May 11, 2020 | Margaret Helder

Latest Tetrapod Ancestor Can’t Stand Up

 

Fame and Fortune:

New fish claims sister taxon status to tetrapods

By Margaret Helder, PhD

Many scientists dream of attaching their name to some really significant achievement. Indeed, within paleontology there are few milestones considered to be more significant than describing the closest taxon to four footed animals. Without the ability to walk on land, the invasion of the land by vertebrate animals would not have been possible, so the thinking goes. Thus the search for the closest sister taxon to tetrapods continues apace. And it continues in 2020 with an article from Nature, which describes the latest candidate for this honor.1 It turns out that Elpistostege watsoni is uniquely found in a geological formation in Quebec, which should be much more famous than it is.

Credit: Earth Magazine

There may be few landmarks that everyone knows from Canada, but the Gulf of St. Lawrence is surely one of them. A finger of land that marks the southern shore of the St. Lawrence River, projecting into the gulf, is called the Gaspe Peninsula. If one travels east along the southern shore of the peninsula, one encounters a bay called Baie des Chaleurs, formerly Scaumenac Bay. On the north shore of the bay, just as one enters the Restigouche River, lies Parc de Miguasha. This is the official name for an amazing deposit of fossilized fish (both large and small) and terrestrial plants. The deposit is called the Escuminac Formation. Largely unfamiliar to English-speaking North Americans, this is one of the most dramatic fossil sites in the world!2

Miguasha National Park (credit: Quebec Original)

Dramatic Fossil Bed

A cliff in the park exposes part of a thick sedimentary bed 385 ft. (118 m) thick. It must have been a disaster scene. Tremendous numbers of fish were engulfed and permanently buried by massive underwater avalanches3 which scoured plants and sediments from the land4, thereby burying the fish in the receiving body of water. The currents bearing the sediments came uniformly from the same direction (northeast) throughout the entire period of deposition.5 For turbidity currents to come from the same direction for the entire time it took to dump hundreds of feet of sediment, is certainly unusual!

All the evidence seems to point to a catastrophic event which engulfed one fish community. The composition of the community is the same throughout these hundreds of feet of strata. Among the creatures were jawless fish, armoured fish that look nothing like what we see today, and lungfish with robust fins. The fauna was so uniform that ninety five percent of the community was dominated by only four fish species: two small species in Bothriolepis and Triazeugacanthus and two lungfish with robust fins, Eusthenopteron and Scaumenacia.6

Eusthenopteron. Drawing by Nobu Tamura.

It is however the condition of the fossils themselves that attests to the sudden permanent burial of the community. Many of the fish are preserved in three dimensions and some large fish like Eusthenopteron are occasionally found in the posture of rigor mortis7 or even in postures suggesting medical distress.8 Also surprisingly for a large fish like Eusthenopteron and others, they are “usually found complete or almost so, which suggests they were interred quickly after death.”9 Other signs of sudden disaster include the very small fish Triazagacanthus deposited in layers at densities of up to 600 specimens per square meter . The small fish Bothriolepis was variously found in a bed lying upside down10 and in other beds with all specimens oriented in the same direction.11 Even more dramatic were the finds of large fish Scaumenacia and Eusthenopteron with post mortem gas rupture of thoracic cavities!12  But one of the most dramatic discoveries involved two specimens of Eusthenopteron. Within a depth of 10 cm (4 inches) two fish were found stacked on top of each other and lying between layers of plant debris above and below.13

The Evolution of Tetrapod-Evolution Theories

Edward Drinker Cope (1840-1897)

It was on this scene of devastation that paleontologists began to focus their attention. Beginning in 1879 the famous dinosaur hunter (also a fish expert) Edward Drinker Cope, for example, declared in 1892 that Eusthenopteron from the Escuminac Formation was a key example of a transitional form between fish and tetrapods. Later Erik Jarvic (1907-1998) of Sweden devoted his whole working career of close to 60 years, to describing Eusthenopteron morphology and characterizing it as a sister taxon to all four limbed animals with backbones. This fish continued to enjoy its exalted status as ‘Prince of Miguasha’ until 1998. Since then, two other fish, one from Latvia and the other from the Canadian north, have claimed the coveted title as sister taxon to the tetrapods. But now in 2020, another fish from Miguasha, this one very rare, has claimed the title. The story of the features of these various fish and how these traits compare to tetrapod morphology, is fascinating indeed.

