Fossils called thunderbolts

By Barry Taylor

Many years ago people often discovered strange cylindrical objects that tapering at one end,these were found in fields that had recently been ploughed. People at the time thought that these must be thunderbolts that had been hurled down by lightening during fierce storms. They were often accompanied by what were considered as toe nails from the devil cast aside during contests with god. We now know that these were fossils of Belemnites and Gryphea, we will concentrate on the former here that were in fact part of a fascinating squid like creature that is now extinct.

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What did the animal look like?

The Belemnites were much larger than the fossil guards or Rostrum we see; these bullet shaped fossils are just the backbone that was presumed to have been used by the creature as a counterbalance, the larges known were up to 3 metres long. The Rostrum was made of Calcite and you can see growth lines in cross sections similar to growth rings in wood (see picture). In front of this bone there usually existed a conical, chambered Phragmacone that sat within a conical depression in the Rostrum. This cone is sometimes preserved, but less often as this was made of the more unstable mineral Aragonite. The creature that had the Rostrum had a much larger body that had 10 tentacles, each equipped with hooks as opposed to the suckers of the related squid and cuttlefish; unconfirmed reports suggest some belemnites had 2 of their arms with suckers. They had 2 large eyes and also a siphon for propulsion, as well as an ink sack as in modern squid to aid escape. It is only through very rare near perfectly preserved fossils that we can see their true form including the rear fins which are even more rarely preserved (see picture). This group of creatures is poorly researched as there were very many smaller types that do not seem to fit the description exactly.

Time Zone IndicatorsBelemnites are good dating fossils and a prime example is the Late Jurassic to Early Cretaceous (Hauterivian) of the Speeton coast in Yorkshire. Here the Late Jurassic deposits can be clearly seen, dating from the Jurassic D beds to the Cretaceous A beds; during this time you can see the development of the species with the variation in the shape and size of the Rostrum, this change over time is indicative of the distinct time zones (see Picture). The shape of the rostrum varies enormously and could also have varied in gender to confuse the definition; this feature is seen in the related Ammonites for example. The cephalopods have largely been studied in Europe from the Jurassic and Cretaceous, but new finds from the Triassic period found miles away from Europe have extended the existence of belemnites back for a further 33 million years, that is before the Triassic/Jurassic mass extinction event.
It also seems that from the Jurassic to the Cretaceous there was a distinct change in the overall size that surely must have been environmental as the climate is known to have warmed in the Cretaceous Era.

Lifestyle
The creatures are part of the fossil group of Belamnoids which were in tern part of the order of Cephalopods that were like the modern squid and Cuttlefish along with the Ammonite and Nautiloid's. The Belemnites lived in the Mesozoic Era and in Europe they existed from the Lower Jurassic (Hettangian age) until the Upper Cretaceous (Masstrichtian age) nearly 200 million years in all. As mentioned above they had existed before the Jurassic. Ancestors with much larger phragmacones also lived in the Carboniferous and even the late Devonian, the Belamnites died out eventually around the time that Dinosaurs went extinct 65 million years ago, although some stragglers lasted for another 15 million years. How they lived can be deduced by the spines attached to the tentacles, they were probably specialist feeders, most likely of shrimp or similar bottom dwelling creatures, hence the Belemnites large eyes.

Collective Group Habits

The Belemnites can be found in clusters the clay deposits, it can be inferred from this that mass spawning events may have taken place, where the adults would have died after laying their eggs, probably at a particular time of year. This mass spawning can be seen in creatures such as the Horseshoe Crabs, Red Crabs and numerous other modern egg laying creatures. Belemnites were so numerous that they were the staple food for many of the aquatic animals from the large fish to the Ichthyosaurs and Plesiosaurs that existed then. As the Rostrum was not able to be digested the predators would it seems have then expelled the indigestible parts, just like the modern owl would regurgitate fur and bones in the same way.
Ps I am not a palaeontologist and therefore the details are very broad but suitable for the armature geologist.

 

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