The Monster Nautiluses of the Palaeozoic Dr Neale Monks (UK)
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The monster nautiluses of the Palaeozoic Dr Neale Monks (UK) he handful of nautilus species today, limited to a few species only ammonites and belemnites did so in found in seas today are found in relatively deep water, but the past. Tsmall, retiring animals that in the past they were quite diverse. Monoplacophorans, like scavenge about at night, foraging Although they look a lot like a limpet, Plectronoceras exile, had shells with for carrion and crustacean moults. their internal anatomy is distinctive, chambers, but no siphuncle. 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 Cambrian that was thin and economical to in Ordovician and Silurian 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, cephalopods, 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 animal. 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 cephalopod 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 squids, the bottom. Rather wonderfully, it Having become buoyant, nautiluses octopuses, cuttlefishes, 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 Endoceras wahlenbergi) is an example of one of the larger orthocone 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 squid do modern nautiluses, this structure is and graptolites. today. called the hyponome. While this tube Therefore, Early Ordovician The Bactritida 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 Orthoceras and Michelinoceras 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 apex 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 Cameroceras 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 fossils 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 Devonian. 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 cuttlefish 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 fossil 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 nautiloid found in the animal less easily seen by its were no different.