Nautiloid
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| Missing image Nautiloid_trilacinoceras.jpg Fossil nautiloid Fossil nautiloid Trilacinoceras from the Ordovician of China | ||||||||
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Palcephalopoda |
Nautiloids are a group of marine molluscs which all possess an external shell, the most well known example being the modern nautiluses. They flourished during the early Palaeozoic era, where they consitituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.
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Taxonomic relationships
The nautiloids are among the group of animals called the cephalopods (class Cephalopoda), which also includes ammonoids, belemnites and nautiloids. The cephalopods are an advanced class of a larger group of animals called the molluscs (phylum Mollusca), which includes gastropods and bivalves.
The ammonoids (a group which includes the ammonites and the goniatites) are extinct cousins of the nautiloids that evolved early in the Devonian period, some 400 million years ago.
Cladistically speaking, nautiloids constitute a paraphyletic assemblage. They are a grade of cephalopods united only by shared primitive (plesiomorphic) features that are not found in more specialised or derived cephalopods.
Defining Characteristics
There are three key features which are common to the shells of the nautiloids. These are the internal chambers, the siphuncle and the sutures of the shell, features that are also found in the shells of all ammonoids.
The siphuncle is a tube which runs through each of the internal chambers of the shell. Nautiloids are classified by the nature of the siphuncle and its position within the shell.
The thin walls between the internal chambers (camerae) of the shell are called the septa, and as the nautiloid grew it would move its body forward in the shell secreting new septa behind it. This process added new camerae to the shell. The body of the animal itself occupied the last chamber of the shell - the living chamber. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers, a process which may have been connected with controlling buoyancy.
Sutures (or suture lines) are visible as a series of narrow wavy lines on the surface of the shell, and they appear where each septa contacts the wall of the outer shell. The sutures of the nautiloids are simple in shape, being either straight or slightly curved. This is different from the "zigzag" sutures of the goniatites and the highly complex sutures of the ammonites.
The Recent Nautilus
Much of what is known about the extinct nautiloids is based on what we know about the modern nautiluses, such as the Chambered Nautilus which is found in the south west Pacific Ocean, from Samoa to the Philippines, and the in the Indian Ocean off of the coast of Australia. It is not usually found in waters less than 100 meters deep and may be found as far down as 500 to 700 meters (2,300 feet).
Nautiluses are free swimming animals that possess a head with two well developed eyes, arms (or tentacles). They each have a smooth shell, with a large body chamber, which is divided into chambers that are filled with an inert gas (similar to air but with more nitrogen and less oxygen) making the animal buoyant in the water. As many as 90 tentacles are arranged in two circles around their mouth. The animal has jaws which are horny and beak-like, and it is a predator, feeding mainly on crustaceans.
Empty nautilus shells may drift a considerable distance and have been reported from Japan, India and Africa. Undoubtedy the same applies to the shells of fossil nautiloids, the gas inside the shell keeping it buoyant for some time after the animals death so that the empty shell was carried some distance from where the animal lived before it finally sank to the sea-floor.
Nautiluses propel themselves by jet propulsion, expelling water from a elongated funnel called the hyponome, which can be pointed in different directsions to control their movement. They do not have an ink sac like that found in belemnites and some of the other cephalopods, and there is no evidence to suggest that the extinct forms possessed an ink sac either. Unlike the extinct ammonoids, the modern nautiluses lack any sort of plate for closing their shell, and no such plate has been found in any of the extinct nautiloids.
The coloration of the shell of the modern nautiluses is quite prominent, and, although it is somewhat rare, the shell coloration has been known to be preserved in fossil nautiloids.
Fossil Record
Nautiloids are often found as fossils in early Palaeozoic rocks (less so in more recent strata). The shells of fossil nautiloids may be either straight (as in Orthoceras and Rayonnoceras), curved (as in Cyroceras) or coiled (as in Cenoceras), or a hellical coil (as in Lorieroceras). Some shells are ornamented with spines and ribs, but most have a smooth shell.
The rocks of the Ordovician period in the Baltic coast and parts of the United States contain a variety of nautiloid fossils, and specimens such as Discitoceras and Rayonnoceras may be found in the limestones of the Carboniferous period in the Republic of Ireland. The marine rocks of the Jurassic period in the United Kingdom often yield specimens of Cenoceras, and nautiloids such as Eutrephoceras are also found in the Pierre Shale formation of the Cretaceous period in the midwestern part of the United States.
Specimens of the Ordovician nautiloid Endoceras have been recorded measuring up to 3.5 meters (13 feet) in length. These large nautiloids must have been formidable predators of other marine animals at the time they lived.
In some localities, such as Scandinavia and Morocco, the shells of fossil nautiloids accumulated in such large numbers that they form Orthoceras limestones. Orthoceras is a generic name gioven to straight shelled nautiloids that lived from the Ordovician to the Triassic periods (but were most common in the early Paleozoic).