Before we begin to discuss the so-called transitional forms, it is important to establish what a tetrapod is, and what a fish is. In the past, mistakes have been made. In 1938 British scientist Thomas Stanley Westoll (1912-1995) described Elpistostege watsoni as an amphibian (tetrapod such as frogs). This identification was based on two small fragments from the skull discovered at Miguasha.14 For the next 47 years Elpistostege was considered a tetrapod. That continued until another fish (Panderichthys) was discovered which exhibited similar skull bones, but was clearly a fish. The book which tells the story of Miguasha, shares with us what happened with Prof. Westoll: “The misinterpretation of the holotype of Elpistostege as a tetrapod on supposedly clear tetrapod features should make everyone cautious about using fragmentary material to decide placement between Elpistostegalia and Tetrapoda.”15 Prof. Westoll had assumed that tetrapods exhibited a distinctive and exclusive set of features, and if one identified any one of these traits, all the rest were sure to be present also. So, what characteristics actually define a tetrapod?

Elpistostege watsoni fossil on display at Miguasha National Park

Canadian Catherine Boisvert, at the time studying in Sweden (and currently in Western Australia), published some definitions of the differences between fish and tetrapods in Nature in 2005 while still a graduate student.16 It turns out that a tetrapod has two sets of limbs, both sets of which are connected to the back bone and the dominant set of limbs is the back (hind) limbs. The hind limbs and the pelvis are attached to the backbone by a sacrum (bone), a novelty not found in fishes. Thus, Boisvert declared of the vertebrate transition from fins to limbs: “This transformation involved not only the generation of morphological novelties (digits, sacrum) but also a shift in locomotory dominance from the pectoral to the pelvic appendage.”17

Digital Imagining

The feature of Eusthenopteron and other fishes that attracted attention as possible transitional traits, was the robust fins, especially the front (pectoral fins). But in Eusthenopteron, the back (pelvic fins) looked fairly substantial as well. Neither set of fins in the fish, however, was attached to the backbone. The pectoral fins were attached to a pectoral girdle that was attached to the skull. This made for very poor flexibility for the head. The pelvic fins were only connected to an independently situated pelvic girdle. Thus, fish lack many essential features of tetrapods such as strong legs and an independently moving head.

Tetrapods have four limbs and digits. Northern Two-lined Salamander Eurycea bislineata (Brian Gratwicke).

Tetrapods also have digits in the limb extremities. This is another issue of concern to the paleontologists. Thus, Boisvert et al. declared in 2008: “Whereas the proximal [close to the body] part of the tetrapod limb skeleton can easily be homologized with the paired fin skeletons of sarcopterygian (lobe-finned) fish, there has been much debate about the origin of digits.”18 Nevertheless, as specialists began to study these fossils with CT scans, many came to the conclusion that fingers are not “a novelty of tetrapods but [are] derived from pre-existing distal radials [bones] present in all sarcopterygian fish.”18 Thus the race was on to discover which fish contained the most “finger-like” bones within their fins.19

To be continued: in the second part of “Fame and Fortune” tomorrow, Dr Helder compares the previous candidates for tetrapod ancestor with the latest one.

Footnotes

  1. Richard Cloutier, Alice M. Clement, Michael S. Y. Lee, Roxanne Noel, Isabelle Bechard, Vincent Roy and John A. Long. 2020. Elpistostege and the origin of the vertebrate hand. Nature 579 #7800 549-554.
  2. P. Schultze and R. Cloutier (editors). 1996. Devonian Fishes and Plants of Miguasha, Quebec, Canada. Verlag Dr. Friedrich Pfeil. Munchen pp. 374.
  3. Schultze and Cloutier p. 26 “mode of deposition by turbidity currents”.
  4. Schultze and Cloutier p. 23 “essentially terrigenous detrital sediments” and p. 28 “massive transportation of material into the basin”.
  5. Schultze and Cloutier p. 30, 33.
  6. Schultze and Cloutier 75 Bothriolepis canadensis 38.5% (small), Triazeugacanthus affinis 29.4% (very small), Eusthenopteron foordi (to 5 ft), and Scaumenacia curta (to 2 ft).
  7. Schultze and Cloutier p. 61, 72.
  8. Schultze and Cloutier p. 72.
  9. Schultze and Cloutier p. 73.
  10. Schultze and Cloutier p. 60,
  11. Schultze and Cloutier p. 64, 73.
  12. Schultze and Cloutier p. 62, 71.
  13. Schultze and Cloutier p. 59.
  14. Schultze and Cloutier p. 316.
  15. Schultze and Cloutier p. 324
  16. Catherine A. Boisvert. 2005. The pelvic fin and girdle of Panderichthys and the origin of tetrapod locomotion. Nature 438 #7071 pp. 1145-1147.
  17. Boisvert p. 1145.
  18. Catherine A. Boisvert, Elga Mark-Kurik and Per E. Ahlberg. 2008. The pectoral fin of Panderichthys and the origin of digits. Nature 456 #7222 pp. 636-638. See p. 636.
  19. Looking for fingers seems irrelevant in appendages that are not even attached to the backbone.

Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.

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