Roger A. Hewitt

Environmental setting of fossiliferous rocks from the uppermost proterozoic—lower Cambrian of central England

Abstract The Hartshill Formation of Warwickshire in England provides a section through the initial stages of the “Cambrian marine transgression”. The sea encroached an irregular land surface of weathered volcanic and plutonic rocks which was probably fringed by sand dunes. The first transgressive deposits reflect this provenance but later sedimentation can be interpreted as comprising a gradual retreat of the shoreline and an increase in water depth; superimposed on this trend are several short periods of regression. the transgressive coastal deposits contain meandering trails, Planolites and Arenicolites traces rather than the Skolithos and Diplocraterion assemblages found elsewhere in similar facies of Cambrian (Tommotian and younger) age. The episodes of deeper water, sediment-starved deposition correspond approximately to an early “ Cruziana facies” in laminated sands and muds, to a shelly and stromatolitic limestone facies with a Tommotian fauna and finally to monotonous offshore muds bearing the first local trilobite fragments.

Architecture and Strength of the Ammonoid Shell

The present chapter concentrates on those aspects of ammonoid morphology that are directly related to the habitat depth of ammonoids and the strength of the shell. Other chapters in the present volume discuss the role of cameral water and ornamentation in locomotion (Chapters 7 and 16, this volume), the growth of septal sutures (Chapter 9, this volume), and ecology (Chapter 16, this volume). The symbols used in this chapter are defined in Table I.

Nautilus Shell Architecture

The familiar shape of the shell of Nautilus pompilius has been described thoroughly (e.g., Stenzel, 1964) and needs little introduction. We wish to consider to what extent the shells design resists water pressures as great as 8.34 megapascals (MPa) [830 m depth equivalent (Kanie and Hattori, 1983)]. The loading of the last septum and phragmocone wall is due to the difference in pressure between the partial vacuum within the gas chambers (<1 atm) and the ambient hydrostatic head of seawater. The strength and stiffness of the aragonite shell permit it to act as a constant-volume hydrostatic apparatus that enables Nautilus to maintain neutral buoyancy. The gas diffuses into the chambers via the siphuncular tube as the cameral water is removed from behind the recently completed last septum of the growing shell. Because the equilibrium gas pressures in the ocean are largely independent of water depth, there is no difference in the final gas pressure of chambers grown at different depths.

RECURRENCES OF HYPOTHESES ABOUT AMMONITES AND ARGONAUTA

The functional morphology of septal sutures and modified body-chambers of ammonoids and their inferred life habits have been popular topics for over a hundred years. Such long history includes the repetition of ideas, some rejected long ago, that originated in the last half of the 19th century mainly in Germany and Austria. Recent examples of such recurrences are the papers by Lewy (1996, 2000, 2002). His nude-ammonites-became-octopods thesis goes back to Suess (1865) and Steinmann (1888); Naef (1922) argued that argonautid octopods later grew an epipelagic shell to replace the ammonite shells when they were no longer available as drifting egg and brood chambers. Lewy, extending these hypotheses, supposed that octopods modified the host apertures and the hatchlings lived off the decomposing host. Lewy (2002) also revived the hypothesis that intricately folded septal lobes served only or mainly to securely attach the animal to the shell (Diener, 1912) in order to withstand the buoyant force between the light shell and the heavier body and, especially, the dragging force produced by arms and tentacles during swimming and predation. He concludes that septal complexity correlated with the ferocity of predatory activity. We will review the history of these and similar hypotheses relating to ammonite body-chambers, septal sutures and possible octopod relations in order to show the dangers in promoting ideas, which have been long forgotten and rejected, in ignorance of their history. Disparate hypotheses have been proposed for the variously uncoiled or recoiled body-chambers of many Jurassic and Cretaceous “heteromorph” ammonoids (reviewed in Westermann, 1996). Lewy (1996, 2000) proposed that most adult body-chamber modifications “must have resulted in death, rather than providing the mature organism with suitable means for living its prime stage” (1996, p. 627). His arguments are the following: 1) The terminal body-chamber of many Cretaceous …

