A four-legged snake

July 26, 2015 • 3:30 pm

by Greg Mayer

It has long been known that some snakes are two-legged, because many modern species have two legs– externally visible hind limbs– a fact we’ve noticed here at WEIT before.

Hindlimbs ('spurs') of a ball python (Python regius). The spurs are next to the anal scale, which covers the vent of the cloaca. (Front of snake is toward top of photo.)
Hindlimbs (‘spurs’) of a ball python (Python regius). The spurs are next to the anal scale, which covers the vent of the cloaca. (Front of snake is toward top of photo.)

These small external legs, capped by keratinous claws, are supported internally by vestigial femurs and a vestigial pelvis. They are larger in males, and are used during courtship. In the fossil record, snakes with much larger hind limbs have been known since Georg Haas described Pachyrachis in 1979 and Ophiomorphus in 1980 from the early Late Cretaceous (about 95 mya). In these the legs were less rudimentary than in modern snakes, having, in addition to the femur and pelvis, a distinct tibia and fibula, and tarsal bones. That legless tetrapods would have legged ancestors is of course expected, and for the caecilians, a modern group of legless amphibians, a four-legged progenitor was described by Farish Jenkins and colleagues several years ago.

In a new paper paper in Science, David Martill, Helmut Tischlinger, and Nicholas Longrich describe a four-legged snake from the late Early Cretaceous (about 120 mya), of Brazil, giving it the rather aptly descriptive name Tetrapodophis, “four-legged snake”. The fore and hind legs are small, but well developed, with five digits on each. The limbs are suggested to have been used during prey capture. They also interpret it as being fossorial. This is significant, as the two major theories of snake origin are that they came from fossorial (burrowing) ancestors, or that they came from marine ancestors (some of the closest known relatives of snakes are extinct aquatic lizards, and Haas’s specimens come from marine sediments).

T. amplectus appendicular morphology. Fig. 4 from Martill et al. (2015). (A) Forelimb. (B) Manus. (C) Hindlimbs and pelvis. (D) Pes. (E) Pelvis. Abbreviations: fem, femur; fib, fibula; hu, humerus; il, ilium; lym, lymphapophysis; man, manus; mc, metacarpal; mt, metatarsals; ph, phalanges; ra, radius; sr, sacral rib; tib, tibia; ul, ulna; un, ungual.
T. amplectus appendicular morphology. Fig. 4 from Martill et al. (2015).
(A) Forelimb. (B) Manus. (C) Hindlimbs and pelvis. (D) Pes. (E) Pelvis. Abbreviations: fem, femur; fib, fibula; hu, humerus; il, ilium; lym, lymphapophysis; man, manus; mc, metacarpal; mt, metatarsals; ph, phalanges; ra, radius; sr, sacral rib; tib, tibia; ul, ulna; un, ungual.

A long, flexible body, recurved teeth, and intramandibular joint (for opening the mouth wide) all suggest to Martill and colleagues that Tetrapodophis was a constrictor, preying on other vertebrates. This also has significance for what it says about the origin of snakes. Under the fossorial theory, the earliest snakes should have been insectivorous or eating other small prey (as are many supposedly primitive burrowing snakes today). Under the marine theory, the earliest snakes should have been predators of prey “bigger than their heads“, having large, extensible mouths, and associated adaptations of the skeleton and musculature– such snakes are called macrostomatan (literally, ‘large mouthed’). Haas’s two-legged marine snakes are macrostomatan. Martill and colleagues have found what they consider to be a very early macrostomatan, yet fossorial, snake– a cross between the two theories.

Tetrapodophis constricting and eating a small mammal, reconstruction by Julius Cstonyi.
Tetrapodophis constricting and eating a small mammal, reconstruction by Julius Cstonyi.

Almost immediately upon its publication, the paper became enmeshed in a series of overlapping controversies, which, while nothing compared to the brouhaha over the insults traded between Nicki Minaj and Taylor Swift, has created quite a stir in the small world of science social media. There are at least three areas in which questions have been raised, so let’s take them one at a time.

