John A. Chamberlain

Mechanical properties of coral skeleton; compressive strength and its adaptive significance

Measurement of the compressive strength and elastic modulus of the skeletal material of three common Caribbean corals suggests that the mechanical properties of coral skeleton are an important factor in the adaptive repertoire of these animals. The strength (stress at fracture) of the specimens tested is 12–81 meganewtons/meter 2 , with material from branched colonies being generally stronger than material from massive colonies. These values are lower than the strength of most other carbonate skeletal materials, but higher than that of carbonate engineering materials like concrete and limestone. The comparatively low strength of coral skeleton may be the result of architectural properties produced by the requirements of competing adaptive factors, such as polyp phototropism, or it may reflect the low probability that a colony will be broken, and therefore need to be stronger, before it achieves reproductive parity. The skeleton of the three species tested here is strongest when stress is applied parallel to the growth direction of the polyps. Strength varies inversely with skeletal porosity. Decreasing porosity in highly stressed colonies represents a potentially valuable adaptation for enhancing strength. The adaptive value of porosity modification may explain differences in porosity and strength between highly stressed branched growth forms and more moderately stressed massive growth forms. Boring organisms reduce the strength of coral skeleton by increasing its porosity. Only minor amounts of boring can produce strength reductions of up to 50%. Specialized, stress-minimizing branch arrangements help maximize resistance of coral structures to mechanical degradation in situations where colony size is unusually large or hydraulic energy dangerously high.

Post-mortem ascent of Nautilus shells: implications for cephalopod paleobiogeography

Analysis of post-mortem buoyancy loss in Nautilus shells suggests that extensive nekroplank- tonic drifting occurs infrequently. Most shells do not reach the surface but settle to the sea floor, after a short period of ascent. This occurs because the rate of water influx into the phragmocone due to ambient hydrostatic pressure is sufficiently rapid in most cases to overcome positive buoyancy before the shell reaches the surface. The resulting geographic distribution of Nautilus shells would therefore mirror the distribution of the live animals. Thus, post-mortem drift in Nautilus cannot be used as a basis for questioning the validity of cephalopod paleobiogeography. Estimate of influx rates in ammonoid siphun- cles indicates that many, if not most, ammonoid shells also would not become nekroplanktonic. This is especially true for small (<5 cm diameter) shells. Cephalopod paleobiogeographic investigation appears less subject to criticism stemming from the supposed obfuscating effects of post-mortem drift than pre- viously thought.

Ammonite shell shape covaries with facies and hydrodynamics: Iterative evolution as a response to changes in basinal environment

Shell shape varies within many ammonoid species, and some ammonoid lineages appear to have evolved in concert with changes in their environment. We report variation within an Upper Cretaceous ammonoid species that correlates with facies differences and is consistent with a hydrodynamic explanation. In the Turner Sandy Member of the Carlile Shale (Turonian) of South Dakota and Wyoming, more compressed morphs of Scaphites whitfieldi Cobban are found in nearshore sandy facies, whereas more depressed morphs occur in offshore muds. We measured drag forces on models of juvenile and adult shells that differed in lateral compression of the shell. Plots of drag coefficient as a function of Reynolds number indicate that thinner, more compressed morphs swam more efficiently at higher velocities and depressed morphs swam more efficiently at low velocities. Higher swimming velocities may be essential for life in nearshore sandy environments, which have higher ambient current velocities. Shelled cephalopods swim most efficiently at low swimming speeds; therefore, lower velocity, more energetically economical swimming should be preferred in more quiescent offshore settings. An analysis of power consumption supports this interpretation. Correlated changes in shell compression and environmental factors, here observed within a species, have been documented in numerous ammonite lineages. These iterative evolutionary changes within lineages may be similarly explained by selection for shell morphologies appropriate to environments that fluctuate cyclically with sea level.

Hydrodynamic properties of cephalopod shell ornament

We investigated the hydrodynamic properties of cephalopod shell sculpture in two ways: 1) flow visualization experiments with sculptured shells; and 2) application of drag coefficient data for simple geometric bodies to cephalopod shells. Results of this work suggest: 1) the hydrodynamic effect of shell sculpture depends primarily on the size of the sculptural elements relative to the size of the shell and on the positions of sculpture elements on the shell and relative to each other. 2) sculpture is detrimental to swimming (reduces hydrodynamic efficiency) if it exceeds the height of the lower part of the shells boundary layer. 3) sculpture is advantageous to swimming (increases efficiency) if it remains immersed in the boundary layer and induces premature conversion to turbulent boundary layer flow. To be hydrodynamically optimal, small shells (diam 10 cm) must have rough (sculptured) surfaces, whereas large shells (diam 100 cm) require smooth surfaces. Thus, in order to maintain maximum efficiency throughout life, the ontogeny of small individuals, or species, should be characterized by progressive roughening of the shell, while large forms should be- come increasingly smooth. Such allometries are observed among many ammonoids. 4) sculpture always has an effect on the flow around a cephalopod shell. In some species this effect was probably negligible, while in others, those with compressed shells especially, it was probably of major importance. In these species, sculpture appears to have functioned primarily to increase swimming ability.

Radiographic observation of chamber formation in Nautilus pompilius

Radiographic observation of four immature Nautilus pompilius shows that new chamber formation commences when approximately half of the cameral liquid of the last formed chamber has been osmotically removed. At this time, the septal mantle is rapidly moved forward in the body chamber, where it reattaches to the internal shell wall and begins calcification of a new septum. Throughout this time, apertural shell growth rates remain constant. The time between successive chamber initiations increased during ontogeny; in the four specimens monitored here, time between two successive chamber formation events ranged between 85 and 132 days and showed no apparent lunar correlation.

