Transitions. Nothing surprising about being preoccupied these days by transitions, given the striking and powerful challenges to this country’s social and political norms. In a situation of such fluidity, one should be hopeful that meaningful change will be coming, but I don’t hold my breath. But among the transitions that have garnered a significant portion of my attention these past several days is one that happened in March of this year and ones that lie back, way back, in the Upper Devonian period, some 383 to 359 million years ago. The first was the death from cancer of paleontologist Jennifer A. Clack (1947-2020); the second is a couplet of transitions: fish to tetrapods, and vertebrates from an aquatic existence to terrestrial living. (Tetrapods are any four-legged animal or any animal whose ancestors had four legs.)
Last month I posted a review of paleontologist Neil Shubin’s new book, Some Assembly Required. Not mentioned in that post was that Shubin occupies a preeminent place among those who study the transitions of vertebrates from water to land, and from fish to tetrapods, particularly with the find in the Canadian Artic in 2004 of the spectacular fossil Tiktaalik, a lobe-finned fish sporting various traits shared by tetrapods, including highly advanced pectoral fins. As Shubin wrote of the animal’s pectoral fins in Your Inner Fish (2009), “Tiktaalik has a shoulder, elbow, and wrist composed of the same bones as an upper arm, forearm, and wrist in a human. When we study the structure of these joints to assess how one bone moves against another, we see that Tiktaalik was specialized for a rather extraordinary function: it was capable of doing push-ups.” (p. 39) And, in that book, Shubin noted that push-ups could be useful if one had to navigate in shallow streams straddled by mudflats. Though clearly a fish living in an aquatic environment, Tiktaalik was a way station en route to tetrapods.
While writing last month’s post, I learned that Jennifer Clack had passed away. See the piece by Per Ahlberg titled Obituary: Jennifer Clack (1947-2020): Palaeontologist Who Described How Vertebrates Moved From Water to Land (Nature, Volume 580, April 30, 2020, p. 587). Ahlberg’s obituary features a wonderful picture of Clack. She’s wearing a broach shaped like an early tetrapod, probably Acanthostega. Clack merits inclusion in my pantheon of paleontological heroes for, among other accomplishments, (1) her seminal work on one of the earliest tetrapods, Acanthostega, fleshing out the morphology and environment of that animal through fieldwork in Greenland and meticulous research on the fossils her expeditions found, and (2) the filling of “Romer’s Gap” with, once again, fieldwork, only this time in Scotland, and careful research of her finds.
Though I am likely to get into trouble by venturing into areas about which I know even less than usual, I will consider both of those areas of accomplishment very briefly (hoping, thereby, to minimize misstatements; I will also try to let others describe her work). The Upper Devonian strata are where fossils reflecting the evolution of fish to tetrapods are to be found. The clade diagram below is based on one that appears in a recent article by paleontologists John A. Long and Richard Cloutier titled The Unexpected Origin of Fingers (Scientific American, June 2020). That’s the title of the print version of the article; online it’s: How a 380-Million-Year-Old Fish Gave Us Fingers which is incorrect since the animal in question is 375 million years old. The focus of this article is a close relative of tetrapods, perhaps the closest yet found, named Elpistostege watsoni. As shown in this clade diagram, Elpistostege occupies a position nearer to tetrapods than Tiktaalik, a position that may have resulted largely from the completeness of a fossil of the former compared to that of the latter.
For reference, Tiktaalik is dated at 380 million years, Elpistostege at 375 million years, and Acanthostega (one of Clack's domains) at 360 million years. As one moves from left to right up the clade diagram from predecessors to Tiktaalik on to Acanthostega, the transition from fish to tetrapod is proceeding. This transition involved various critical morphological changes, among others, the evolution of limbs from fins and the appearance of a neck.
In 1987 and 1998, Clack undertook fieldwork in Greenland, initially searching an area from which fossils of two very early intermediate animals (Ichthyostega and Acanthostega, the latter known at the time only from skull roof fragments) had been found in the late 1920s and early 1930s. In that first expedition, she found fossils of Acanthostega which included post-cranial features. And therein lay treasure because the fossils showed that the limbs of this taxon had the universal tetrapod arrangement of limbs. To quote Shubin, this is a “one bone-two-bones-little bones-digits pattern” which “is a grand anatomical theme, an ancient pattern that underlies the diversity of every creature with a limb skeleton.” (Some Assembly Required, p. 119-120.) Significantly, these limbs of the Acanthostega tetrapod sported more than five digits, broadening the ends of the limbs.
Clack described the importance of the Acanthostega fossils for our understanding of the transition of vertebrates from fish to tetrapod, and that from water to land:
Here was a creature that had legs and feet but that was otherwise ill equipped for a terrestrial existence. Acanthostega’s limbs lacked proper ankles to support the animal’s weight on land, looking more like paddles for swimming. And although it had lungs, its ribs were too short to prevent the collapse of the chest cavity once out of water. . . . In other words, this animal, though clearly a tetrapod, was primarily an aquatic creature whose immediate forerunners were essentially fish that had never left the water. (Clack, Getting a Leg Up on Land, Scientific American, December 2005, p. 102.)