Where (and thus when) is the type specimen of Tetrapodophis from? It turns out that the authors of the paper don’t actually know the provenance of the specimen. They have made an inference about it (and they may very well be right), but the fact that they do not even mention the assumed nature of the specimen’s provenance in the paper is shocking. A specimen’s provenance is absolutely crucial information in systematic biology; it is especially so for fossil specimens, because in most instances it is only by examining the geological context of the discovery (the associated fossils within the bed, and the nature of the over- and underlying beds) that we can date the fossil. In this case, we are not really sure where the specimen came from, and thus we cannot be certain of when the specimen died and was entombed in sediment.

I read the paper, and did not realize the provenance was uncertain. The uncertainty, and the argument for why the authors felt they could identify the provenance, are buried in an online supplement. To some it may seem like I’ve been a bit “hey you damned kids, get off of my lawn” about it”, but I’ve long complained of the growing practice of journals, especially Science, of burying key details of papers in ephemeral online sources, and in this case such warnings have come home to roost. Where (and thus when) it’s from is just about the most basic thing you can say about a fossil, and to hide the fact that in this case it’s unknown in an online supplement is unconscionable.

In the online material, Martill and colleagues state that “no notes as to its [the specimen’s] acquisition or provenance are available.” However, in an interview with the BBC, Martill says that he first saw the specimen at the Museum Solnhofen in an “exhibition of Brazilian fossils”, so some notes on provenance seem to have been available to the Museum. Another source states that the exhibit was of fossils specifically from the “Crato Formation”.

Is the fossil, as the authors claim, from the Nova Olinda member of the Crato Formation of Ceara, Brazil? It might well be. Martill is an expert on the formation. Certain fossil localities do have a distinctive lithology and preservation– I can (usually) recognize Green River Formation fossils myself. But to not mention this up front, and provide the justification for the assignment to provenance in the paper, is beyond the pale.

Is Tetrapodophis even a snake? In a news posting on Science‘s website, Michael Caldwell alleges that the specimen is not a snake; in fact, he says, it’s not even a reptile. Rather, the article says, Caldwell thinks it might be a surviving member of a “group of extinct amphibians that died out during mass extinctions about 251 million years ago, long before Tetrapodophis appeared on the scene.” (I’m not sure what amphibians he’s thinking of– perhaps microsaurs or lysorophids?) This would be astonishing– that a group survives 150 million years without a fossil record and then reappears (which does, though, have a partial precedent in the coelacanth), and that the authors and reviewers of a paper in Science could be so wildly off in the identification of the subject of the paper. This of course is not impossible, but it would be surprising. I have only seen the published figures, but Martill and colleagues do discuss and defend the characters by which they assign the specimen to the snakes. Caldwell has published on early snakes and should know their morphology, but he has not seen the specimen either, so it’s hard to give full credence to his views. We’ll have to wait till others get to look at the specimen more closely, or perhaps for a monographic treatment by Martill and colleagues. This is the scientifically most important controversy (although the first controversy is right up there, because much of its significance as a four-legged snake depends on its supposed time of occurrence).

Should fossil collecting and/or exporting require a permit or license? Since the specimen has no collecting data accompanying it, it is unclear if the specimen was collected legally. Fossil collecting in Brazil has required a license since 1942 (or perhaps 1988– recent sources diverge on the date). This is of course not a scientific question, but a question of public policy that has implications for science. To a great extent, the controversy is an old one in paleontology– does  amateur and commercial collecting enhance or retard the growth of scientific knowledge?  There are strong opinions on both sides. In the United States, this argument flared up over Tyrannosaurus Sue, which was discovered and collected by commercial collectors, but eventually seized from them without recompense (one even went to jail). The downside of such regulation is that many specimens will never come to light, or, if found, will be tossed aside and left to degrade, as their possession would be illegal. A commercial black market may develop, in which the best fossils may be found, but then disappear, unstudied, into private collections. The upside is that specimens will have known provenance, and be of maximal scientific value. Martill has long argued (also here) that in Brazil the permitting system has become so corrupt that scientists are driven out of the field, and that, through bribery, commercial trade flourishes, while many fossils are left to erode and break, as no one may legally save them. He also says it was not always so– for years he worked successfully under Brazil’s regulations. His view:

‘Protecting fossils’ criminalises palaeontologists. Laws banning fossil collecting and private fossil collections deter amateur palaeontologists, drive them underground and stifle curiosity. Fossils left in the ground weather away and are lost. Banning commercial collecting loses tax revenue.