Post-mortem behaviour of Early Paleozoic nautiloids and paleobathymetry

KurzfassungDie vielgestaltigen und oft vollständig erhaltenen Schalen von ordovizisch-silurischen Nautiloideen konnten wahrscheinlich nicht lange auf der Meeresoberfläche treiben, sogar wenn die Tiere dort starben. Das Lumen des Phragmacons war groß verglichen mit dem verlorenen Weichkörper, und die Kammern enthielten im lebenden Tier nicht nur Flüssigkeit sondern auch Gas mit weniger als 1 atm. Nach dem Tode und Entfernung oder teilweiser Verwesung des Weichkörpers verursachte dieser Unterdruck in wenigen Stunden oder Tagen das Fluten eines Teils der Kammern durch Meerwasser. Während oder nach dem Sinken der leeren Schale konnten die dünnen Septen von oberflächennah lebenden Nautiloideen unter umgekehrtem, zur Mündung gerichtetem, hydrostatischen Druck leicht nachgeben und brechen. Diese Art von Druckladung und Implosion der Septen geschah wahrscheinlich durch plötzlichen Bruch (?Boden-Aufschlag) des Siphos in einer apikalen Kammer, im Bereich der maximalen Lebenstiefe der jeweiligen Art. Das schnell folgende Einbrechen der adoralen Septen endete wahrscheinlich an relativ stärkeren Septen, die dicker waren und/oder durch teilweises Fluten der dahinter liegenden Kammer gestützt waren. Nach diesem Szenario betrug die minimale Ablagerungstiefe der silurischen Nautiloideen von Böhmen etwa 65 m. Alternative Szenarios, wo dieselben Septen unter (Membran-)Spannung (adapikal) brechen, wie bislang angenommen, oder wo Druckunterschiede in benachbarten Kammern durch Flutung entstanden, resultieren in 160 m minimale Wassertiefe für diese Fazies. Die Septen vieler kleinerer Silur-Nautiloideen, die in über 100–300 m Wassertiefen lebten, waren zu dick um nachzugeben und brachen auch kaum unter Spannung, weil die Gehäuse bei dem hohen Wasserdruck wahrscheinlich schneller geflutet wurden als sie absanken.AbstractIt is unlikely that the intact or commonly preserved varieties of Ordovician-Silurian nautiloid shells were able to drift for any distance at the surface of the sea even if they died there. Their cameral capacity was much larger than the volume of the extracted or decayed body, and it would have contained a partial vacuum and cameral liquid when they were alive. The closely spaced and thin septa of the shallow-water adapted species were liable to buckle in compression and then implode in local tension during reverse hydrostatic loading by water pressure. This reverse loading and internal implosion of the septa was probably initiated by the sudden cameral refilling of an apical chamber caused by the depositional rupture of the apical siphuncle at or near the maximum habitat depth of these species. The instantaneous buckling of the more adorai septa was potentially terminated by variations in the septum thickness and cameral fill-fractions at that time, and they imply that some of the Silurian nautiloids from Bohemia were deposited at a minimum depth of about 65 m. Alternative interpretations involving the breakage of the same septa in tension, or buckling due to the difference in pressure between adjacent flooded chambers, set a maximum depth limit of about 160 m for the same facies. Many of the smaller Silurian nautiloids were unlikely to buckle during refilling, and they were potentially flooded faster than they could sink, below a depth of 100–300 m.

Growth rates of ammonites estimated from Aptychi

Abstract The study of Laevaptychus and Lamellaptychus from the Kimmeridgian of Spain, the Lower Tithonian of Spain, Germany and Argentina, and the Berriasian of Spain, shows that the mature modification of the ammonite body chamber was a product of retarded submatures growth rates. The size variation of mature Aspidoceratids and Oppeliids reflects differences in their submature growth rate as well as age.

Nautiloid septal strength: revisited and revised concepts

Recent critiques of all aspects of nautiloid septal strength indices have in effect questioned the adaptation of these shells as minimum weight structures and the use of this concept to deduce the habitat depth limits of fossil nautiloids. It is suggested here that the method is actually more precise than the original work suggested, with most of the error lying within the measurements made by the palaeontologist and the generalised nature of the originally formulated septal strength index. There may be little or no difference between the calculated implosion depth and maximum habitat depth of juvenile, as opposed to mature, Nautilus.

Bathymetric limits of a Carboniferous orthoconic nautiloid deduced by finite element analysis

The last septum and adjacent portions of a Carboniferous nautiloid Michelinoceras unicamera (Smith) have been subjected to a finite element analysis involving 387 nodal points concentrated at the septal suture of the last septum. At a depth of about 1125 m and an external water pressure of 11.35 MPa, the shell would have imploded almost simultaneously in the septum, the sutures and the shell walls overlying the septum. Stress distributions support the validity of the highest septal strength index so far obtained from any nautiloid and suggest the Michelinoceras unicamera lived in basins and continental slope environments near southern Oklahoma. Our analysis shows the structural importance of the mural ridge underlying the septal suture: without it, the implosion depth would have been halved.

Nautiloid shell taphonomy: Interpretations based on water pressure

Abstract Nautiloid taphonomy is a difficult and widely neglected topic acting as a test of many of the assumptions made in nautiloid paleobiology, phylogenetic interpretations, and geological facies analysis. The Eocene nautiloids from the offshore London Clay Formation of England are considered from the perspective of flooding and implosion experiments on Recent Nautilus . The deeper water facies are often characterized by body chambers in approximate life position, one or two complete septa, and various types of shell debris. Intermediate depth facies show intact and often vertically embedded phragmocone walls, with zones of internally broken and spar cemented septa. Taphnomic characteristics of the former facies probably result from the implosion of shells at depths of 200 or 300 m on the seafloor, while those of intermediate depth facies result from the sinking of gas-filled shells to the seafloor and subsequent bending stress failure of septa left unsupported by sediment in the last whorl. Evidence for the removal of the body from the largely unbroken shell by octopus illustrates the potential for a biological influence on the taphonomic history of nautiloids through the Phanerozoic. Tertiary nautiloids were probably unusually prone to extensive post-mortem transport, a process which tends to remove shells from the fossil record under most geological circumstances.

Phosphatic connecting rings and ecology of an Ordovician ellesmerocerid nautiloid

The phosphatic siphuncle of Bactroceras latisiphonatum Glenister, a Late Ordovician member of the ancestral order of nautiloid cephalopods, displays the outer ‘chalky’ and inner ‘horny’ tubes seen in the aragonitic connecting rings of Recent Nautilus and Carboniferous Orthocerida. The specimens were collected from the Malongulli Formation of central New South Wales. Bactroceras resembles Mesozoic ammonoids in having a marginal phosphatic connecting ring that is partly a product of diagenetic phosphatization. Both the septa and connecting rings were apparently able to resist greater hydrostatic water pressures than other ellesmerocerids and had a potential implosion depth comparable to that of mature Nautilus. The septal necks and siphuncle position resemble the morphology of Bactrites; but it is unlikely that Bactroceras and ‘Eobactrites’ were directly ancestral to bactritoids and ammonoids.