Evolutionary History
Nautiloids are first known from the late Cambrian Fengshan Formation of northeastern China, where they seem to have been quite diverse (at the time this was a warm shallow sea rich in marine life). However, although four orders have been proposed from the 131 species named, there is no certainty that all of these are valid, and indeed it is likely that these taxa are seriously oversplit.
Most of these early forms died out, but a single famil;y, the Ellesmeroceratidae, survived to the early Ordivician, where it became the ultimate ancestor of all subsequent cephalopods. In the Early and Middle Ordovician they underwent an evolutionary radiation (perhaps due to the new ecological niches made available by the extinction of anomalocarids at the end of the Cambrian). Some eight new orders appeared at this time, covering a great diversity of shell types and structure, and ecological lifestyles.
Nautiloids remained at the height of their range of adaptations and variety of forms throughout the Ordovician, Silurian, and Devonian periods, with various straight, curved and coiled shell forms coexisting at the same time. They began to decline in the Devonian, perhaps due to competition with their descendants and relatives the Ammonoids and Coleoids, with only the Nautilida holding their own (and indeed increasing in diversity). Their shells became increasingly tightly coiled, while both numbers and variety of non-Nautilid species continued to decrease throughout the Carboniferous and Permian, and the last straight-shelled forms died out at the end of the Triassic. From that time on only a single suborder, the Nautilina continued throughout the Mesozoic, where they co-existed quite happily with their more specialised ammonoid cousins. Most of these forms differed only slightly from the modern Nautilus. They had a brief resurgence in the early Tertiary (perhaps filling the niches vacated by the Ammonoids in the end Cretaceous extinction), and maintained a worldwide distribution up until the middle of the Cenozoic era. With the global cooling of the Miocene and Pliocene, their geographic distribution shrank and these hardy and long-lived animals declined in diversity again. Today there are only 3 or 4 living species, all belonging to the single genus Nautilus (the pearly nautilus).
Classification
The following 1988 classification by Curt Teichert, updates the 1964 version in the Treatise on Invertebrate Paleontology, and is based mostly on shell structure (Teichert 1988, p.19)
Subclass: Orthoceratoidea Kuhn, 1940
- Order Plectronocerida Flower, 1964 (Cambrian)
- Order Yanhecerida Chen & Qi, 1979 (Cambrian)
- ?Order Protactinocerida Chen & Qi, 1979 (Cambrian)
- Order Ellesmerocerida Flower, 1950
- Suborder Ellesmerocerina Flower, 1950 (Cambrian to Ordovician)
- Suborder Cyrtocerina Flower, 1964 (Ordovician)
- Order Orthocerida Kuhn, 1940 (Ordovician to Triassic)
- Order Ascocerida Kuhn, 1949 (Ordovician to Silurian)
Subclass Actinoceratoidea Teichert, 1933
- Order Actinocerida Teichert, 1933 (Ordovician to Carboniferous)
Subclass Endoceratoidea Teichert, 1933
- Order Endocerida Teichert, 1933 (Ordovician to Silurian)
- ?Order Injetocerida Balashov, 1960 (Ordovician)
Subclass Nautiloidea Agasiz, 1947
- Order Tarphycerida Flower, 1950
- Suborder Tarphycerina Flower, 1950 (Ordovician to Silurian)
- Suborder Barrandeocerina Flower (Ordovician to Devonian)
- Order Oncocerida Flower, 1950 (Ordovician to Carboniferous)
- Order Discosorida Flower, 1950 (Ordovician to Devonian)
- Order Nautilida Agassiz, 1847
- Suborder Rutocerina Shimanskiy, 1957 (Devonian to Triassic)
- Suborder Lirocerina Shimanskiy, 1957 (Carboniferous to Triassic)
- Suborder Nautilina Agassiz, 1847 (Triassic to Recent)
A further order, Bactrida, are sometimes considered nautiloids close to the Orthocerida, sometimes very primitive ammonoids, and sometimes placed in a subclass of their own.
A more recent interpretation by Theo Engeser (Engeser 1997-1998) suggests that nautiloids, and indeed cephalopods in general, fall into two main groups, the Palcephalopoda (including all the nautiloids except Orthocerida and Ascocerida) and the Neocephalopoda (the rest of the cephalopods).
References
- Engeser, T., (1997-1998) The Palcephalopoda/Neocephalopoda Hypothesis (http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautpalneocephalopoda.htm)
- Teichert, T. (1988) "Main Features of Cehalopod Evolution", in The Mollusca vol.12, Paleontology and Neontology of Cephalopods, ed. by M.R. Clarke & E.R. Trueman, Academic Press, Harcourt Brace Jovanovich,
External Links
- The Fossil Nautiloidea Page (http://userpage.fu-berlin.de/~palaeont/fossilnautiloidae/)
- Palaeos (http://www.palaeos.com/Invertebrates/Molluscs/Cephalopoda/Nautiloidea.htm)