Are spiral bromalites enterospirae or coprolites? A brief review and some additional evidence

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SVP 2020
Are spiral bromalites enterospirae or coprolites? A brief review and some additional evidence
David J. Ward, Christopher J. Duffin and Alison E. M. Ward
Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Introduction
An academic fascination with bromalites, more commonly referred to as
coprolites, fossil faecal material, goes back almost two centuries. In the
draft of Henry de la Beche’s cartoon Duria Antiquior, translated as “A
more ancient Dorset” all the marine reptiles were immortalised in the
process of defaecation. In the finished work, an editorial paintbrush had
reduced the number of putative coprolites heading for the sea floor. At
the time, it was assumed that spiral coprolites, common at Lyme Regis,
were the product of marine reptiles, namely ichthyosaurs and
plesiosaurs. We now attribute them to sharks.
FOR: Some bromalites from the early
Jurassic of Dorset. demonstrate a clear
calcite layer approx. 300 to 700 microns
wide, between the individual whorls of
sectioned bromalites. Fish scales within the
faecal matrix do not penetrate from one
whorl to another. This is consistent with the
individual whorls being separated by a thin
mucosal membrane and would be difficult to
explain if these were passed coprolites.
It was William Buckland who in the 1830’s, by filling the intestine of a
dogfish with Roman cement, demonstrated that spiral coprolites could
have been produced by sharks and rays. He suggested that they were
formed by a plastic ribbon of faeces leaving the spiral valve and coiling
up within the confines of the distal colon.
However, in 1907,Anton Fritsch introduced the concept of
“enterospirae”, that some coprolites were not excrement but fossilised
spiral valves of fish.
In 1972 Michael Williams produced histological evidence of mucosal
folds preserved in an early Permian specimen. However, he did accept
that some bromalites were true coprolites.
This view was re-evaluated by James McAllister in 1985, who
regarded Williams’ mucosal folds as dubious and noted that the number
of coils seen in coprolites greatly exceeded the number of coils in the
spiral valve of a Recent dogfish. In turn, McAllister did not rule out the
possibility that some coprolites were true enterospirae.
Details from Henry de la Beche’s draft of Duria Antiquior. The green
arrows indicate coprolites. The purple arrows indicate spiral coprolites
which appear to have been added in pencil as an afterthought. EVIDENCE FOR or AGAINST
ENTEROSPIRAE
Two of the original spiral intestines of a ray
filled with Roman cement by William
Buckland, still preserved in the collections of
Oxford University Museum, UK.
FOR: Some bromalites appear far too consistent
in morphology to have been a passed coprolite.
AGAINST: These two bromalites,
probably from large rays. came from the
Mid-Cretaceous Kem Kem Formation in
southern Morocco. As they are made up
of numerous thin coils, more than are
present in the spiral valve of a Recent
shark or ray, they are most likely to be
coprolites rather than enterospirae.
Conclusions
Most bromalites are coprolites but
true enterospirae probably exist.
AGAINST: Two late Cretaceous spiral coprolites. In both cases they comprise the first three
whorls of what could have been a large coprolite. The faecal ribbon appears to have terminated
prematurely leaving a partial, apically concave coprolite in the distal colon. Peristaltic contractions
of the colon would have cause it to rotate and fold onto itself, enabling to pass more readily through
the narrower rectum and anal sphincter. This last action would result in a fusiform shape,