A group of Brazilian paleontologists led by Max Langer, in a strong riposte to one of Martill’s pieces, wrote

Instead, the Brazilian perspective is that taking fossils out of the country is depleting its scientific resources. Brazil has a growing, but still minor scientific community. For palaeontology, keeping the fossils in the country is a way of promoting scientific opportunities. International partnerships are most welcome, but simply allowing fossils to leave Brazil to be studied by foreign scientists mostly helps science in the other countries.

On the issue of regulation, my own view is an in-between one. An analogy can be made (one which Martill disputes) with wildlife conservation. Many species and natural areas require protection. It is often scientists who are at the forefront of advocating for such protections, even though it will add to the difficulty of doing scientific work on the protected species and habitats.  It is true that sometimes such regulations can be over-zealously enforced against scientists (in part because they are so visible and have no economic clout), while ignoring the truly endangering factors. But when scientifically informed and sensibly applied, these protections are welcomed by scientists. And, indeed, Martill describes a formerly good relationship with the Brazilian authorities. I do not know enough about the situation in Brazil to have an informed opinion on whether Brazilian policy and practice on this matter has achieved the right balance to encourage discovery and scientific research, while maintaining proper stewardship of their resources.

Another analogy with wildlife conservation issues is what to do with illegally collected specimens. It is standard for wildlife enforcement agencies to donate such materials to museums or educational institutions (although their value as scientific specimens is lowered by the frequent lack of provenance). More controversial is what to do with seized specimens of commercial, but no scientific, value. For example, some advocate the destruction of seized elephant ivory, while others argue that that only drives up prices, leading to more poaching. In the case of fossils, what scientific value there is in them can be extracted by describing them (again subject to the constraints of knowledge of provenance) and placing them in museums (not, by any means, destroying them!).

Regarding the issue of whether fossils should be exported, I am sympathetic to the need to develop scientific institutions throughout the world, and thus to build local collections and relationships between foreign and local institutions and researchers. This must be tempered by recognition that not all places are in a position to care for important collections or engage in collaboration. In this regard, I would note that Brazil has at least one distinguished student of early snake evolution, Hussam Zaher, and at least some excellent museums, although I do not know enough about the situation with the Crato fossils to have an informed opinion on that specific case. I would point to the recent return of Tiktaalik to the Canadian Museum of Nature after 11 years of study in the U.S., and Costa Rica’s policy of a division of collected specimens among foreign and Costa Rican institutions as policies that seem to be working.

The Brazilian journalist Herton Escobar has conducted an email interview with Martill. In it, Martill’s frustration with being denied further access to his field sites in Brazil is evident. He asserts (correctly, in my opinion) that the scientific value of the fossil is not affected by the legality of its collection (to which, I should add, there is no suggestion that Martill or his colleagues were involved in its collection– he first saw it in Museum Solnhofen during a class field trip); its value is affected by the lack of certain provenance. In response to a question as to whether he had sought a Brazilian collaborator, Martill said he did not. Not seeking a Brazilian collaborator is fair enough– he worked with colleagues in England and Solnhofen, close to him and the fossil, and he had at best difficult personal relationships with Brazilian workers at the time. But Martill also goes off on an odd rant about ethnic and sexual diversity in research groups– not at all what Escobar was asking about.

Haas, G. 1979. On a new snakelike reptile from the Lower Cenomanian of Ein Jabrud, near Jerusalem. Bulletin du Museum national d’Histoire naturelle 4: 51–64.

Haas, G. 1980. Remarks on a new ophiomorph reptile from the lower Cenomanian of Ein Jabrud, Israel. pp. 177–192. In Jacobs, L.L.,
ed., Aspects of Vertebrate History. Museum of Northern Arizona Press, Flagstaff, Arizona.

Martill, D.M., H. Tischlinger, and N.R. Longrich. 2015. A four-legged snake form the Early Cretaceous of Gondwana. Science 349:416-419.

h/t Matthew

Farish A. Jenkins, Jr., 1940-2012

November 13, 2012 • 11:42 pm

by Greg Mayer

Farish A. Jenkins, Jr., Curator of Vertebrate Paleontology and Alexander Agassiz Professor in the Museum of Comparative Zoology at Harvard University, died on November 11, 2012. Farish made major contributions to vertebrate paleontology, functional morphology, and evolutionary biology. He had been ill with cancer for some time, but had continued to work productively, and his death came quickly following a recent reverse. (See update below.)

