Evolution of cephalopods
The
The class developed during the middle Cambrian, and underwent pulses of diversification during the
Traditional views of origin
The cephalopods were once thought to have evolved from a
Early shell record
Understanding of early cephalopod origins is by necessity biased by the available fossil material, which on the whole consists of shelly fossils. Critical fossils are detailed below; since their stratigraphic age has guided the interpretation of the fossils,[18] they are listed in descending order of age.
Cambrian
With the exception of the shelly genera
Tannuella
Tannuella is the oldest fossil to have been assigned to the cephalopods, dating from the Early Cambrian (Atdababian and Botomian), ~522 million years ago. Its position in this group is suggested based on its shape and the presence of chambers.[20] Under this hypothesis, it would be a precursor to the hypseloconids and then genera such as Knightoconus that eventually gave rise to the cephalopods.[21]
Knightoconus
Plectronoceras
Plectronoceras is arguably the earliest known crown-group cephalopod, dating to the Upper Cambrian.[1] Its 14 known specimens hail from the basal
Yochelcionellids
The
Ellesmerocerida
The earliest true cephalopod order to emerge was the Ellesmerocerida, which were quite small; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes.[4] This marks an important difference from the earlier cephalopods, whose siphuncle was at the edge of the septum and against the shell wall.[18] On the basis of muscle scars preserved in such genera as Paradakeoceras and Levisoceras, these animals are reconstructed with a straight body and dorsal shell, with the head at the anterior, concave surface of the shell, and the funnel (consisting of a pair of folds in the foot at the rear), not juxtaposed with the head as in later, oncocerid-like forms.[14]
Early Ordovician diversity
The Ellesmerocerids were the only shelled cephalopods known to have survived the end-Cambrian extinction; all subsequent cephalopods are thus thought to be derived from these forms, which diversified throughout the Ordovician period.
Early cephalopods had fine shells that could not cope with the pressures of deep water.[4] In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida (with narrow siphuncles).[4]
By the mid Ordovician these orders are joined by the
Curved shells brought a number of benefits. Firstly, minerals are not required in as large quantities, as each successive whorl builds on the one before. Also, the organism is more stable (its centre of mass coincides with its centre of buoyancy) and more manoeuvrable.[24]
Early cephalopods were likely predators, near the top of the food chain.
Fossils mistaken for cephalopods
A number of fossils have historically been considered to represent components of the cephalopods' history, but been reinterpreted on the basis of additional material.
Volborthella
When it was discovered in 1888,[18] it was thought that the early Cambrian Volborthella was a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica); neither was it septate.[27] This illusion was a result of the laminated texture of the organisms' tests.[28] Therefore, Volborthella’s classification is now uncertain.[29]
Shelbyoceras
Because the characters differentiating monoplacophora from cephalopods are few, several monoplacophora have been mistaken for cephalopod ancestors. One such genus is
Pohlsepia
Kirengellids
The
Hyoliths
Hyoliths such as Allatheca have been interpreted as cephalopod ancestors,[34] but hyoliths proper are now recognized as brachiopods.[35]
Coleoidea
The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago. We know this because the orthocerids were the first known representatives of the neocephalopoda,[36] were ultimately the ancestors of
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters.[13]: 36 The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit.[38]: 289 This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure.[38]: 289 Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.
Early fossils
The Early Devonian Naefiteuthis has been interpreted as the earliest fossil coleoid, and its shell may be in a partly internalized state.[37] Belemnoids proper appear slightly later in the Early Devonian, and represent the first unambiguous coleoids.[31]
The Mazon Creek biota contains a decapod, Jeletzkya, which had ten arms, but the status of its shell is ambiguous as it has not been extracted from the concretion that preserves the only fossil. Accordingly, it has been interpreted as both an internal and an external shell; the specimen may represent a 'squid' or a belemnoid,[31] although due to preservation its affinities are not known well.[32]
The Late Mississippian Bear Gulch Limestone contains some important genera of early coleoids. Gordoniconus has large internal conch which looks similar to external shell that can be seen in bactritid, and this genus probably shows how external shell become into internal conch.[39] Syllipsimopodi is considered as belonging to Octopodiformes which contains modern octopuses and vampire squids.[32]
Organ origins
The tentacles of the ancestral cephalopod developed from the mollusc's foot;[40] the ancestral state is thought to have had five pairs of tentacles which surrounded the mouth.[40] Smell-detecting organs evolved very early in the cephalopod lineage.[40]
The earliest cephalopods,[b] like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards.[38]: 289 Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods, such as by contracting their funnels or moving the head in and out of the chamber.[38]: 289
Exceptional preservation
The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic,
Footnotes
- ^ a b Endogastric means the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curved so as the ventral side is longitudinally convex (belly out). Exogastric coiling allows the funnel to be pointed backwards, beneath the shell.[23]
- ^ Ordovician orthocone nautiloids are the first for which trace fossil evidence is available.
References
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- ^ a b c Wingstrand, K.G. (1985). "On the anatomy and relationships of recent Monoplacophora" (Link to free full text + plates). Galathea Rep. 16: 7–94.
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- ^ Hagadorn, J.W.; Waggoner, B.M. (2002). "The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range". In Corsetti, F.A. (ed.). Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section (PDF). SEPM Book. Vol. 93. Society for Sedimentary Geology (SEPM). pp. 135–150. Archived from the original (PDF) on 1 October 2008. Retrieved 1 June 2010.
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Kröger, Björn (2006). "Early growth-stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia". Lethaia. 39 (2): 129–139. doi:10.1080/00241160600623749. Archived from the originalon 2012-10-20.
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Further reading
- Chen, J. Y.; Teichert, C. (1983). "Cambrian cephalopods". Geology. 11 (11): 647–650. ISSN 0091-7613.