Clack flipped the orthodoxy concerning the timing of these two transitions which had posited that moving onto land occurred before or concurrent with becoming a tetrapod. In fact, the intermediate taxa were becoming and became tetrapods before they had to navigate on land. When that began, the morphological changes they’d already undergone were to serve them in good stead. Yes, this is another instance of what Darwin posited would be true of some evolutionary developments: they could occur "by a change of function." (This is the core premise of Shubin’s Some Assembly Required.)
As for Romer’s Gap, Clack filled it in with early tetrapods. In her book, Gaining Ground: The Origin and Evolution of Tetrapods (second edition, 2012), she described how paleontologist A.S. Romer had observed that the fossil record contained a 30-million-year gap immediately following the Devonian. This gap from about 359 to 331 million years ago fell early in the Carboniferous period and, as Romer noted, presumably covered the time when the actual movement of vertebrates fully from water to land occurred. Among the explanations for the gap that scientists had proffered was that the absence of fossils in this time period reflected an actual absence of terrestrial animals due to environmental challenges.
Though Clack acknowledged that the mass extinction at the end of the Devonian played a role in what the fossil record showed, she demonstrated that the gap in the record could be addressed in large part by looking hard for fossils in the right places. To that end, she undertook the TW:eed Project, searching in Scotland, north of Berwick-upon-Tweed. Ahlberg describes the success of the project as follows:
These sandstones and mudstones promptly started yielding fossils of previously unknown tetrapods. Six genera have been described so far and more material awaits description. They look set to revolutionize our understanding of the early diversification of tetrapods by filling in the wide morphological gap between very primitive ones, such as Acanthostega, and the more modern tetrapods of the Carboniferous. (Obituary, p. 587.)
Above I mentioned Clack’s book Gaining Ground. This book is a signal accomplishment, providing a comprehensive, authoritative examination of the transition from fish to tetrapods, delineating the morphological changes of these animals in detail, and also describing the environments within which they lived and its influence on their development. Shubin describes the book as “magisterial” (Some Assembly Required, p. 224) and as “[t]he bible of this transition” which “will bring a novice to expert status quickly” (Your Inner Fish, p. 212.) The book is not for the faint of heart given how fully Clack entered into the skeletal complexity of the many taxa she considered. At the same time, during the course of quite serious analyses of tetrapods and tetrapods-in-the-making, she, on occasion, offered light asides. I approached the book with high expectations having conflated “magisterial” with “magical" or "majestic;” sadly, I certainly remained a novice after reading it. The book, in fact, lies somewhere between a paleontology textbook and a lengthy volume of popular science. It’s neither, though it shares more attributes of the former than the latter. Dare I say, it is a transition volume.
I’ll close with a tiny bit of biography drawn from the obituary by Ahlberg and from Clack’s own website. She was born in Manchester, England, and studied zoology as an undergraduate at the University of Newcastle upon Tyne. Though she would have liked to pursue Ph.D. study of paleontology after graduation, financing wasn’t available. So she earned a certificate in museum studies and went to work at the Birmingham City Museum and Art Gallery. After several years there, looking for a paleontological side project, she approached paleontologist Alec Panchen, whose paleontology option she’d taken at the University of Newcastle upon Tyne. This would profoundly change the course of her professional life. Here’s how she described that experience.
I approached Alec Panchen for ideas. He said "What about trying to get hold of the holotype of Pholiderpeton scutigerum from Bradford Museum? It's the last remaining embolomere [Early Carboniferous tetrapod] specimen undescribed in this century, and I've never been able to borrow it to work on." I approached Bradford Museums, who, in the intervening years, had changed their policy on loans of material. They recommended that the specimen be transferred to Newcastle to be worked on in Alec's lab, and I took three weeks' study leave from Birmingham to work on it. That was the beginning of my good luck in palaeontology, which seems to have been with me ever since.I began to prepare the material, and almost at once, discovered it was more complete than had been previously realised. Having uncovered a curious-looking lump on the previously hidden side of the specimen, I showed it to Alec, who paused and then said "Well I'm damned - it's a braincase!" and then later "There is probably a PhD in this. If you'd be interested, I'll try and get a grant." Was I interested? Thoughts of bears and woods...
Thoughts of bears and woods!!
In 1981, her Ph.D. work at the University of Newcastle upon Tyne led to a position as Assistant Curator at the University Museum of Zoology, Cambridge. She was a tenured member of the Zoology Department at Cambridge and remained there for the rest of her career. As she noted on her website:
As Curator, I have several different strands to my work: curation, teaching, administration and research. Each of these is supposed to take about one third of my time (!).
How she managed to fulfill all of her professional obligations while being so fully and productively engaged in her paleontological research staggers the imagination. Ahlberg probably captured the essence of her character that enabled her to do what she did:
A happy convergence of brilliance, tenacity, opportunity, generosity and modesty enabled Clack (née Agnew) to rejuvenate an entire research field.
I will close by quoting from the author description that accompanied her December 2005 Scientific American article (p. 105). These two revealing sentences are a delight:
A fellow of the Linnean Society, Clack’s outside interests include choral singing (particularly of early sacred music) and gardening. She is also a motorcyclist and rides a Yamaha Diversion 900.
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