Endocerida

Endocerida Temporal range: Floian–Hirnantian[1][2] PreꞒ Ꞓ O S D C P T J K Pg N
The massive endocerid Cameroceras (middle) alongside other nautiloids
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Superorder:	†Endoceratoidea
Order:	†Endocerida Teichert, 1933
Families
† Cyrtendoceratidae † Endoceratidae † Proterocameroceratidae † Yorkoceratidae

Endocerida is an extinct

Endocerids had a relatively small

Endocerids reached enormous body sizes. The largest confirmed specimen, belonging to Endoceras giganteum, is 3 metres (9.8 ft) long as preserved, but is missing a substantial portion of its aboral end.^[3][4] The reconstructed length of the shell is nearly 6 metres (20 ft).^[3][4] An alleged endocerid specimen 30 feet (9.1 m) long is unconfirmed.^[3]

Ecology

The mode of life of endocerids is debated.^[5] Endocerids may have been the apex predators of the Ordovician, probably living close to the sea floor, and preying on trilobites, molluscs, brachiopods and other bottom-dwelling organisms.^[1] They were probably not active nektonic swimmers, but rather crawled over the floor of epicontinental seas or lay there in ambush.^{[citation needed]} Although there is study that supports filter feeding ecology,^[5] according to hydrostatic properties, it is not likely and still supports benthic predators.^[6]

Reproduction

Endocerids laid relatively large eggs, and hatched at a relatively large body size.

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 Silurian.^[2] 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

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

Restudy of piloceratid-like families with complex endocones has suggested that Endocerida in its broadest form is polyphyletic, 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.^[10] Bisonocerida may still be related to endocerids within Endoceratoidea.^[9]

See also

• Cephalopod size

References