Endocerida, from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct

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

order, a group of

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

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

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

. Endocerida was a diverse group of

s that lived from the Early

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

possibly to the Late

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

. 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'' attained shell lengths close to . The related '' Cameroceras'' 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 The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which depo ...

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 predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

s of the Ordovician, probably living close to the sea floor, and preying on

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s, molluscs,

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s and other bottom-dwelling organisms. They were probably not active nektonic swimmers, but rather crawled over the floor of epicontinental seas or lay there in ambush. Although there is study that supports

filter feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a spe ...

ecology, according to hydrostatic properties, it is not likely and still supports benthic predators.

Reproduction

Endocerids laid relatively large eggs, and hatched at a relatively large body size. It is likely that endocerids were

demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer o ...

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 (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 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 '' Endoceras'' and '' Cameroceras''. In another lineage (now known as Bisonocerida), the siphuncle grew more complex, resulting in genera such as '' Chihlioceras'' and '' Allotrioceras''.

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 () 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 recognized ...

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 Proterocameroceratidae, Manchuroceratidae, and Emmonsoceratidae, while Endocerina included the Piloceratidae and Endoceratidae. 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 Orthoceratoidea, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly ...

, which are ancestral to ammonoids (ammonites) and coleoids (squid, octopus, etc.). However, their nautilosiphonate connecting ring structure and lack of cameral deposits are more similar to living

nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

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 that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

, 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. Bisonocerida may still be related to endocerids within Endoceratoidea.