Locomotion of Nautilus

As an agent of rapid transportation, jet propulsion has no significant competitor. Although technologically it is but a latecomer, biologically jet propulsion is of great antiquity, probably having originated in the late Precambrian («700 Ma) with the advent of medusoid cnidarians and animals of similar grade.

Marine Cretaceous-Tertiary boundary section in southwestern South Dakota

A distinctive zone of disrupted strata, which we interpret as a distal manifestation of the end-Cretaceous Chicxulub impact event, occurs over 300 km 2 in southwestern South Dakota. This disrupted zone is within the Fox Hills Formation, ranges from 0.5 to 5 m in thickness, and contains large-scale slump-roll structures, clastic dikes, flame structures, and massive, homogenized beds. The zone is ∼0.5 m above a belemnite fauna Sr dated as 67.6 ± 0.5 Ma, contains scaphitid ammonites characteristic of the Jeletzkytes nebrascensis ammonite zone of the Fox Hills Formation, and is capped by a 0.5–4-cm-thick brownish-black mudstone that contains spherules. Pollen of the late Maastrichtian Wodehouseia spinata palynostratigraphic zone occurs immediately above and below the disrupted zone. The disrupted zone is overlain by an additional 25 m of marine Fox Hills Formation. These stratigraphic relationships suggest that the upper part of the Fox Hills Formation in this part of South Dakota is Paleocene; that the Western Interior Seaway was locally present well into the Paleocene; and that scaphitid ammonites may range the Cretaceous-Tertiary (K-T) boundary.

CHONDRICHTHYANS FROM THE LOWER FERRON SANDSTONE MEMBER OF THE MANCOS SHALE (UPPER CRETACEOUS: MIDDLE TURONIAN) OF EMERY AND CARBON COUNTIES, UTAH, USA

Abstract The Lower Ferron Sandstone Member of the Mancos Shale in southeastern Utah preserves a chondrichthyan assemblage of at least 13 taxa that include: Hybodus sp., Ptychodus cf. P. mammillaris Agassiz, 1843, Ptychodus whipplei Marcou, 1858, cf. Chiloscyllium sp., Scapanorhynchus raphiodon (Agassiz, 1843), Cretodus crassidens (Dixon, 1850), cf. Leptostyrax sp., cf. Cretalamna appendiculata (Agassiz, 1835), Squalicorax sp., Pseudohypolophus mcnultyi (Thurmond, 1971), Protoplatyrhina hopii Williamson, Kirkland and Lucas, 1993, Ischyrhiza schneideri (Slaughter and Steiner, 1968), and Ptychotrygon triangularis (Reuss, 1844). Although this assemblage is typical of other Turonian chondrichthyan faunas in North America, fossil teeth are preserved in two unique facies associations that consist of arenitic sandstones with mud interclasts and rounded chert, feldspar, and quartz pebbles. The coarser beds within these facies associations are previously interpreted to represent storm events and turbidity flows associated with a sea level lowstand. Chondrichthyan teeth occurring within these coarser beds are indicative of extensive transport and reworking and attest to the durable nature of chondrichthyan teeth for biostratigraphic and paleoecological interpretations. Similar studies of chondrichthyan teeth in shelf marine settings may also provide new insights for facies interpretations related to sequence stratigraphy and regional stratigraphic correlations.

Is cephalopod septal strength index an index of cephalopod septal strength

Septal strength index has become a prime means of establishing maximum living depths of fossil cephalopods with simple, concave septa. This method calculates palaeobathymetric limits from the septal geometry of Nautilus, based upon the premise that septa are the weakest structural .unit of the shell. We examined this proposal by evaluating data relating to ontogeny, septum formation, septal geometry, and microstructure of Nautilus. Our main results are: 1) septal strength index of embryonic and juvenile septa are not representative of the depths at which such specimens live, 2) septa become fully functional in resisting hydrostatic pressure at thicknesses significantly less than that used to compute strength index, 3) no correlation has been demonstrated between septal strength index and rupture pressure, and 4) shell microstructure does not show sufficient uniformity among cephalopods to permit an undocumented assumption of uniformity in mechanical properties of cephalopod shell material. These observati...

Beryciform-Like Fish Fossils (Teleostei: Acanthomorpha: Euacanthopterygii) from the Late Cretaceous - Early Tertiary of New Jersey

ABSTRACT. Well-preserved fin spines, ornate opercular bones and thick ctenoid scales recovered from the unconsolidated marine sediments of the Cretaceous-Tertiary boundary interval in Upper Freehold Township, New Jersey, USA derive from at least one species of enigmatic euacanthopterygian fish. Within this group the fossils are most similar to corresponding bones of some beryciforms. The fin spines are less than 1.5 cm long, have complex basal articulation structures, and a narrow posterior sulcus extending almost to the distal tip. Most spines have lateral grooves, and some have prominent anterior dentations. A few are attached to broadly keeled pterygiophores. Beryciforms and more basal clades of Acanthomorpha first appear in the Late Cretaceous. The New Jersey fossils may expand the known geographic distribution of beryciforms across the K/T boundary beyond the better known Late Cretaceous beryciforms from the Western Interior of North America, Europe, and the Middle East.