Hidden in Stone
~
A DAY
IN THE LIFE
OF A BIG DEAD
FISH ~
Part
One
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…
Further
Reading:
(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
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