W. Bruce Saunders

Morphology and morphologic diversity of mid-Carboniferous (Namurian) ammonoids in time and space

Morphologic analysis of 281 species of ammonoids from Great Britain, the North American mid-continent, and the South Urals, at eight successive levels within the Namurian Series (ca. 18 Myr duration), using bivariate plots and principal-components analysis, permits definition of morphologic diversity and identification of morphotypic patterns in time and space. Namurian ammonoids exhibit the same general range of shell geometry that characterizes ammonoids as a whole; there were few post- Namurian innovations in the basic geometry of planispiral ammonoids. Within this overall range of geometry, there are eight preferred morphotypes: two were phylogenetically monopolized by long-ranging forms; three were generalized and reoccur in successive horizons; two others were homeomorphically utilized at different times by different lineages; and one represents morphologic innovation followed by radiation. Such patterns seem to represent combined effects of function, phylogeny, and ecology. Synchro- nous variations in isolated successions suggest global controls such as eustatic sea-level fluctuations, whereas provincial differences in diversity may be attributable to paleogeographic and ecologic factors. We predict that the Namurian record of ammonoid morphologic diversity and change will be found to be distinctive and differentiable from earlier and later intervals.

Calculation and simulation of ammonoid hydrostatics

The buoyancy, stability, and orientation of a shelled cephalopod in water are the predictable products of shell geometry, body chamber length, and such physical parameters as shell, tissue, and water densities. Given such physical characteristics as shell geometry, shell, tissue, and water densities, and shell thickness, the hydrostatic characteristics of planispiral shelled cephalopods, including orientation, centers of mass and buoyancy, stability, and neutrally buoyant body chamber length, can be calculated and simulated using microcomputer-based techniques. Individual variables such as geometry, body chamber length, and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability. Living Nautilus provides a means of testing the model and for making hydrostatic comparisons between ammonoids and nautiloids. The close agreement between calculated versus observed body chamber lengths in five species of Mississippian ammonoids shows that neutral buoyancy, and (with one exception) Nau- tilus-like orientations, were at least feasible for these species.

Shell morphology and suture complexity in Upper Carboniferous ammonoids

Principal components analysis of Upper Carboniferous (Pennsylvanian) ammonoids (all 117 genera), using 21 variables to measure shell geometry, sculpture and suture complexity, shows that following a sharp decline (∼30%) in generic diversity after the mid-Carboniferous boundary, seven morphotypes persisted throughout the Pennsylvanian (ca. 30 m.y.). Six of these were polyphyletically adopted at different times, while the seventh was monopolized by the prolecanitids, a group whose evolution accelerated during the Pennsylvanian and later gave rise to Mesozoic ammonoids. Innovations in suture geometry distinguished at least 17 of 39 (44%) Pennsylvanian ammonoid families. Average suture complexity increased almost threefold; this was achieved by various methods (lobe serration, insertion of umbilical elements, prong subdivision, lobe trifurcation, and secondary bifurcation), which were recurrent and crossed morphotype boundaries. The Pennsylvanian record supports suggestions that Paleozoic ammonoids were confined to a certain suite of basic shell geometries, showing preference for a limited number of sites in the spectrum of available morphospace. However, these morphic constraints did not, with one possible exception (the prolecanitids), control the emergence of increasing sutural complexity during the Pennsylvanian, which occurred among different lineages in all seven morphotypes.

Septal complexity in ammonoid cephalopods increased mechanical risk and limited depth

The evolution of septal complexity in fossil ammonoids has been widely regarded as an adaptive response to mechanical stresses imposed on the shell by hydrostatic pressure. Thus, septal (and hence sutural) complexity has been used as a proxy for depth: for a given amount of septal material greater complexity permitted greater habitat depth. We show that the ultimate septum is the weakest part of the chambered shell. Additionally, finite element stress analyses of a variety of septal geometries exposed to pressure stresses show that any departure from a hemispherical shape actually yields higher, not lower, stresses in the septal surface. Further analyses show, however, that an increase in complexity is consistent with selective pressures of predation and buoyancy control. Regardless of the mechanisms that drove the evolution of septal complexity, our results clearly reject the assertion that complexly sutured ammonoids were able to inhabit deeper water than did ammonoids with simpler septa. We suggest that while more complexly sutured ammonoids were limited to shallower habitats, the accompanying more complex septal topograhies enhanced buoyancy regulation (chamber emptying and refilling), through increased surface tension effects.