Farish Jenkins in the vertebrate paleontology collection of the Museum of Comparative Zoology, holding a skull of Massetognathus, a Triassic cynodont (an advanced mammal-like reptile) from the Chanares Formation, Argentina. Photo by Hilary Rosner, Tooth & Claw.

Although Farish published on many subjects, the part of his work likely to be of most interest to WEIT readers is that on transitional forms. Farish worked on three great transformations in the history of tetrapods, including two that have become classic case studies in the origin of higher taxa. First, he worked on the origin of mammals, often in collaboration with his  MCZ colleague, A.W. “Fuzz” Crompton. That the ancestors of mammals were to be sought among a particular group of fossil reptiles known as synapsids had been known since the 19th century. What Farish, Fuzz, and many colleagues helped to show was how this transition occurred, and how the bones of the reptilian jaw joint of synapsids moved in to the middle ear of mammals to become ear ossicles, while a new jaw joint, the mammalian jaw joint, evolved. It is a favorite tactic of creationists, even today, to ask how possibly could the jaw of a reptile come unhinged, and a new joint develop, with the reptile bones passing into the ear? Well, the answer is, we know exactly how they did it, because we have the fossils- read Crompton and Jenkins, and look at the pictures!  (For the latest on mammalian ear evolution, see this paper by Luo Zhe Xi.)

Farish was one of the triumvirate who, along with Neil Shubin and Ted Daeschler, described Tiktaalik, the fish-tetrapod intermediate from Arctic Canada that made the front pages of newspapers around the world when it’s discovery was publicly announced in 2006. Neil and Ted got most of the media appearances, but it was Farish who was the old hand at arctic paleontological exploration (in the video below, look for Farish at 1:45). Although describing Tiktaalik taxonomically and morphologically was but a small part of his copious output, Farish may be best remembered for this work.

Most recently, Farish and colleagues completed a monographic account of Eocaecilia, a caecilian with limbs (which they had named and briefly described years earlier). Caecilians (not to be confused with the edible variety) are a group of tropical amphibians which today lack limbs, and Eocaecilia is a form that is transitional from fully-limbed ancestors to the modern condition.

Eocaecilia micropodia (‘the tiny-footed dawn caecilian’) from Jenkins and Walsh, 1993.

Both Jerry and I knew Farish from our days at the MCZ. I last saw him on a visit a year or two ago, after he was diagnosed with cancer, but he was his usual voluble self; Jerry saw him at the MCZ just a few months ago. Always impeccably dressed and charming, he had the demeanor of what I imagine a retired officer of the Royal Horse Guards would be like. He helped organize and lead a superb graduate course on vertebrate paleontology (I cannot recall now whether I enrolled or just attended) in the comfortable environs of the Romer Library, named for one of his distinguished predecessors at the MCZ, Alfred Sherwood Romer. I do recall stories of Arctic fossil hunting, with high powered rifles a necessity, as one man stood guard for polar bears, while others peered at the rocks. In addition to his teaching duties in the Faculty of Arts and Sciences, Farish taught human anatomy at the medical school. His comparative and evolutionary approach was not only appreciated by medical students, but also provided an opportunity for vertebrate morphology graduate students, by either taking the course or assisting in its teaching (or both), to gain the experience and background in human anatomy that would allow them to go on and train generations of physicians, as well as commanding the much higher salaries found in medical school anatomy departments. The Nature News Blog has some nice recollections of Farish by Hopi Hoekstra, the MCZ’s curator of mammals. The science writer Hilary Rosner has posted an endearing reminiscence of her encounters with Farish, along with a number of fine photographs, at her blog, Tooth & ClawAs another MCZ colleague put it to me earlier today, “His lectures were legendary…He was a scholar and a gentleman, and truly one of kind.”

A symposium in Farish’s honor, Great Transformations, was held last June. Like Ernst Mayr, also of the MCZ, who got to attend and speak at his 100th birthday symposium, Farish too was able to attend and speak at this gathering to celebrate his achievements. I understand there is a festschrift of the contributions in the works, but unfortunately Farish will now not see it.

Update. More accounts and reminiscences well worth reading have appeared in the Harvard Gazette, Boston Globe, and at Postcardsfrom Farish.


