A Big Dead Fish Story
~
A BIG DEAD FISH STORY ~
Dec
4, 2012
C. F. Mansky, Curator, Blue Beach Fossil Museum
Every
fisherman seems to have a good story about ‘the one that got away’. This all-too
familiar scene has been repeated in books and art; in theatres and on film. The
so-called ‘big fish story’ has, in short, become something of a North-American
tradition. So by this point you’re probably already thinking “…just about heard
them all”, and you could be forgiven for reacting sceptically to the above
title. Trust me, this is a fish story you probably never heard
before.
For
several years now my work at Blue Beach, Nova Scotia (Canada) has involved,
among other things, uncovering a truly big fish that’s been extinct for hundreds
of millions of years. Fossils of this giant fish, belonging to the family
Rhizodontida, have grown into a ‘massive’ collection that has spilled out of the
Blue Beach Fossil Museum near Hantsport; and now also happens to occupy the
greater part of our basement. The ongoing discovery of thousands of skeletal
parts at Blue Beach, in 350 million year-old sedimentary rocks, is providing new
information on these mysterious and lurking beasts. Were it not for the powerful
tides of the Bay of Fundy these rare and spectacular fossils would seldom be
found. They are, however, now accumulating quite nicely.
Rhizodonts
were enigmatic ‘lobe-finned’ fish that grew up to 8 meters in length, larger
than your average sport-fishing boat (1). 
Weighing
in at several tons, it’s probably good they no longer terrorize marshes and
lakes. For one thing they’d probably give the crocodiles and alligators a real
run for their money, because they’d be virtually occupying the same ecological
niche. Rhizodonts were big ambush predators whose behaviour and feeding strategy
in many ways mirrored that of today’s crocs. These huge fish could evidently
also crawl on their powerful front fins, gulping air into primitive lungs and
even, presumably, gave chase. This remarkable feat doesn’t seem so surprising
when one looks at the skeletons of their fins. Rhizodonts possessed arm
bones.
Weighing
in at several tons, it’s probably good they no longer terrorize marshes and
lakes. For one thing they’d probably give the crocodiles and alligators a real
run for their money, because they’d be virtually occupying the same ecological
niche. Rhizodonts were big ambush predators whose behaviour and feeding strategy
in many ways mirrored that of today’s crocs. These huge fish could evidently
also crawl on their powerful front fins, gulping air into primitive lungs and
even, presumably, gave chase. This remarkable feat doesn’t seem so surprising
when one looks at the skeletons of their fins. Rhizodonts possessed arm
bones. 
Another
way to characterize rhizodonts is to say they were more closely related to us
than any living fish is today. Rhizodonts have recently been resolved as 'basal
members of the Tetrapodomorpha’ (2), which literally means
changing towards four-leggedness. They were part of the evolutionary
history of vertebrates when land-animals separated from their fish-ancestors.
The rhizodonts can be likened to the first 100 meters of the off-ramp leading
towards a new, four-legged condition. They’re not the ones who actually changed
their fins into the first limbs – they’re the start of the movement. Several
families down the road would be the ones that made the final changes, and thanks
to this we vertebrates eventually conquered land.
Scientists
still know less about this enigmatic group of fishes than they do about any
other lobe-finned group. The main reason for this is purely taphonomic, or
because of the realities of fossil preservation. Large animals are less-likely
to be fossilized ‘intact’ than small animals. Adult rhizodonts were of such
size, the likelihood of 'their’ preserving as intact skeletons is slim. To
compound this problem, rhizodont skeletons were rather weakly connected
throughout – adaptations to improve flexibility and reduce weight. Their
skeletons were therefore ‘quite prone to coming apart’, or disarticulating. To
this day the complete adult rhizodont skeleton remains undiscovered.
Paleontologists must therefore work with individual rhizodont parts,
assembling these like a puzzle to eventually conceive an entire skeleton. At
Blue Beach, where twice per day the highest tides in the world are working to
steadily expose fossils, we’ve finally gathered more parts than ever
seen before. This research is resulting in a previously-obscure fossil fish from
Blue Beach eventually becoming the best-known example of the mysterious
Rhizodontida.
