Endocerida

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]

Endocerids laid relatively large eggs, and hatched at a relatively large body size.[7] It is likely that endocerids were demersal after hatching, as large eggs would make an easy target for predators in the pelagic zone.[7] Endocerids may have migrated from their habitat in the open ocean to shallower water to lay their eggs.[5]

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.

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 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, which are ancestral to ammonoids (ammonites) and coleoids (squid, octopus, etc.).[8] However, their nautilosiphonate connecting ring structure and lack of cameral deposits are more similar to living nautilus and their proposed ancestors, the subclass Multiceratoidea. Some studies have re-established Endoceratoidea to clarify that endocerids occupy a unique subclass of nautiloids.[9]

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]