Belemnites
Belemnites (Bel-Em-Nights) are probably
the most well known extinct cephalopod after the ammonites. They are quite
common fossils and have a worldwide distribution. They are a very
characteristic and easily recognizable fossil usually resembling a bullet in
shape, although this only represents the extreme 'tail' of the animal.
The name 'Belemnite' is derived from the Greek word belemnon which means javelin or dart due to the obvious resemblance
in the shape of the fossil. It was a common folklore tale that belemnites were
formed from the point of strike of lightning bolts into the ground; hence they
are frequently referred to as 'thunderbolts'.
Belemnites are grouped along with the squid, octopus, cuttlefish and argonaut.
Belemnites were very squid-like in shape, sharing the same streamlined torpedo
shape, but this came about through convergent evolution rather than squids
being descended from belemnites.
In fact, the closest living relatives to
the belemnites are probably the cuttlefish and the strange little squid Spirula, both of which have a chambered
internal shell structurally similar to that of the belemnite though in both
cases highly modified in their own ways.
Although the method of employing a
chambered shell for buoyancy control amongst the belemnites is similar to many
cephalopod groups, (e.g. Nautilus, Spirula, ammonoids), the use of a
counterweight at the rear of the body was an unusual feature.
Coleoids and ammonoids are more closely related to each other than either are to
the nautiloids, despite the superficial similarities of ammonoids with
nautiloids.
However, the coleoids and ammonoids
diverged long ago, probably during the Silurian Period, with the ammonoids
being the dominant group until their extinction at the end of the Cretaceous.
Physiology
Perhaps somewhat surprisingly, much more information is known about the inner
workings of the belemnite than the ammonite.
This
is because unlike the ammonite, examples of belemnites displaying soft-bodied
anatomy have been discovered. Notable examples in the 155 million year old
Jurassic Solnhofen limestones in Germany and the Late Jurassic Oxford clays in
the UK display creatures with ten arms, each equipped with 30-50 hooks that are
slightly recurved to ensnare prey and prevent it from struggling free.
As
with almost all modern hooked squids these hooks are normally arranged in pairs
on each arm and are arranged in a V-shape. With those particular species
although both male and female belemnite have mainly identical hooks, the males
have in addition a few large smooth hooks; and it has been speculated that
these may have been used to grasp hold of the female during mating.
Unlike
most squid belemnites are not found with tentacles and all ten arms tend to be
of same length. Belemnites and their close relatives do seem to have two fins
on either side of the mantle and some German examples have also displayed
traces of ink sacs.
The belemnite had a complex and complete internal shell that was divided into
three sections.
- Rostrum (or guard),
- Phragmocone,
- Pro-ostracum.
The rostrum (pl.rostra) was the posterior bullet shaped section of the shell and is
the fossil that is most commonly found. It is normally straight sided and
tapers to a point, although in some species the rostrum can be conical or even slightly curved and bladelike in
profile.
If sliced open the rostrum often displays concentric rings much like a tree trunk; this almost certainly represents growth rings.
Pyritic belemnite phragmacones, from the Lower Oxford Clay.
(C)opyright 2008 - UKGE Limited, UK Fossils Network and Deposits Magazine: www.ukfossils.co.uk/guides/belemnites.htm
The rostrum is composed of calcite, a
weighty material that probably acted as a counterbalance for the head and arms
whilst swimming.
Some
genera of belemnite have long ventral grooves cut into the rostrum (e.g. Actinocamax).
The slight differences in profile of the rostrum,
the shape of the cross section and shape of the tip are one method
palaeontologists use to differentiate species of belemnite. The rostrum is believed to have accounted
for roughly a third of the length of the animal. |