Trace
fossils: new perspectives
The
rhizodont project at Blue Beach involves additional avenues of inquiry such as
the ‘trace fossil’ evidence. Trace fossils are not actual parts of
animals like the bones are, but some act of an animal, usually some
kind of markings. Traces include tracks, trails, tunnels, burrows and borings on
one hand – but also things like fecal remains, stomach contents, vomitus, webs,
nests, and so forth. Trace fossils can also be defined as activities or
behaviours, usually the work of some animal. They inform on aspects of ancient
life after the body fossils have nothing more to tell.
One
of the most unusual traces at Blue Beach are some recently
discovered ‘intestinal-contents’of the big dead fish (3). These show
the rhizodonts apparently caught and ate whatever came too close, as they
contained an unusual quantity of
vertebrate
bones, including smaller members of their own species. Also not safe from the
hungry rhizodonts were numerous small, shark-like fishes with bony fin-spines,
and shiny-scaled little palaeoniscoid fishes: together their bones made up the
bulk of what these rhizodonts were eating. Finally, a single leg bone belonging
to an early amphibian or tetrapod, shows that land-animals were also preyed
upon; perhaps sometimes even after a brief chase onto the land.
The
idea that lobe-finned fishes were occasionally using their pectoral-fins to
‘crawl’ has long been accepted because of the obvious limb-like structure of
their fins. However, theories about ancient crawling behaviour based solely on
features of the fins never managed to make it past the theoretical stage. Any
obvious and deliberate crawling activity by the fishes should have left its fair
share of markings in the trace-fossil record, but no verified cases have been
reported. By sharp contrast, the footprints of very early tetrapods were first
recognized over 170 years ago. As coincidence would have it, this first
Carboniferous-aged footprint discovery was made here at Blue Beach (also called
‘Hortons Bluff’) by Sir William E. Logan in 1841. Blue Beach has therefore
already played a very significant role in the early history of ichnology (the
study of trace fossils).
The
long-awaited confirmation of ‘crawling behaviour’ in lobe-fins can now be said
to exist in some recently recognized rhizodont-trails at Blue Beach (3,
6). These traces have been observed by scientists on many occasions, but
they have never been properly identified for what they are. Numerous
fin-imprints, belly-resting imprints and tail-drags are preserved on several
conspicuous layers that represent ancient mudflats. More than one set of these
fin-impressions form continuous trails; one has an apparent length of over 50
meters. This impressive example (with fin-imprints larger than a human foot),
was first discovered in the early 1960’s, and was later described as the track
of a large amphibian 4, 5). Some now believe otherwise, (Spencer G.
Lucas of the New Mexico Museum of Natural History, and Science and I, for
example). We regard 'the giant trackway’ to be the trail of a big
fish.
As
paleontologists have recently come to appreciate, trace fossils often provide
new insight where the body fossil record has nothing more to add. Subtleties
regarding trace-maker behaviours, as well as vital clues to their ecology and
environmental preferences: only trace fossils can inform us on these. The
utility of trace fossils for adding these new dimensions to the reconstructs of
ancient ecosystems has now become widely recognized as an important tool for
solving many paleontological questions.
Acknowledgements:
I
especially wish to thank Sonja E. Wood, my wife and co-founder of the Blue Beach
Museum, for all her time and effort and for her evaluation and improvements in
this article. I also wish to thank Dr. Spencer G. Lucas for his continued moral
and technical support.
www.bluebeachfossilmuseum.com
(1)
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.
(2)
Ahlberg,
P. E. and Johanson, Z., 1998, Osteolepiformes and the ancestry of
tetrapods, Nature,
(Oct 22, 1998), v. 395, p. 792-794.
(3)
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 Coprolites,
New Mexico Museum of Natural History and Science, Bull. 57, p.
161-170.
(4)
Sarjeant,
W. A. S. and Mossman, D. J., (1978), Vertebrate footprints from the
Carboniferous sediments
of Nova Scotia: a historical review and description of
newly-discovered forms;
Palaeogeography,
Palaeoclimatology, Palaeoecology, v. 23, p. 279-306.
(5)
Mossman,
D. J. and Sarjeant, W. A. S., (1980), How we found Canada’s oldest
known footprints;
Canadian Geographic, (Oct/Nov, 1980), 100 (5), p. 50-53.
(6)
Lucas,
S. G.,Hunt, A. P., Mansky, C., and Calder, J. H., 2004a, The oldest tetrapod
footprint ichnofauna,
from the Lower Mississippian Horton Bluff Formation, Nova Scotia,
Canada; Geological
Society of America, (Annual Meeting, Denver 2004), Abstracts with
Programs,
v.
36 (5), p. 66.
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