Hidden in Stone
~ A DAY IN THE LIFE OF A BIG DEAD FISH ~
Dec 20, 2012 C. F. Mansky, Curator, Blue Beach Fossil Museum
The last time we discussed the ‘big dead fish’ it was, I’m pleased to say, only a surficial introduction to this Beast of the Minas Basin. There are many avenues left for discussion about how the ancient rhizodonts were interacting with their environment and each other, or about the anatomy and evolution of a huge fish that could crawl onto the mudflats and sandbars like a walrus, gulping air in great swallowing motions - snapping up anything too surprised to flee in time…
But where should one begin? In order to properly understand the ferocious fish scenario just described we need to break it down to the facts as we know them. The facts can only be obtained from a single source, the fossils. Fossils are the only actual evidence there ever is for us to talk about life on earth in the distant past. Only rocks are old enough to record these ancient happenings…everything else decays. The only alternative to not decaying is therefore to become a fossil of some kind.
Rhizodonts were first known by their fossils before the science of paleontology could even be said to have gotten underway (ie., Ure, 1793). These early discoveries were beyond the comprehension of the first naturalists, and no great attention seems to have been drawn to them. Then in 1828, at an English coal mine in a place called Burdiehouse, Mr. S. Hibbert made a series of discoveries that would begin our fossil saga – would start the ‘paleontology’ of rhizodonts. (Paleontology in a nutshell: (1) first develop some models of ancient organisms or environments; (2) interpret these models as best you can; (3) begin to reconstruct a snapshot of how this ancient world looked and operated).
Hibbert found several enormous teeth that we now know belong to rhizodonts. They were so large at first these were believed to belong to some kind of reptile, like one of the Mesozoic dinosaurs being found in and around that time. Hibbert and others soon realized they were dealing with a big fish, not a saurian, and proceeded to seek more of their bony parts. Generous rewards were soon offered to the miners, and the collections from Burdiehouse and other localities began to grow. Different scientists began describing teeth from one place, and scales from another, and these British rhizodonts were soon known as
either Rhizodus or as Strepsodus, with new
species being described left and right. This becomes one of paleontology’s most
famous confusions, with new species for every new form of tooth or scale. By the
time this frenzy of species-naming was over, nobody had a clue what a real
Rhizodus or Strepsodus was really all about. The result was
science being unable, for over 150 years, to name a complete rhizodont skeleton
if one was found. We were restricted to comparing only the new teeth and scales
to this British material, knowing these were of little to no value in
determining one species of fish from another.
Fortunately, these British rhizodonts weren’t the only fossils being found, and in 1843 James Hall described a complete pectoral fin of a rhizodont from Pennsylvania that we know today as Sauripterus. Late Devonian in age, Sauripterus had a few small differences in his shoulder and humerus, and was obviously different than the British rhizodonts. The scientific world was shocked by the arm-like design of its fin. There was a ball-and-socket shoulder, upper and lower arm bones with elbow and wrist; there were finger-like bones. All of these bones were stout and built for bearing weight. Their shapes included areas for attaching muscles, implying they were employed like arms. Sauripterus is responsible for starting many theories on tetrapod origins, where we first began to consider lobe-finned fishes as a distant ‘mother’.
However as we now realize, the rhizodonts weren’t the best fish to use when looking for clues on the tetrapod transition because they were several families too primitive to be a good model. Rhizodonts were the base of the movement, not the final act. Being the basal part of the tree isn’t a bad place to be at though, meaning the rhizodonts are therefore still very informative on the fundamental beginnings of an evolutionary miracle.
So what did these early scientists understand by that point? At first they could say they thought the teeth were from reptiles, then they figured out it was a huge fish instead, following which they become appreciated as very special big fish – with arm-like fin skeletons. There is still a lot of anatomy that hadn’t been explained. Fast forward the story to the 1970’s and 80’s, to a new generation of researchers on a mission to understand the many kinds of lobe-finned fishes. Enter one Shirley Mahala Andrews, whose study of British rhizodonts was several decades in the making. Andrews never quite completed all of her ambitions, namely: she never finished a complete review of all the known rhizodonts, having spent so many years ‘extracting’ and interpreting the British material. Her accounts of rhizodonts were still incomplete ones, barely fleshing-out their basic details, but several landmark papers were released, giving us our first functional picture of how these fish were built, and how they operated. This preliminary ‘reconstruct’ of the rhizodonts tied together hundreds of little observations, each may seem so insignificant by themselves, but correctly interpreted they spoke volumes. Much of what follows comes from the works of S. M. Andrews, especially Andrews (1985).