Crompton, A.W. and F.A. Jenkins, Jr. 1973. Mammals from reptiles: a review of mammalian origins. Annual Review of Earth and Planetary Sciences 1:131-155.

Crompton, A.W. and F.A. Jenkins, Jr. 1979. Origin of mammals. Pp. 59-73 in J.A. Lillegraven, Z. Kielan-Jaworowska, and W.A. Clemens, eds., Mesozoic Mammals: The First Two-Thirds of Mammal History. University of California Press, Berkeley.

Daeschler, E.B., N.H. Shubin, and F.A. Jenkins, Jr. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440:757-763.

Downs, Jason P., Edward B. Daeschler, Farish A. Jenkins, Jr., and Neil H. Shubin, 2008. The cranial endoskeleton of Tiktaalik roseae. Nature 456: 925-929.

Jenkins, Jr., F.A and A.W. Crompton. 1979. Triconodonta. Pp. 74-90 in J.A. Lillegraven, Z. Kielan-Jaworowska, and W.A. Clemens, eds., Mesozoic Mammals: The First Two-Thirds of Mammal History. University of California Press, Berkeley.

Jenkins, F. A., Jr., and D. M. Walsh. 1993. An Early Jurassic caecilian with limbs. Nature 365:246-250.

Jenkins, F. A., Jr., D. M. Walsh, and R. L. Carrol, 2007. Anatomy of Eocaecilia micropodia, a Limbed Caecilian of the Early Jurassic. Bulletin of the Museum of Comparative Zoology, 158 (6): 285-365. pdf

Luo, Z.-X. 2011.  Developmental patterns in Mesozoic evolution of mammal ears.  Annual Review of Ecology, Evolution and Systematics 42: 355–80. pdf

Shubin N.H., E.B. Daeschler, and F.A. Jenkins, Jr. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440: 764-77.

Burrowin’ lizards, Batman!

May 19, 2011 • 1:42 pm

by Greg Mayer       (Update below)

Lizards are far and away the most species-rich group of living reptiles, with over 7000 species. One of the first things you learn if you’re a little boy interested in such creatures is that snakes are lizards. One of the other things you learn is that snakes are not the only group of legless lizards. There are, in fact, many groups of lizards with reduced or missing legs, such as the European slow worm and American glass snakes (now preferably called glass lizards).  Snakes are just the most evolutionarily successful such group of lizards, comprising 3000 or so of the species of lizards. One of the most distinctive of the non-snake legless lizards are the worm lizards, or amphisbaenians, a group of about 150, mostly tropical, burrowing species. Perhaps our greatest student of the group, the late Carl Gans, thought them so distinctive that he championed a classification in which they were ranked equally with lizards and snakes within the Squamata (the taxon which includes lizards and all their derivatives, including snakes and amphisbaenians), although most other workers did not accept this ranking.

A worm lizard, Amphisbaena sp.

Gans wrote in his Biomechanics (he was a functional morphologist and physiologist as well as a systematist) that:

Unfortunately, we lack fossils intermediate between the Amphisbaenia and other groups, and can only speculate what their ancestors looked like.

A paper published in Nature today by Johannes Muller and colleagues (abstract only) goes a long ways towards constraining our speculations. In the paper, they describe a new species of lizard from the Eocene Messel shale of Germany (Messel is a famous lagerstatte: a deposit with extraordinary fossil preservation) as a transitional form from ‘normal’ lizards to the amphisbaenians.
Cryptolacerta hassiaca, holotype, from Nature 473:365.

Ever since Charles L. Camp’s 1923 classic, “Classification of the lizards”, amphisbaenians have bounced around a bit in terms of who their closest relatives are (this proposal being the most heterodox), but recent molecular work (summarized here and here by Blair Hedges and Nicolas Vidal) has connected them to the Lacertidae, a group of typical-looking Old World lizards (‘lacerta’ is Latin for ‘lizard’). In describing the new species, known from a single, well-preserved, and nearly complete specimen, Muller and colleagues write that the species shows “a mosaic of lacertid and amphisbaenian anatomical characters”. The skull, like that of amphisbaenians, is strongly constructed, and evidently adapted for a semi-fossorial life, while the limbs, though well developed proximally, are fairly short and have miniaturized digits. The body is not elongated. Morphometric comparison to modern lizards show that Cryptolacerta was likely a cryptic, leaf litter dwelling form.

