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Home , Bactritida, Cameroceras, Endoceras, Michelinoceras, Orthoceras, Plectronoceras

The monster nautiluses of the Palaeozoic Dr Neale Monks (UK)

he handful of nautilus species today, limited to a few species only ammonites and did so in found in seas today are found in relatively deep water, but the past. Tsmall, retiring that in the past they were quite diverse. Monoplacophorans, like scavenge about at night, foraging Although they look a lot like a limpet, exile, had shells with for carrion and crustacean moults. their internal anatomy is distinctive, chambers, but no . Were However, nautiluses were not with unusual features such as serial the walls that formed the chambers always so insignificant and during repetition of the gills, kidneys and for some other purpose? Did they the first half of the Palaeozoic Era reproductive organs along the body. work like bulkheads, so that the especially, nautiluses were major At least some monoplacophorans had monoplacophoran could build shell predators, occupying the same niches chambered shells. The Late that was thin and economical to in and seas as species, Knightoconus antarcticus construct, but just as strong as a sharks do today. was one such species, but, unlike thicker, unchambered shell? Perhaps, , the chambers were all but recent work on the genetics The first nautiluses sealed off from one another. Only of the molluscs suggests that Compared to their cousins, the cephalopods have a siphuncle, a cephalopods are more closely related ammonites, the Palaeozoic nautiluses tube-like structure that runs through to scaphopods (tusk shells) than they are relatively unfamiliar animals. the chambers, allowing them to be are to monoplacophorans. Modern That is a shame, because they are truly connected to the living body of the scaphopods are small, specialised remarkable, in all likelihood being the . The siphuncle is important animals that live buried in the first really big predators to evolve on because it allows cephalopods seafloor catching microscopic prey. Earth. But, to understand how they to replace water inside the shell While their genes might tell us they reached the top so quickly, we need chambers with gas. By filling the are closely related to the cephalopods, to look back at their ancestors, the chambers in this way, the there are no obvious anatomical or floating ‘snails’ of the Cambrian. reduces its overall density and, once it ecological similarities. Nautiluses are the most primitive acquires the same density as seawater, of all the cephalopods, the group of it neither floats to the top nor sinks to Early success molluscs that also includes , the bottom. Rather wonderfully, it Having become buoyant, nautiluses , , ammonites simply hangs in midwater and can found themselves able to explore and belemnites. Nautiluses appeared swim off in any direction it wants. midwater habitats free from serious during the Late Cambrian, about Compare this to other molluscs, competition. As the Cambrian yielded 500mya, but what their ancestors which have to drag their heavy shells to the Ordovician, most animals might have been remains uncertain. around with them. Nautiluses and were stuck on the seafloor. The The traditional explanation is cuttlefishes continue to operate this other molluscs crawled about on the that the first nautiluses, such as way today, bottom, as did worms Plectronoceras exile, were derived a n d and echinoderms, from monoplacophorans. These and most of the are snail-like trilobites and m o l l u s c s c r u s t ac e a n s. While there w e r e certainly f i s h

Dideroceras wahlenbergi (formerly wahlenbergi) is an example of one of the larger nautiluses. Complete shells are rare, but fragments 30 to 45cm long are common at certain horizons. Photo by Jens Rydell.

