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How many “nautiloid” specialists exist in the world ? What are the current research themes of nautiloid specialists? What are the next promising topics? – Occasionally people ask me these question on conferences or in emails. Here, I try to compile a short overview about current developments in “nautiloid research” relevant to paleontology.  Of course my list is not comprehensive, but it will be more so by discussion…

What are “nautiloids”?

This question is easily answered by neontologists: Nautiloidea (subclass) are the others beneath the soft bodied cephalopods, the Coleoidea (subclass). Of the several hundred known living cephalopod genera [Felley et al. 2003, 1], only two are nautiloids, Nautilus, and Allonautilus. For neontologists, nautiloids are the few cephalopods which have primarily an outer shell [Lindgren et al. 2004, 2].
More problems arise for the classification of fossil cephalopods. Because nautiloids, are simply considered as stem-group cephalopods, nautiloids in the broad, classical sense are thus all old cephalopods that are not coleoids or ammonoids (this is the practice of the Treatise). However, the evolutionary history that lies between the appearance of the first cephalopod and between the appearance of the first coleoids is long and complex, thousands of extinct genera are known. Here, the neontological bipartition of cephalopods into nautiloids, and coleoids causes a great dilemma for the classification of fossil cephalopods, because it does not allow for the erection of new total groups that would express phylogenetic hypotheses, above the subclass level fixed by the Coleoidea.
There are only few exceptions. Some extinct large monophyletic offshoots (plesions) can be excluded and treated as additional subclasses (e.g. ammonoids). However, for several proposed subclasses, such as actinoceroids, and bactritoids it is not clear if they represent paraphyletic/grade groups, or even polyphyletic groups [Kröger and Mapes, 2007a, b ]. For others, such as the Orthoceratoidea it would be appropriate to include new higher taxa above the subclass and below the class level, in order to classify extinct cephalopods in total groups that are transitional between coleoids and Nautilus.
One approach to do this is that of Lehmann and Hillmer (1980), which distinguish between the Palcephalopoda and Neocephalopoda. The Neocephalopoda contain the ammonoids, coleoids and the diverse cephalopods with straight and coiled shells that are classified within the “Orthoceroidea”, and “Bactritoidea”, and the Lituitida. The Palcephalopoda are the remaining stem-group cephalopods.
When accepting the bipartition of cephalopods into Neocephalopods and a stem-group grade than, the Palcephalopoda would  be a subjective junior synonym of the Nautiloidea. In fact, this is the implicit usage of the term “nautiloid” that I use since a few years in my own papers and that also has been practiced by the e.g. Manda and Turek (2009): nautiloids are stem group cephalopods exclusive ammonoids, bactritoids, coleoids and orthoceroids. Here I will call them “nautiloids sensu stricto”. In contrast the classical nautiloids of the Treatise, and that of the neontologist are all non-ammonoid cephalopods with an external shell. Here I will call them “nautiloids sensu lato”. A completely different classification was suggested by Shevyrev (2005), who divided cephalopods into eight subclasses. The subclasses itself comprise very different groups (paraphyletic/grade groups, plesions, and a crown group) many of them are considered as problematic and need serious revision [see here].

Who is currently working on fossil nautiloids sensu lato?