Thus, the burrowing head evolved before the fully fossorial life style, while the body was as yet unenlongated, and the limbs still fairly well developed. We should not be surprised to find limbs in a transitional form from the well-limbed lacertids, but it is also the case that three extant species of worm lizards, the members of the Mexican genus Bipes, retain short front legs. Though very short, the limbs are well-developed for mole-like burrowing.

Amphisbaena sp. (left) and Bipes biporus

The New York Times has a story on this, which gets the gist of the story right, but the headline (“Fossil Sheds Light on the Lizard-Snake Divide”) and lede (“The origin of snakes is a perplexing matter”) are way off: the paper concerns the origin of amphisbaenians, not snakes.

h/t: Matthew Cobb

UPDATE: Burrowing lizards seem to be all the rage this week, as alert readers Dominic and James C. Trager have pointed out two other burrowing lizard events in the comments below. First, a new species of blind skink, Dibamus, has been described by Thy Neang and colleagues in the journal Zootaxa (BBC piece here). There are about ten species of dibamids, which lack forelimbs, but have flap-like hindlimbs. Like amphisbaenians, they have bounced around a bit in their classification; the latest work (see papers by Hedges and Vidal below) places them as the earliest branch within the lizards. I’m not sure why this new species merited news coverage, except insofar as all new species are newsworthy. One of the authors of the new species is Lee Grismer, whose alpha taxonomic exploits we’ve noted here at WEIT before.

The second item is a paper by Steve McAlpin and colleagues at Macquarie University in Plosone, describing heretofore unknown complexity in lizard social behavior (NY Times piece here). I’ll let the abstract speak for itself:

Here we provide the first example of a lizard that constructs a long-term home for family members, and a rare case of lizards behaving cooperatively. The great desert skink, Liopholis kintorei from Central Australia, constructs an elaborate multi-tunnelled burrow that can be continuously occupied for up to 7 years. Multiple generations participate in construction and maintenance of burrows. Parental assignments based on DNA analysis show that immature individuals within the same burrow were mostly full siblings, even when several age cohorts were present. Parents were always captured at burrows containing their offspring, and females were only detected breeding with the same male both within- and across seasons. Consequently, the individual investments made to construct or maintain a burrow system benefit their own offspring, or siblings, over several breeding seasons.

Complex social behavior is well known in crocodilians and, of course, birds (which are glorified reptiles), but this is a unique case for squamates (so far). They don’t seem to be eusocial though, which, in addition to overlapping generations, requires cooperative care of the young (there is at least some indirect parental care here), and a reproductive division of labor. The skinks involved are burrowing, but well-limbed.

A social skink, Liopholis kintorei, from Australia. Adam Stow,via NY Times.


Camp, C.L. 1923. Classification of the lizards. Bulletin of the American Museum of Natural History 48:289-481. (pdf)

Hedges, S. B. and N. Vidal. 2009. Lizards, snakes, and amphisbaenians (Squamata). Pp. 383-389 in S. B. Hedges and S. Kumar, eds., The Timetree of Life,  Oxford University Press, New York. (pdf)

McAlpin, S., P. Duckett and A. Stow. 2011. Lizards cooperatively tunnel to construct a long-term home for family members. Plosone 6(5):e19041, 4pp. (pdf link)

Muller J., C.A. Hipsley, J.J. Head, N. Kardjilov, A. Hilger, M.Wuttke and R.R. Reisz. 2011 Eocene lizard from Germany reveals amphisbaenian origins. Nature 473:364-367. (abstract)

Neang, T., J. Holden, T. Eastoe, R. Seng, S. Ith, and L.L. Grismer. 2011. A new species of Dibamus (Squamata: Dibamidae) from Phnom Samkos Wildlife Sanctuary, southwestern Cardamom Mountains, Cambodia. Zootaxa 2828:58-68. (abstract)

Vidal, N. and S. B. Hedges. 2009. The molecular evolutionary tree of lizards, snakes, and amphisbaenians. Compte Rendus Biologies 332:129-139. (pdf)

Your ear bones came from your jaws

October 15, 2009 • 6:22 am

by Greg Mayer

Although the mammals and reptiles most people know are quite distinct– mammals are hairy, warm-blooded, live-bearers, that suckle their young, while reptiles are scaly, cold-blooded, egg-layers– a wider knowledge of the modern forms reveals that the differences are less absolute. There are many live-bearing reptiles, for example, and platypuses and echidnas lay eggs and are nipple-less. And it has long been known that mammals are descended from a particular group of fossil reptiles:  both the great British anatomist Richard Owen and the American paleontologist and zoologist Edward Drinker Cope noted this in the 1800s (Cope doing so in a paper with the wonderful title “The theromorphous Reptilia”, “theromorphous” meaning, roughly, “beast-shaped”).