22. around at the time, they were still living chamber pointing downwards. Why couldn’t they use their jet armour plated and lacked a swim Perhaps these cephalopods drifted propulsion to swim forwards? All bladder, so these were also mostly across the seafloor, picking off prey cephalopods are limited in this confined to the sea bottom. The only much like cyrtocone nautiluses. Or, regard, because the outlet from their large animals commonly found in perhaps, they were plankton feeders, jet is below the head, usually through midwater were drifting animals that moving up and down the water a muscular tube of some sort. On fed on plankton - things like jellyfish column, as many deep sea do modern nautiluses, this structure is and graptolites. today. called the hyponome. While this tube Therefore, Early Ordovician The were not the only works very well at directing the jet nautiluses found themselves with a straight-shelled cephalopods though. away from shell, bending the tube unique combination of large size and Indeed, the Bactritida were rather an backwards under the shell reduces mobility. They quickly became quite unimportant group, so far as we can the flow of water, in much the same diverse and seemingly occupied a tell, and it was other straight-shelled way as tight bends in hose pipes slow variety of different ecological niches. nautiluses that were ecologically down the flow of water. Therefore, Of course, such assumptions are much more significant. Known as modern cephalopods can swim based purely on the shapes of their ‘orthocone nautiluses’, their remains quickly in reverse, but only slowly shells - as with ammonites, we know are very common in Ordovician and going forwards - orthocone nautiluses virtually nothing about the soft body Silurian deposits, to the degree that were presumably no different. parts of Palaeozoic nautiluses. In any certain limestones are known as case, what we do know of Ordovician ‘orthoceratite limestones’, precisely Ordovician giants! nautiluses is that their shells came because of their abundance. Most and in lots of different shapes and sizes. Among the best-known genera are were of moderate size, up to 30cm One of the earlier types had coiled Orthoceras, Michelinoceras and being typical, though exceptional shells like those of Plectronoceras Endoceras. Instead of hanging head- species reached lengths of around exile, but larger and more elongate, downwards, they were oriented along a metre. However, there were much so that they looked a bit like elephant the horizontal plane, like most modern larger orthocone nautiluses swimming tusks. Cyrtoceras ellipticum is a squid. They solved the orientation around the Ordovician seas. Some well-known example of this type problem by weighing down the species of Endoceras got to over three and, consequently, these nautiluses of the shell with mineral deposits. metres in length, and fragments of are referred to as having ‘cyrtocone Just as the tissue running through the some suggest they were shells’. Because such shells would siphuncle provided a mechanism for even larger, perhaps as much as nine have rocked forwards and backwards removing water from the chambers, metres in length. if the nautilus tried to swim quickly, so too was the siphuncle used for Were such giants active midwater the assumption here is that these secreting these counterweights. swimmers or did they mostly drift nautiluses swam slowly above the The shell now operated like a see- above the seafloor, only jetting away seafloor, picking off sessile (that is, saw, with the counterweight at the when harassed? Did they swim non-mobile) prey such as clams and ‘tail end’ of the shell, balancing the after large prey, or were they slow brachiopods. These nautiluses were body at the ‘head end’. Therefore, grazers that fed on sessile animals usually rather small, with shells no orthocone nautiluses were able to or plankton? These are difficult more than 10 to 15cm in length. swim - backwards at least - as quickly questions to answer and, to be honest, Quite early on, some nautiluses and easily as its jet propulsion would the do not tell us very much evolved tightly coiled shells similar allow. either way. to those of ammonites and, indeed, modern nautilus species. For example, the Middle Ordovician species, Estonioceras imperfectum, has a coiled shell with successive whorls almost touching. Interestingly, these coiled nautiluses were not the ancestors of the ammonites, which instead seem to have evolved from a peculiar group of straight-shelled cephalopods known as the Bactritida, which appeared during the Early . Because of their straight shells, members of the Bactritida, such as Bactrites gracilis, seem to have lived a head-downwards life. This is because the shell was less dense than the head, and so, when floating in the water, the animal would pivot around the centre of the shell, the Dideroceras wahlenbergi is a common constituent of what geologists often call ‘orthoceratite apex pointing upwards and the limestone’. Photo by Jens Rydell.

23. One problem with assuming they So, were they the sharks of the How they appeared were active predators is that they Ordovician seas? Perhaps, but more could only swim rapidly in reverse. like bottom-dwelling nurse sharks and saw the world As we have seen already, cephalopod than midwater mackerel sharks. Top jet propulsion is great for swimming predators, yes, but not necessarily One of the interesting things about away from predators, but not so good agile or fast-moving ones. Palaeozoic nautiluses is that the for swimming forwards towards shells of some specimens have prey. Squid and solve this Distribution revealed traces of colour markings, problem by using their fins to swim Ordovician and Silurian typically concentric or zigzag bands. forwards, while octopuses prefer to nautiluses often have a very wide What were these colour markings crawl about when hunting. Modern distribution. One of the most for? Most likely, they acted as some nautiluses mostly use their tentacles familiar species to collectors sort of camouflage, breaking up to pull themselves along and, in all will be Michelinoceras michelini, the outline of the shell and making probability, Palaeozoic nautiluses an orthocone found in the animal less easily seen by its were no different. Central Europe and North Africa. predators or its prey. On the other hand, their large size Polished specimens from Morocco It is assumed the Palaeozoic nautiluses would have made them formidable are staples of the fossil trade, being had simple pinhole eyes like predators, even if they were not sold in gift shops, museums and those of modern nautiluses, in which swimming particularly fast. Like at geological shows of all types. case, their eyesight was relatively other cephalopods, they were Orthoceras regulare can be found poor compared to the superb eyes equipped with muscular tentacles across a broad region of Europe, possessed by squids and octopuses. and a strong beak, and it is hard to from to the . Whatever else the colours on their imagine a three metre Endoceras Dideroceras wahlenbergi (formerly shells were for, they were not for having any problems at all tackling Endoceras wahlenbergi) has been communication between individuals, a slow-moving animal like a trilobite collected from sites as far apart as unlike the bright colour patterns or, for that matter, sessile prey like and China. that octopuses, squids and especially brachiopods. It is also important Unfortunately for palaeontologists, cuttlefish now use to interact with to remember that they were not some scientific names have been each other. competing with very much in terms used rather loosely and, because Modern nautiluses hunt primarily of midwater predators of equivalent orthocone nautiluses are quite by smell rather than sight. Did size. Midwater fish were few and difficult to identify to species level, Palaeozoic nautiluses operate in generally small. So, as clumsy as reports of things described as the same way? Perhaps they swam it might have been by modern Orthoceras sp. or Endoceras sp. are across the seafloor, using their standards, a large, jet-propelled very common. Fossil collectors with tentacles to search for suitable prey. Endoceras looking for food was specimens sold under such names Nautilus tentacles are both sensors a pretty dangerous animal by the should treat these identifications as and grapplers, so as soon as they standards of the Ordovician! strictly provisional. detected anything edible, it would be quickly entangled and dragged up to the mouth. Regardless of how they sensed their environment, one thing the Ordovician nautiluses did bring to the world was something approaching intelligence. Modern cephalopods are famously ‘brainy’ animals, being adept at solving man-made puzzles in labs as well as performing all sorts of sophisticated behaviours in the wild. Modern nautiluses are not quite so smart as cuttlefish or octopuses, but, by the standards of most other invertebrates, they have large brains and complex nervous systems. Handling food with their tentacles, while processing visual and taste data at the same time, requires a fair amount of brain power. If we could go back in time and take a swim in an Ordovician sea, the big orthocone nautiluses would very likely be the only animals that would be curious enough to approach and, in their Estonioceras imperfectum was one of the coiled nautiluses of the Ordovician, superficially similar to the much later ammonites. Photo by Jens Rydell. own way, look back at us.

