Endocerida is an extinct

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...

order, a group of

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s from the Lower Paleozoic with cone-like deposits in their

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, a ...

. Endocerida was a diverse group of

s that lived from the Early

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

possibly to the Late

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

. Their shells were variable in form. Some were straight ( orthoconic), others curved (cyrtoconic); some were long (longiconic), others short (breviconic). Some long-shelled forms like ''

Endoceras ''Endoceras'' ( Ancient Greek for "inner horn") is an extinct genus of large, straight shelled cephalopods from the Middle and Upper Ordovician that gives its name to the Nautiloid order Endocerida. The cross section in the mature portion is slig ...

'' attained shell lengths close to . The related ''

Cameroceras ''Cameroceras'' ("chambered horn") is a genus of extinct, giant orthoconic cephalopod that lived mainly during the Ordovician period. It first appears during the middle Ordovician, around 470 million years ago, and was a fairly common component ...

'' is anecdotally reported to have reached lengths approaching , but these claims are problematic. The overwhelming majority of endocerids and nautiloids in general are much smaller, usually less than a meter long when fully grown.

Morphology

Endocerids had a relatively small body chamber as well as a proportionally large

, which in some genera reached nearly half the shell diameter. This suggests that much of the visceral mass may have been housed within the siphuncle itself rather than just in the body chamber as with other nautiloids (Teichert, 1964). Endocerids are primarily distinguished by the presence of conical calcareous deposits, known as endocones, found in the more apical portion of the siphuncle. They are thought to act as a counterweight for the animal’s body. The chambers ( camerae) of endocerids are always free of organic deposits, unlike orthoceratoid cephalopod orders such as the Orthocerida and Actinocerida.

Body size

Endocerids reached enormous body sizes. The largest confirmed specimen, belonging to '' Endoceras giganteum'', is long as preserved, but is missing a substantial portion of its aboral end. The reconstructed length of the shell is nearly . An alleged endocerid specimen long is unconfirmed.

Ecology

The mode of life of endocerids is debated. Endocerids may have been the apex predators of the Ordovician, probably living close to the sea floor, and preying on

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s, molluscs,

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s and other bottom-dwelling organisms. They were probably not active

nekton Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...

ic swimmers, but rather crawled over the floor of epicontinental seas or lay there in ambush. However, the extremely long, straight shells of endocerids would have made them slow and not very maneuverable, poorly suited for an active predatory role or a life near the sea floor. Rather, endocerids may have been pelagic filter feeders, similar to other gigantic sea animals such as

baleen whales Baleen whales ( systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea ( whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in t ...

and

whale sharks The whale shark (''Rhincodon typus'') is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, Cosgrove J, ...

. However, according to hydrostatic properties, encocerids are not likely to be filter feeders but benthic predators.

Reproduction

Endocerids laid relatively large eggs, and hatched at a relatively large body size. It is likely that endocerids were demersal after hatching, as large eggs would make an easy target for predators in the pelagic zone. Endocerids may have migrated from their habitat in the open ocean to shallower water to lay their eggs.

Diversity

Endocerids were among some half a dozen cephalopod orders that appeared in the Lower Ordovician. They reached their greatest diversity during the Lower to Mid-Ordovician, but were already in decline by the middle of this period with most genera becoming extinct by the end of the

Sandbian The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species '' Nemagraptus gracilis'' around million yea ...

(late Ordovician), while some rare hangers on lasted into the

. In any case, the endocerid lineage became completely extinct relatively early on in cephalopod history.

Classification

Evolution

Endocerids evolved from the earlier ellesmerocerids, most likely from a genus similar to '' Pachendoceras''. This ellesmerocerid gave rise to '' Proendoceras'', the earliest representative of the

Proterocameroceratidae The ''Proterocameroceratidae'' were the first of the Endocerida. They began early in the Ordovician with ''Proendoceras'' or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor. Proterocamerocerat ...

and hence of the Endocerida. Endocerids evolved from ellesmerocerids by reduction of siphuncle diaphragms and the development of endocones. In the early part of the mid-Lower Ordovician, the Endocerida quickly diversified into many different families. In true endocerids, there was a trend of overall increasing size, eventually resulting in massive orthoconic genera such as ''

'' and ''

''. In another lineage (now known as

Bisonocerida Bisonocerida is an order of Ordovician to Silurian nautiloid cephalopods. Members of this order were originally placed in the order Endocerida, but later investigation argued that this broad usage of Endocerida was a polyphyletic assemblage en ...

), the siphuncle grew more complex, resulting in genera such as '' Chihlioceras'' and ''

Allotrioceras ''Allotrioceras'' is a tubular fossil from the Middle Ordovician of the state of New York, collected by Rousseau H. Flower; included by him in the Endocerida and placed in a new family, the Allotrioceratidae. ''Allotrioceras'' is characterized b ...

''.

Taxonomy

Citing its diversity, Curt Teichert (1964) placed the Endocerida in its own subclass called the Endoceroidea or Endoceratoidea (which some Russian paleontologists ranked as a

superorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

instead. Rousseau Flower rejected this separation on the grounds that endocerids were no more diverse or complex than any other order. He considered them to be simply another order within the Nautiloidea. Flower (1958) divided the Endocerida into two suborders, the Proterocamerocerina and the Endocerina. As he defined the two suborders, Proterocamerocerina included the

, Manchuroceratidae, and Emmonsoceratidae, while Endocerina included the Piloceratidae and

Endoceratidae Endoceratidae is a family of large to very large straight shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician. They include the largest known Paleozoic invertebrates, represented by ''E ...

. Endocerid classification since then has relied on a dichotomy between short-shelled forms with complex endocones and long-shelled forms with simple endocones. Endocerid relationships have been difficult to establish both within the order and relative to other nautiloids. Their generally orthoconic shell shape and dorsomyarian muscle scars are similar to the subclass Orthoceratoidea, which are ancestral to ammonoids (ammonites) and coleoids (squid, octopus, etc.). However, their nautilosiphonate

connecting ring The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, an ...

structure and lack of cameral deposits are more similar to living

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...

and their proposed ancestors, the subclass Multiceratoidea. Some studies have re-established Endoceratoidea to clarify that endocerids occupy a unique subclass of nautiloids. Restudy of piloceratid-like families with complex endocones has suggested that Endocerida in its broadest form is

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

, with piloceratid-like and proterocameroceratid-like members having independent origins from ellesmerocerids. In light of this issue, the numerous piloceratid-like families were placed within a new order,

. Bisonocerida may still be related to endocerids within Endoceratoidea.