• Matilde S. Beresi, Mendoza, Argentina, (taxonomy of Middle Ordovician cephalopods, Argentine Precordillera, papers)
• Regis Chirat, Lyon, France (shell morphology and ontogeny of Mesozoic nautilids, papers)
• Marcela Cichowolski, Buenos Aires, Argentina (taxonomy of Ordovician cephalopods and Mesozoic nautilids, Southern Hemisphere, papers),
• David Evans, Peterborough, UK (taxonomy and phylogeny of Ordovician and Silurian cephalopods, UK, Ireland and cephalopods from former Gondwana, Avalonia, Perigondwana regions, papers)
• Robert C. Frey, Columbus, OH, USA (Upper Ordovician cephalopods of the USA, papers)
• Kathleen (Catherine) Histon, Italy (taxonomy of Late Carboniferous cephalopods Carnic Alps)
• Mauricio Gnoli (taxonomy and paleogeography of Silurian-Devonian cephalopods of Italy and the Carnic Alps,  papers)
• Christian Klug, Zürich, Switzerland (paleobiology of Triassic nautiloids of Germany, papers)
• Björn Kröger, Berlin, Germany (Paleoecology and Phylogeny of Paleozoic non-ammonoid cephalopods, with focus on the Early Paleozoic, papers)
• Stepan Manda, Praha, Czech Republic (taxonomy, paleoecology, paleogeography, phylogeny of Silurian-Devonian cephalopods of the Prague Basin, papers)
• Royal Mapes, Athens, OH, USA (Carboniferous-Permian cephalopods of the USA, bactritoid specialist, papers)
• Harry Mutvei, Stockholm, Sweden (cephalopod shell ultrastructure and siphuncular morphology with focus on Paleozoic nautiloids, papers)
• Shuji Niko, Higashihiroshima, Japan ( taxonomy of Late Paleozoic non-ammonoid cephalopods of Japan, papers),
• Ian G. Percival, Sydney, Australia (taxonomy and paleogeography of Australasian Ordovician cephalopods, papers)
• Jan Audun Rasmussen, Copenhagen, Denmark (Ordovician cephalopods of Greenland, and Norway)
• Voitech Turek, Praha, Czech Republic (taxonomy and phylogeny of Silurian-Devonian cephalopods of the Prague Basin, papers)
• Ryoji Wani, Yokohama, Japan  (Taphonomy of Nautilus, papers)
• Markus Wilmsen, Dresden, Germany (Cretaceous nautilids, papers)
• Cheol-Soo Yun, Kyungpook, Korea (taxonomy of Ordovician cephalopods of Korea, papers)
• Yunbai Zhang, Nanjing, China (Taxonomy and Morphology of Ordovician and Silurian cephalopods of China)

There are some anonymous workers (Theo Engeser, Berlin, is probably among them) which continuously complete, and keep up to date the fossil cephalopod pages at the  Palaeontology Portal of Wikipedia.
Austin Hendy, and me (very slowly) compile fossil cephalopod occurrences into the Paleobiology Database.

Current “hot” topics & interesting nautiloid papers

Living nautilids

Some of the remarkable papers about living nautiloids that appeared during the last years are: The complete mitochondrial genome of Nautilus macromphalus was published by Boore (2006).
Shigeno et al. (2008) analyzed the early development of Nautilus and suggested that early in ontogeny the morphology resembles the body plans of monoplacophorans and basal gastropods, and that the tentacles of Nautilus “develop from simple serial and spatially-patterned bud-like anlagen along the anterior-posterior axis, indicating that origins of digital tentacles or arms of all other cephalopods develop not from the head but from the foot.”
Crook et al. (2009) analyzed the spatial memory of Nautilus and found that its performance is comparable to coleoids.
Staples et al. (2000) demonstrated that Nautilus can suppress its metabolic rate 16-fold in low P O2, low temperature environments, supporting earlier evidence of an extremely high hypoxia tolerance of Nautilus.
Wani et al. (2008) reviewed the fossil history of Nautilus.
The taphonomic experiments of Wani and colleagues [2005, 2006, 2007] are important for the understanding of the interpretation of fossil cephalopod occurrences.

Aturia

The worldwide distribution and phylogeny of the Paleocene “goniatite” Aturia remains to be a discussed topic [Chirat, 2000; Nielsen et al., 2009]. Are the shells of Aturia drifted or represent the original distribution? A  paper of Tsujino and Iwata (2009) in Cretaceous Research reports a Late Creataceous Aturia from Japan.

