Reconstructing a 460-million-year-old Cephalopod

[lpetrich]

Reconstructing fossil cephalopods: Endoceras – Incertae Sedis -  Endoceras - was a kind of straight-shelled cephalopod that lived in the Middle and Upper Ordovician, some 445 - 470 million years ago.

The orthoceratoid Endoceras is the both the longest extinct cephalopod and largest Paleozoic invertebrate. A shell of E. giganteum (MCZ unnumbered) from the Late Ordovician of New York is the current record-holding specimen. As preserved it measures 3 meters in length, with an estimated complete length of 5.73 meters (Teichert & Kummel, 1960; Klug et al. 2015). There is even an anecdotal report of a 9.14 meter shell that was destroyed in the field (Flower, 1955a). Despite its impressive size, Endoceras is not particularly well-represented in paleoart. The few depictions are plagued by the lack of research found in most art of extinct cephalopods.

Author Tyler Greenfield composed a nice reconstruction painting of it for that page.

Some simplified phylogeny first:

• Palcephalopoda
  • Orthoceratoidea X
    • Endocerida X - incl. Endoceras
    • Orthocerida X
  • Nautiloidea - the nautilus
• Neocephalopoda
  • Ammonoidea X - ammonites
  • Coleoida
    • Decabrachia - squid
    • Octobrachia - octopus, vampire squid

X = extinct

So first off, Endoceras is closer to the nautilus than to octopuses and squid. Was Endoceras a sort of super nautilus with a straight shell instead of a spiral one?

 

[lpetrich]

First, the limbs. Informally, cephalopod limbs are called both arms and tentacles. But in squid, some of them are longer than others, and there is a convention in cephalopod biology of calling the long ones tentacles and the short ones arms. But the arms are tentacles in a general sense, like elephant trunks and vertebrate tongues.

So let's look at what cephalopods are known to have.

Though cephalopod arms typically have at least partial radial symmetry, cephalopods are overall bilaterally symmetric, with a dorsal side, the eye side, and a ventral side, the funnel side. Squid have 10 arms, and from dorsal to ventral, the arm pairs are designated I, II, III, IV, and V. In most squid (Decabrachia), pair IV is elongated to make tentacles with expanded ends. Looking in Octobrachia, in vampire squid, however, pair II is reduced and used as sensory filaments, and in octopuses, pair II is lost.

Belemnites, which were some extinct coleoids or close coleoid relatives, are known to have had 10 arms.

Nautiluses have 90 arms, but they have 10 embryonic arm buds.

Arm imprints have been found for a Late Ordovician cephalopod, likely the orthocerid Treptoceras. It also had 10 arms.

So the ancestral cephalopod likely had 10 arms, and Endoceras likely did also.

 

[lpetrich]

Tyler Greenfield concludes that Endoceras had 10 equal-length arms.

What was on those arms?

Living coleoids have suckers on their arms, and belemnites had hooks on theirs. Nautiluses have ridges on their arms with adhesive on them. TG reconstructs Endoceras as having had them.

Did they have an operculum or shell-opening door on top of their heads?

The nautilus has one and there is evidence that orthoceratoids also had one. So TG reconstructs Endoceras as having had one.

What kind of eyes did they have?

Coleoids and ammonoids all had lens-camera eyes, and that is a likely ancestral state for Neocephalopoda.

The nautilus, however, has a pinhole-camera eye, and TG reconstructs that kind of eye for Endoceras

In further reconstruction, TG has Endoceras be an ambush predator that liked to sit on the seafloor with its shell pointing upward.

Thus, the painting.