24. Book review Extinction Surprisingly perhaps, the Jon Trevelyan (UK) managed to hang on for The Ordovician–Silurian extinction quite a long time, the last species being Fossils of the Gault Clay event set the orthocone nautiluses known from sediments of Field guide to fossils no. 12. back significantly. To be fair, the age (though some palaeontologists really big species in the genera place these late species in a related, It is always exciting when Palass Endoceras and Cameroceras but distinct group of their own, the publishes a new field guide to fossils. belonged to the order , order Pseudorthocerida). However, However, their last (Silurian Fossils of a group that went into decline through their descendants, the the Pentland Hills, Scotland, Field Guide from the middle part of the Orthocerida have continued to to Fossils No. 11), despite being of the Ordovician onwards. However, maintain their importance. expected quality, suffered from being the order Orthocerida included The Bactritida mentioned earlier somewhat esoteric in its subject matter. most of the small and medium- were an offshoot of the Orthocerida This one, number 12 in the series, is back sized species, including Orthoceras and, while the Bactritida themselves on form and, like their guide to the chalk and Michelinoceras. For whatever are pretty obscure, two very (No. 2) is for ‘cenomaniacs’, this is likely reason, these straight-shelled important groups of cephalopods to be the constant companion for anyone, nautiluses managed to pass through branched off from them. These were who (like me) loves the Gault Clay. the Ordovician–Silurian extinctions the ammonoids and the coleoids. The Like others in the series, it has the relatively unscathed and were quite ammonoids include the ammonites usual excellent black and white diverse in the Silurian. and their relatives, and they were photographs of beautifully prepared From the middle of the Devonian terrifically important from the later specimens. Each chapter is authored onwards, even the Orthocerida part of the Palaeozoic through to by a specialist in the relevant subject, started to decline in diversity. The the very end of the . The containing an introduction, followed by evolution of fast, agile midwater fish, coleoids are the belemnites and their detailed systematic descriptions of each with strong jaws, was probably the modern day relatives, the cuttlefishes, specimen. key issue. Sharks and placoderms octopuses and squids. However, after the beautifully produced were more agile swimmers than Ammonites & other Cephalopods of orthocone nautiluses, and their jaws So, while the orthocone nautiluses the Lower Albion of the would have made short work of their of the Palaeozoic may be long gone, South East of England (Fred Clouter, shells. their descendants live with us still. published by The Medway Fossil & Mineral Society) was published in D 2007, is there a need for another guide to the Gault? The answer is that this publication (as always with the Palass guides) is a more academic piece. It covers the full range of fossils to be A found in the Gault of the entire UK, whereas Fred’s book is more populist and limited in range (to cephalopods and the southeast). Therefore, for the keen amateur as well as the B professional, there are good reasons to have both publications on the shelf. If I have one criticism, it is that geologists, including many of those who contributed to this excellent tome, use a private language that is unintelligible to anybody outside of the geological ‘elite’ or not in possession of a geological dictionary. I believe it is time to acknowledge that the academic language of geologists is often unnecessary and counterproductive. One contributor, who does not fall C into this trap, is Deposits’ own Neale Monks, who authors the chapter on heteromorph ammonites. I wonder if there is any connection!

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UKGE Code: BK0014 Michelinoceras michelini is a familiar fossil, widely sold polished or sectioned. The famous black

marble which, contain, Orthoceras specimens, is also worked into ornaments: A) Orthoceras carved into a boat, B) black marble bowl with polished Orthoceras, C) large slab of published ISBN: 978-1-4443-3542-2. Orthoceras, D) carved statue of Orthoceras. Published by: The Palaeontological Association25.