Classification/Phylogeny

Orthocerida, Pseudorthocerida, Dissidocerida, Actinocerida – These orders are somehow related, but how, this is not clear. Are the orthocerids a paraphylum that consist partly of pseudorthocerids, dissidocerids, and actinocerids? Is there a higher monophyletic taxon, such as the “Orthoceratoidea” that includes all these orders plus the Lituitida (Mutvei, 2002)? In two papers by D. Evans [Evans, 2005] and me [2008] the Orthocerida and their relationships are discussed but no conclusion is reached. The revision in prep. of a Tremadocian cephalopod fauna with several orthocerids from the Montagne Noire by David Evans and me is promising. Orthocerids appear much earlier than previously thought and Bactroceras is now the earliest orthocerid, and neocephalopod. But because two very different transitional forms between ellesmerocerids and orthocerids exists already in the middle Tremadocian (Rioceras and Slemmestadoceras) a polyphyletic/paraphyletic origin of the orthocerid, and pseudorthocerid/actinocerid lineages cannot be excluded at the time.
Bactroceras is not related to Bactritites, the type of the Bactritoids. It is clear now, that bactritoids are only slightly younger than ammonoids, they evolved from Spaerorthocerids in the latest Lochkovian or Pragian [Kröger and Mapes, 2007].
The origin of the Nautilida is still an open question. But Manda and Turek [2009] revised the Rutoceratidae, coiled oncocerids that are probably the direct ancestors of the Nautilida and found evidence for a single monophyletic higher taxon of coiled oncocerids (the superfamily Rutoceratoidea) that strongly radiated during the Pragian.
As a consequence of these findings a major Early Devonian evolutionary pulse becomes visible during which pseudorthocerids [Kröger, 2008], and coiled oncocerids strongly radiated, and the bactritoids, the ammonoids, and probably also nautilids and coleoids had their first appearance.
Additionally the reviews in [Kröger and Landing, 2008; , Kröger et al. 2009], and my diversity analysis of Ordovician cephalopods reveal that during the narrow time interval between the middle Tremadocian and the early Floian, no less than 5 cephalopod orders had their first appearance; the Endocerida, Tarphycerida, Oncocerida, Orthocerida, and Discosorida.
Together these data show that the evolution of cephalopods during the Paleozoic was not a continuous process but occured in pulses (see also Kröger and Zhang, 2009).

Paleoecology

The first cephalopods of the pelagic realm appeared during the Tremadocian, pelagic cephalopods strongly diversified and became more common during the Middle Ordovician [Kröger et al. 2009]. This expansion of cephalopod habitat is parallel to other organism that entered the pelagic realm during the latest Cambrian-earliest Ordovician.

The Early Ordovician diversification of cephalopods was not only accompanied by an increase in disparity but by a general increase in cephalopod biomass at least at the shallow water carbonate platforms of the former Laurentia [Kröger and Landing, 2009].
The strong diversification of coiled shells during the Early Devonian is interpreted as adaptive response to an increased predatory pressure by durophages and as tendency toward an increased mobility of the nekton [Kröger, 2005; Manda and Turek, 2009; Klug et al in press; ].

Wish list

Morphological reconstruction

For many nautiloids sensu lato indirect data for soft body organization, and life habit are available (see e.g. Manda, 2007; Kröger, 2007), new data on the early ontogeny of Nautilus (Shigeno et al. 2007) would allow for nice reconstructions of ancient cephalopod, that could be used in popular presentations. Together with Slava Bizikov, I have a paper in prep. with reconstructions of some of them. Who will help us with its 3D/illustrative skills?

Treatise

The systematics of nautiloids sensu lato is partly messy (see for instance here, and here) and completely not to overview for a non-specialist. The Treatise is absolutely not up-to-date anymore, after more than 40 years since his publication. Lots of new genera were added since then and many taxa are revised. A new Treatise Nautiloidea is needed.

Paleogeography

Currently it is impossible to use Ordovician cephalopods for paleogeographic reconstructions because cephalopods from many paleo-regions of Gondwana and Perigondwana are in need of a revision or need to be described at all. Moreover, the (rich) Late Ordovician cephalopod faunas of Baltica are poorly described and lots of taxa are in the collections which never were described. These taxonomic descriptions and revisions are necessary in order to use cephalopods for reliable paleogeographic reconstructions.