Here’s some of Andrews’ little traits, and how they came to revolutionize the model of a big dead fish:
(1). Lateral Lines: There were these curious little pores that pierced some of the bones, and some of the scales too. They were especially dense around the edges of the mouth and snout, and they formed continuous ‘lines’ of closely-space openings that travelled back from the jaws, across the cheeks, and down the length of the body in parallel arrays. Andrews’ interpretation is that these are ‘lateral lines’, or openings for a specialized system of nerves used by fishes to detect faint electro-magnetic fields (or vibrations): a sort of radar. Lateral lines are common to many fishes, so it was not surprising to find out rhizodonts had them. What surprised everyone was their degree of development in rhizodonts, who perhaps had the most elaborate lateral-line system ever seen. Andrews translated this as rhizodont = super-predator, able to detect elusive prey in dark or muddy waters, no problem…
(2). Neck Muscles: Most fishes had heads firmly attached to their bodies without muscles connecting their ‘shoulder’ to their head. Early amphibians had muscles in this area, as did all of their descendants, but the muscles in rhizodonts didn’t attach to the same part of the shoulder as amphibians. In fact, rhizodont shoulders are so specialized, no other fish or amphibian has ever repeated this design. So why did they have these unusual shoulders and muscles? Andrews speculates this is related to their feeding habits. After catching their prey, they would lift them from the water and use violent head-shaking movements to help rend them to pieces. Alligators do it today, and when they lift another animal out of the water its body mass is sixfold what it was underwater. Alligators subdue and reduce large prey in this manner so they can obtain smaller, swallowable pieces. In order to effectively ‘thrash’ your prey like this, you need to be much larger than the thing you want to mess with, and you need a firm grip on the mud beneath you. Andrews saw how all these related structures in the rhizodonts were perfectly suited to this task, including the unusual presence of neck muscles in a fish.
Dr. Andrews did so much, with so very few fossils, to create an understanding of the enigmatic big fishes of Hibbert. Her studies could have been easier if a few complete skeletons were available, but rhizodonts are famous for being very fragmental almost all the time, which is why they remain so enigmatic. Andrews never finished her grand review of the rhizodonts, but she passed the torch before she left us. In 1999 Cambridge University awards a doctorate degree to a newcomer in rhizodont studies, John E. Jeffery, and he will be the first to finally clarify over 150 years of confusion on this fish group, opening the way for Blue Beach and other rhizodont localities to begin naming and reconstructing their fossils.
But that is just the start of yet another round of tales, and ones that we should probably leave for another occasion. For now you should be content to know we are still only exploring the beginnings of a rich and complex chronicle, and should remember that nothing worth really telling should ever be pushy or rushed, rather lingered on like we would a fine repast. Next time we will explore the new-wave of understanding that the twenty-first century brings in the study of rhizodonts. Personally, that my favourite part; it’s where Blue Beach and our work enters the story. We’ll call it Part Two…
(1) Andrews, S. M. + Westoll, T. S., 1970b, The postcranial skeleton of rhipidistian fishes excluding Eusthenopteron; Transactions of the Royal Society of Edinburgh, v. 68 (12), p. 391-489.
(2) Andrews, S. M., 1982, The discovery of fossil fishes in Scotland up to 1845: with checklists of Agassiz’s figured specimens; Royal Scottish Museum, Edinburgh, Royal Scottish Museum Studies, 87 p.
(3) Andrews, S. M., 1985, Rhizodont crossopterygian fish from the Dinantian of Foulden, Berwickshire, Scotland, with re-evaluation of this group; Transactions of the Royal Society of Edinburgh, Earth Sciences, v. 76, p. 67-95.
(4) Ahlberg, P. E. and Johanson, Z., 1998, Osteolepiformes and the ancestry of tetrapods, Nature, (Oct 22, 1998), v. 395, p. 792-794.
(5) Mansky, C. F., Lucas, S. G., Spielmann, J. A. and Hunt, A. P., (2012), Mississippian bromalites from Blue Beach, Nova Scotia, Canada; in Hunt, A. P. et al. (Eds.), Vertebrate