Ecology, Distribution, and Population Characteristics of Nautilus

Until recently, knowledge of the ecology of Nautilus was largely based on trapping results, observations of captured animals held in shallow water, and speculation and hearsay. Despite the limitations imposed by such sources, a considerable amount of information regarding the depth range, diet, and geographic distribution of Nautilus had been assembled by the turn of the century. This was ably summarized by Henryk Stenzel (1957), in a short contribution that includes an excellent annotated bibliography, in the Treatise on Marine Ecology and Paleoecology. This was followed a few years later by the excellent chapter by Stenzel (1964) in the Treatise on Invertebrate PaleontoJogy. The latter is a concise synthesis of almost everything known about Nautilus at that time.

Function and shape in late Paleozoic (Mid-Carboniferous) ammonoids

-Analysis of an exhaustive data base of Namurian ammonoid shell characters indicates that the morphology of the Goniatitida can be explained in terms of functional constraints, resulting particularly from hydrostatic and hydrodynamic properties. Modes of life ranging from benthic to pelagic are inferred on this basis for various goniatitid morphotypes; all morphologic features and facies associations are normally compatible with these inferences. Neutral buoyancy is shown to have been likely for all goniatitids. By contrast, the prolecanitids (Order Agoniatitida) exhibit a number of hydrostatic and morphologic anomalies; these anomalies are not explicable using the same principles and remain problematic. This is noteworthy, in that prolecanitids survived the Permian/Triassic extinctions and gave rise to the diverse ceratitic radiation in the Triassic. The applicability of these results to ammonoids outside the Namurian is assessed, and, in particular, morphologic parallels with Mesozoic ammonites are discussed. Andrew R. H. Swan. Department of Geology, University College of Swansea (Present address: School of Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston-upon-Thames, Surrey KT1 2EE, U.K.) W. Bruce Saunders. Department of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010 Accepted: May 14, 1987

Allonautilus : a new genus of living nautiloid cephalopod and its bearing on phylogeny of the Nautilida

Living ectocochliate cephalopods have long been thought to be restricted to a single genus, Nautilus Linnaeus, 1758, comprising five or six extant species. The shells of two species, N. scrobiculatus Lightfoot, 1786, and N. perforatus Conrad, 1847, are quite distinct, but no soft-parts were known until 1984, when N. scrobiculatus was seen alive for the first time. Dissections show that significant anatomical differences exist between N. scrobiculatus and other Nautilus species, including differences in gill morphology and details of the male reproductive system. These differences, along with phylogenetic analysis of extant and selected fossil nautiloid species, indicate that N. scrobiculatus, and N. perforatus should be distinguished from Nautilus as a newly defined genus, Allonautilus. This analysis contradicts previous phylogenies proposed for the Nautilida, which placed Nautilus as the last-evolved member of the order. We surmise that Allonautilus is a descendent of Nautilus, that the latter is paraphyletic, and first evolved in the Mesozoic, rather than in the late Cenozoic, as has been previously suggested.

Nautilus movement and distribution in palau, Western caroline islands.

Long-term movement of up to 150 kilometers in 332 days by tagged, living Nautilus, and postmortem shell drift of 1000 kilometers in 138 days, corroborate and explain the cosmopolitan distribution of many fossil shelled cephalopods.

Genetic divergence and geographic diversification in Nautilus

Despite exhaustive investigation of present-day Nautilus, the phylogenetic relationships of the five or six recognized species within this genus remain unclear. Mitochondrial and nuclear DNA sequence data plus a suite of morphological characters are used to investigate phylogenetic relationships. Systematic analysis of the morphological variation fails to characterize described species as independent lineages. However, DNA sequence analysis indicates that there are three geographically distinct clades consisting of western Pacific, eastern Australian/Papua-New Guinean, and western Australian/Indonesian forms. The morphologically and genetically distinct species Nautilus scrobiculatus falls outside the three geographically recognized assemblages. Members of the genus Nautilus also exhibit low levels of sequence divergence. All these data suggest that Nautilus is currently undergoing diversification, which may have begun only several million years ago. These data also suggest that some of the morphological features used to define Nautilus species may simply represent fixed variations in isolated populations within the same species.

The ammonoid suture problem; relationships between shell and septum thickness and suture complexity in Paleozoic ammonoids

The most widely cited explanation for the functional enigma of sutural complexity in ammonoids, the Buckland hypothesis, has related septal folding and fluting to buttressing, providing increased shell strength against implosion, along with increased efficiency and decreased weight in shell and septum construction. In Paleozoic ammonoids, sutures ranged from simple to extremely complex. Comparison of shell and septum thickness (in polished sections) with sutural complexity in 49 Paleozoic ammonoid genera (Middle Devonian–Upper Permian) indicates that no significant reduction in either septum thickness or shell thickness accompanied a one-hundred-fold increase in sutural complexity. These preliminary results fail to support the Buckland hypothesis, suggest there may have been alternative incentives for increasing sutural complexity, and add support to views that septal fluting may have been related to buoyancy control.