It is time for some news from the „nautiloid“ research
Three topics: the origin of Nautilus, Nectocaris and hyoliths as hypothetical cephalopod ancestors,
No more nautiloids?
First of all, there is growing evidence that „nautiloids“, like the good old „pisces“ is a term that is obsolete for taxonomy because it is in use for a paraphyletic group. Remember that „fish“ is term for aquatic animals with a gill with a certain fish-like appearance. It is negative exclusive, because tetrapods like mice or man, which are a clade of the Ostheichtyes (bony fish) are not fish.
Nautiloids are classically considered as „primitive“ cephalopods with an outer shell. In contrast, coleoids are the group of internally shelled cephalopods that dominate modern group diversity (see here). The term „nautiloid“ was used by paleontologist for non-ammonoid externally shelled cephalopods with the exclusion of bactritoids and, sometimes, also of orthocerids (this is more or less synonymous with the concept „palcephalopoda“).
This concept was based on the classical hypothesis that Nautilus is a descendent of a group of Paleozoic cephalopods that are called „Oncocerida“.This hypothesis was questioned several times [e.g. Dzik, 1984; Korn & Dzik, 1992], but nobody worked on this problem ever since.
Recent molecular divergence dates additionally and independently are in contrast to the classical oncocerid-origin hypotheses [see here]. These divergence estimates point toward a quite late divergence of nautilus/coleoids (416 Ma, latest Silurian). Because coleoids verifiable are late descendents from orthocerids and orthocerids and oncocerids split already by the early Ordovician it appears reasonable to argue that Nautilus is an indirect descendent of orthocerids (we did this here).
As a consequence the classical use of the term „nautiloids“ would, like in „fishes“, be a paraphyletic clade that excludes coleoids. (see our figure below). This is a little bit a dilemma, because: How to name the various paleozoic stem group cephalopods?
Our new paper makes it obvious that it needs some effort to puzzle out the origin of the Nautilida (the group of Nautilus).
Nectocaris is not a cephalopod
In may 2010 a paper came out that spectacularly claimed that primitive coleoid-like cephalopds existed already during the Cambrian. The authors interpreted the fossil Nectocaris as a shell-less cephalopod. In two recent [Mazurek & Zaton, 2011; Kröger et al. 2011] papers convincing arguments were found that this interpretation is wrong. See also discussions here, here, and here.
Hyoliths are not cephalopod ancestors
Another exotic hypothesis on the origin of cephalopods is the idea that swimming cone shape Cambrian organisms (Hyoliths) were its ancestors. I call this the planktonic-origin hypothesis. It was expressed repeatedly by Dzik [ eg. 1981, 2010] and appears as an interesting alternative to the more widely accepted hypotheses that cephalopods originated from benthic monoplacopohoran like mollusks [see our review here and this discussion].
Recently Ed Landing and me submitted a paper that finds arguments against the planktic-origin hypothesis. These are our arguments:
Cardinal processes, conch microstructure, protoconch, and small apical angle
distinguish hyoliths from the oldest cephalopods. No evidence for an evolutionary transition between hyoliths and early cephalopods exist.
Dzik‘s hypothesis is based on the idea that primitive cephalopods had small spherical initial shells and that the earliest cephalopods had planktic/plankton feeding hatchlings. In our paper we compile evidence that early cephalopods had no such planktotroph hatchlings. Instead the earliest spherical, small initial chambers which are indicative of planktotrophy of hatchlings are known from early Ordovician orthocerids.
