is are two definitions for ‘stem’ taxa, one in the wider sense and one in the narrower sense, the one is was familiar with. Learn more at Wikipedia here.
Usually I cover published academic papers
here at PterosaurHeresies.WordPress.com. Today we’ll cover a Tetrapod Zoology blog post published online by Dr. Darren Naish a month ago. Unfortunately the post was sprinkled with traditional misconceptions.
the Naish text is copied in italic and his captions are copied in their original ALL CAPS. Remarks are in red. You can see the original blogpost here. This is how all good referees mark up submitted manuscripts, with precise comments intended to help the writer improve the next draft. To that end, Naish notes he is currently writing a book that includes this subject.
The Stem-Mammals–a Brief Primer
Mammals are but the only surviving members of a far grander, older lineage
By Darren Naish on September 20, 2016
“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. Not true. According to the large reptile tree (LRT) all descendants of the first reptile/amniote, Gephyrostegus, are also reptiles, and that includes mammals and their long list of descendants. Unfortunately Naish is repeating an old and invalid tradition. 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. If so, it is important that Naish restrict his discussion to just the immediate precursors of mammals, not the long list going back to basal synapsids, but that is not what he does.
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. Not true. Several clades by convergence developed such a skull opening including 1. the millerettid clade and their descendants from Oedaleps to Australothyris, including the caseids. Emeroleter and Lanthanosuchus had that fenestra. So did bolosaurids. And then there are the prodiapsids from Heleosaurus to Archaeovenator and the last common ancestor of synapsids and diapsids, Vaughnictis. 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. Actually finback pelycosaurs are an offshoot clade, not in the lineage of mammals, which proceeds from a sister to Ophiacodon to Cutleria without including finbacks. 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. Those animals were featured on both versions of Cosmos.
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. If you want to see the lineage of mammals going back to stem tetrapods, click here then peruse at your leisure the taxa that interest you.
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. But you can go online here for the latest data. 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).
The oldest stem-mammals date to the Moscovian part of the Carboniferous (here again, an inappropriate use of the term ‘stem’) and have conventionally been depicted as very reptilian in appearance. That’s because they are or were reptiles, as recovered by the LRT. 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. Likely without scales, based on the scant evidence at hand, but living dinosaurs are also without scales, except for those transformed from feathers. 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. Not true. The LRT recovers a clade of pelycosaurs, a resurrected clade Pelycosauria.
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. Why waste time on these non stem-mammals? 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. Why waste time on these non stem-mammals? Even in these animals there are indications of social behaviour and parental care (Botha-Brink & Modesto 2007, 2009).
Dimetrodon – one of the most familiar and famous of all stem-mammals (Not true, merely an offshoot)– 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.
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. Still not talking about stem mammals here. When are we going to get to them? The text does not follow the headline.
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 specializations. Now we’re getting closer to the stem-mammals, members of the clade Tritylodontia within the Cynodontia!
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… WAIT! Naish never once wrote about or illustrated a stem-mammal here! I read this whole blog post without learning anything new about the stem-mammals, the Tritylodontidae and their immediate predecessors.. As we’ve seen before, Naish sometimes cruises on the invalid past rather than exploring today’s cutting edge data and latest discoveries. Pity, all that talent going for the low-hanging fruit. Darren, as you write your book on synapsid relationships, feel free to reference ReptileEvolution.com and the large reptile tree. It will help you understand the issues and enigmas generated in Kemp’s 1982 book.
Stem-mammals have been covered on scant occasions at Tet Zoo. But see…
- Survivors, diggers, herbivores, first giant terrestrial vertebrates: the caseids
- http://scienceblogs.com/tetrapodzoology/2007/07/18/the-answers-we-seek-on-goodbye/ The answers we seek: on ‘goodbyes’, the necks of caseids, and weird mystery sauropods
Sometimes Dr. Naish referees manuscripts offered for academic publication. With his stuck-in-the-past bias, good luck if he referees your submission. I would not want wish that on my worst enemy, especially if you’re promoting new hypotheses.
Many scientists like to play it safe, resisting and waiting for the tide to shift on advancing new hypotheses before jumping on the bandwagon. Don’t be like that. Follow the data. Test as much as you can yourself. Be a skeptical Scientist, not a nodding Journalist.
What do I expect from these remarks?
Based on his vocal antipathy toward the results recovered by the LRT, Dr. Naish will probably cling to his invalid traditions. After all, based on his writings, he has ‘painted himself into a corner’ from which he cannot escape without an about face apology and acknowledgment. That’s something primates, like us, do very very rarely. PhDs are not wired for it. But if Naish did run the tests he would find what I found. If not, I’d like to hear why not.
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.