Mollusca		Nautiloidea
CYRTOSOMA		Nautilida

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Nautilida

Cephalopoda
|--NAUTILOIDEA
|  |--TARPHYCERIDA
|  |  |--Tarphycerina  
|  |  `--BARRANDEOCERINA
|  `--ONCOCERIDA 
|     |--Oncoceratidae  
|     `--NAUTILIDA    
|        |--NAUTILIDA i.s.
|        `--NAUTILINA
`--Neocephalopoda
   |--Ammonoidea
   `--Coleoidea

Introduction

The Nautilids include the modern pearly nautilus and its evolutionary ancestors and relatives. During the late Paleozoic and the Mesozoic this was a large and successful group.  Today it is represented only by several species of the genus Nautilus.  

Conch Shape

Although in Devonian times the shells of nautilids were varied in shape, ranging from cyrtoconic through gyroconic to tightly coiled and involute, later on the loosely coiled and cyrtoconic shells became rare or disappeared altogether, except in the superfamily Aipocerataceae. Nautiloids thenceforth conformed to the standard planispiral evolute, convolute, and involute conch shape (the modern pearly nautilus displays the tightly coiled involute form)

Explains Teichert

The Nautilida exhibit an infinite variety in degrees of coiling (with no deviations from the planispiral model), whorl cross sections, and surface ornamentations, but no overall trends in any of these features. Sutures were straight or only weakly undulating in almost all genera, with the exception of a few that developed in the carboniferous among the Trigonoceratidae, in the Late Triassic in the Clydonautilidae, Gonionautilidae, and Siberionautilidae, and during the Late Cretaceous and Early Tertiary among the Hercoglossidae and Aturiidae, all of which have sutures similar to those of certain Upper Devonian goniatite genera. Characteristically, siphuncles are orthochoanitic and anywhere from subventral to subdorsal in position. Only few forms have marginal siphuncles, with those of the Late Paleozoic Aipocerataceae and of Triassic Menuthlonautilus being ventral and that of Early Tertiary Aturia being dorsal in position.

Phylogenetic Relationships

Although the ancestors of the Nautilida have previously been considered among straight-shelled ("Orthoceras") cephalopods which became gradually coiled nautilids (Schindewolt; 1942), or from coiled Silurian forms of the Barrandeocerine Tarphycerids (Barrandeoceratidae Flower and Kummel 1950 or Lechritrochoceratidae Dzik 1984, it is now generally accepted that the Oncocerida gave rise to the order Nautilida. Teichert 1988 suggests that the best candidates would be the Acleistoceratidae and the Brevicoceratidae, both of whom have exogastrically curved or coiled shells and cyrtochoanitic septal necks, characteristic of early rutoceratids and ptenoceratids nautilida.  

Evolutionary History

The Nautilida have a rich and complex evolutionary history, but the details of taxonomy and phylogenetics remain obscure. But the general pattern is clear. This clade, the last of the major groups of Palcephalopoda to evolve, are also the most successful. The following account is by Teichert:

From modest beginnings in the Lower Devonian (Teichert et al., 1979), the number of genera increased from 1 or 2 in the Gedinnian to at least a dozen in the Enisian and about 22 in the Middle Devonian. After a decline in the Late Devonian, with only 3 genera recorded, the order rose to great prominence in the Carboniferous, when some 75 genera and subgenera in some 16 families are known (Shimanskiy, 1967, with additions). After the Carboniferous we witness a steady decline of the order to about 55 genera in 10 or 12 families in the Permian, and about 35 genera in 8 families in the Triassic. Interestingly, the nautilids were much less affected by the great wave of extinctions at the end of the Permian than the contemporary ammonoids: 3 families and at least 4 possibly 8 genera of nautilids passed from the Permian into the Triassic. Beginning in the Triassic, morphological differentiation of the order declined to a severe setback in the Late Triassic, when, just as today, only one genus, Cenoceras (above), survived (Kummel, 1953). This was followed by a slight recovery in the Late Jurassic and, especially, Cretaceous with 24 genera (Shimanskiy, 1975; Whetstone and Teichert, 1978), and a final decline during the Cenozoic, at the end of which the order is reduced to a single genus Nautilus.

Interestingly, the nautilids were not affected by the wave of extinctions at the Cretaceous - Tertiary boundary which wiped out their close relatives, the ammonoids; three families and at least five genera of nautilids crossed this boundary and the Paleocene and Eocene saw a resurgence of nautilids, with several new genera added, most of which were of worldwide distribution (Miller, 1949; Kummel, 1956; Shimanskiy, 1979).

Lifestyle and ecology

Modern nautilids (and it can be assumed many extinct types too) live at some depth but can rise to shallower waters.  They feed on fish and invertebrates.  There are 38 arms which form a circle around the head. The eyes are small and primitive, and functions rather like a pinhole camera.  Above the head and the arms is a leathery, protective hood, that acts like an operculum to protect the animal when it withdraws inside its shell.  They produce only a few large and well-developed off-spring at a time.  Development time inside the egg of more than a year in the Recent Nautilus.  It takes three or four years to reach maturity.  The adults can spawn several times and may live up to 20 years.  

Families and Genera

Shimanskiy (1962) divided the Nautilida into 34 families (later (1979) reduced to 30) and 184 genera, whilst Kummel in the Treatise (1964) recognized 24 families and 165 genera. Both agree that the Nautilida is the largest order of the (sensu lato) Nautiloidea.