Because the vertebrate fossil record consists mainly of bones, paleontologists need an osteological distinction between mammals and reptiles, and the definition of mammals is that our jaw joint is between the squamosal bone of the skull and the dentary bone of the lower jaw, while in reptiles the joint is between the quadrate and the articular.

Mammal and reptile jaw joints
Mammal and reptile jaw joints, from Wikipedia by Philcha

The stages in the picture above were about all that were known to Cope and Owen, but they could still see the connection between the groups. (The lower picture is of a pelycosaur, an early type of synapsid reptile, the synapsids being the group of reptiles from which mammals eventually evolved; Dimetrodon was a pelycosaur). Cope’s identification of early synapsids as the ancestors of mammals could be considered a prediction that intermediate forms would be found (I leave out Owen, because his views on evolution were equivocal). Later work has abundantly confirmed this, and the reptile-mammal transition is now probably the best documented of all higher level transitions in the vertebrates. A classic paper by A.W. ‘Fuzz’ Crompton and Farish Jenkins, teachers of mine from grad school, summarized the first 100 years of work on the subject.

Here’s a diagram of one of the intermediate forms. Note that it has a double jaw joint, and the bones in the lower jaw have become much smaller. If you look above to the mammal, you will see that these bones have become even smaller still, and detached from the lower jaw.

Double jaw joint
Double jaw joint from Wikipedia by Philcha. This figure is not quite right. The dentary/squamosal contact is actually much nearer to the quadrate/articular contact. The two joints are lateral and medial to one another, not anterior-posterior.

What has happened is that two bones of the lower jaw (the angular and the articular), and the quadrate of the upper jaw, of reptiles have become (some of) the ear bones of mammals– the tympanic, malleus, and incus, respectively (mammals have another ear bone, the stapes, which is the only ear bone in reptiles). This reduction in size and detachment from the jaw occurred in many gradual steps over many millions of years, all documented in the fossil record. Clifford Cuffey has a nice set of figures of some of these, and Karen Peterson of the University of Washington has posted class notes with some very nice figures. What makes this even neater is that the jaws themselves are derivatives of the anteriormost parts of the branchial (gill) arch skeleton, a subject I’ve mentioned before, and thus we can trace the history of these bones from the branchial apparatus to the ear by way of the mouth.

Just as Matthew was inspired to post about sponges after lecturing about them to one of his classes, I bring up the ear bones because I was lecturing to my vertebrate zoology class about the branchial skeleton and its derivatives this past Tuesday. It was also the very day that the New York Times had an article by Natalie Angier on the evolution of the mammalian ear bones inspired by a recent paper in Science (subscription required for full article) by Qiang Ji and collaborators. They describe the jaw of an early Cretaceous mammal that had a persistent reptile-like connection of the ear bones to the jaw.  The authors propose, quite reasonably, that this is a paedomorphic condition, that is, that it is the retention into the adult of an embryonic condition: mammalian embryos pass through a stage in which their jaw/ear bones resemble those of reptiles.

The working out of the history of these bones is one of the great triumphs of vertebrate comparative anatomy. Neil Shubin (sorry Jerry!) summarizes the highlights nicely in chap. 10 of Your Inner Fish.

Transitional fossils

June 7, 2009 • 10:02 am

The latest issue of the free online journal, Evolution: Education & Outreach, highlights transitional fossils.  There are several good articles here: I especially liked Jenny Clack’s piece on the fish-to-reptile transition, which is now documented by many more fossils than just the famous Tiktaalik, and Thewissen et al.’s article on the evolution of whales from terrestrial mammals. The articles have nice figures for teaching purposes. This is a great journal for laypeople and scientists alike to keep up with education in evolutionary biology, and it’s free.