Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
In a paper in press in the Proceedings of the National Academy of Sciences, Neil Shubin, Ted Daeschler, and the late Farish Jenkins describe the pelvis and partial hind limb of Tiktaalik roseae, the lobe-finned fish from the Canadian high arctic that they discovered in 2004 and described in Nature in 2006. Tikataalik is a transitional form from fish to tetrapods, and presents such a suite of advanced (for a fish) features that Neil dubbed it the “fishapod”. The newly reported finds show that Tiktaalik had a very substantial pelvic girdle and limb, which must have had a significant role in locomotion.
Of course, it’s not a surprise that Tiktaalik had hind limbs– most vertebrates do– but the nature of the hind limbs in this, the most tetrapod-like of fish, is of especial interest. It’s been known for a while that Neil et al. had found the hind limb, and their publication on it has been eagerly awaited. The most important find, a pelvis and part of the associated limb, was actually found on the original holotype specimen (the one from which the species was described) found in 2004; four other isolated pelvises were found in later years. Since the first publication, preparator Fred Mullison has been working to free all the bones from the encasing rock.
Comparison of the girdles of Tiktaalik to those of Eusthenopteron (a ‘standard’ lobe finned fish) and Acanthostega (one of the earliest known amphibians)
So, what have we found out? The pelvis is robust, with an ilium and pubis, and a large acetabulum for receiving what must have been a substantial femur. There’s no ischium (the third bone in a typical tetrapod pelvis). The Tiktaalik website has 3D scans of the pelvis which you can rotate to see the full morphology.
Tiktaalik pelvis from below: ilium on left, the rounded acetabulum for reception of the head of the femur, pubis on right. The pubis is directed medially.
Only a portion of the hind limb was preserved: the intermedium, two radials, and several bony fin rays (lepidotrichia). We can tell from the acetabulum though that the femur must have been robust.
Hind limb of Tiktaalik from Shubin et al. 2014. The thin rays are lepidotrichia; the upper rectangular bone is the intermedium, the lower pair are radials.
Although no femur was found, Tiktaalik‘s fin rays and several other bones suggest the hind fin was comparable in size and complexity to the front fin. The shape and size of the hip socket reveal that the fin was capable of a wide range of movements, from swimming to supporting weight and rotating more like a tetrapod limb. But the overall structure of the pelvis is still more fish-like. Whereas tetrapods have a pelvis made of three parts, Tiktaalik‘s pelvis is still made of one, like fish. …
Overall, the mix of fish and tetrapod characteristics show us that the structures and mechanisms necessary for the invasion of vertebrate life on land evolved in the water first. Not only that, but before this discovery, we thought the front fins held the key to how vertebrates began to walk on land. The “front wheel drive” theory that fish dragged themselves out of the water with strong front fins and puny hind fins no longer holds. It appears that an “all-wheel” or even a “rear-wheel drive” system is a more appropriate analogy as the hind fins were just as important and may have even been involved in a walking behavior first.
Shubin, N.H., E.B. Daeschler and F.A. Jenkins, Jr. 2014. Pelvic girdle and fin of Tiktaalik roseae. Proceedings of the National Academy of Sciences in press. pdf
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 & Claw. As 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.
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.
