Peter D. Ward

Stable isotopic studies of Nautilus macromphalus sowerby (new Caledonia) and Nautilus pompilius L. (Fiji)

Abstract Two specimens of Nautilus macromphalus Sowerby from New Caledonia and two specimens of Nautilus pompilius L. from Fiji have been selected for stable isotope analysis of all septa. These animals were captured at depths of 80 m and 200 m, respectively. The early-formed septa were also analyzed from three additional specimens of N. macromphalus captured in New Caledonia at 300–400 m. Oxygen isotope compositions of septa from both populations are characterized by (1) an abrupt increase in δsu18O of 1–3.5‰ from septa 6 to 8, and (2) septum-to-septum variations in δ13 and δ18O which are roughly sympathetic and larger for carbon than oxygen. Septa from the Fiji specimens are slightly depleted in 18O relative to those from New Caledonia. The isotopic “step” between septa 7 and 8 is interpreted to record the hatching of the embryo after development of the seventh septum. Marked 18O-depletion of septa 1–7 evidently reflect non-equilibrium, metabolic isotope fractionation during embryonic groth stages or, perhaps more likely, a difference in the isotopic compositions of nepionic fluid and sea water. The oxygen isotope compositions of septa 10 and higher and shell walls appear to closely approach isotopic equilibrium with sea water, as demonstrated by a comparison of calculated isotopic and known temperatures for aragonite precipitation by an aquarium-grown specimen. A slightly warmer average environment of growth is indicated for the Fiji samples.

Late Cretaceous (Santonian-Campanian) stratigraphy of the northern Sacramento Valley, California

The Upper Cretaceous (Coniacian–lower Campanian) Chico Formation of the northeastern Sacramento Valley, California, includes three newly defined members at the type locality: (1) cobble conglomerate of the basal Ponderosa Way Member, (2) coarse-grained conglomeratic sandstone of the overlying Musty Buck Member, and (3) fine-grained silty sandstone of the uppermost Ten Mile Member. Other outcrops of the Chico Formation exhibit the same three members plus an additional unit, the Kingsley Cave Member, composed of mudstone. The Chico Formation resulted from a transgression onto the Sierra Nevada basement in the Late Cretaceous. The Kingsley Cave Member was deposited locally in quiet-water conditions below wave base in an intrashelf basin and may represent a marine connection with regions farther northeast. Macrofossils allow correlation of the Chico Formation with marine deposits of the Great Valley sequence exposed along the western Sacramento Valley. Strata of the Guinda and Forbes Formations represent submarine fan-channel to outer-fan deposits of a shallowing Late Cretaceous fore-arc basin. The Santonian Guinda Formation, a massive sand unit deposited in fan-channel to distal-fan turbidite environments, is correlative with the Musty Buck and Kingsley Cave Members of the Chico Formation. The Dobbins Shale Member of the Forbes Formation, a widespread mudstone unit of hemipelagic outer-fan to basin-plain deposits, is correlative with the Kingsley Cave Member of the Chico Formation and reflects a Santonian transgressive event in the northern fore-arc basin. Mudstones and turbidites of the middle of the Forbes Formation are equivalent to the shallow-marine strata of the Ten Mile Member of the Chico Formation. Correlatives of younger Forbes Formation strata have not been positively identified from the Chico Creek region but may be represented by unfossiliferous, cross-bedded, coarse-grained sandstones found above sections of the Chico Formation.

The structure of the chambered nautilus siphuncle: The siphuncular epithelium

The siphuncle of the chambered nautilus (Nautilus macromphalus) is composed of a layer of columnar epithelial cells resting on a vascularized connective tissue base. The siphuncular epithelium taken from chambers that have not yet begun to be emptied of cameral liquid has a dense apical brush border. The great number of apical cell junctions (zonula adherens) compared to the number of nuclei suggests extensive interdigitation of these cells. The perinuclear cytoplasm of these preemptying cells is rich in rough endoplasmic reticulum. The siphuncular epithelium of both emptying and “old” siphuncle (which has already completed emptying its chamber) both show little rough endoplasmic reticulum but do contain extensive systems of mitochondria‐lined infoldings of the basolateral plasma membranes. Active transport of NaCl into the extracellular space of this tubular system probably entrains the water transport involved in the chamber‐emptying process. Both emptying and old siphuncular epithelium also show large basal infoldings (canaliculi) continuous with the hemocoel, which appear to be filled with hemocyanin. The apical cell junctions of emptying and old siphuncular epithelium contain septate desmosomes that may help to prevent back‐flow of cameral liquid into the chambers.

Magnetostratigraphy of Upper Cretaceous strata of the Sacramento Valley, California

A magnetic polarity zonation has been developed for nine stratigraphic sections of the Upper Cretaceous portion of the Great Valley Group in the Sacramento Valley of California. Collectively, the sections include five biostratigraphic range zones, based on ammonites and other mollusks. The zones are all of Santonian or Campanian age, and each section contains at least two zones. A total of 681 fully oriented samples was collected from 252 horizons, spanning more than 3 km of section. The samples were subjected to alternating-field demagnetization after it was found that the rocks were not amenable to thermal demagnetization. Rock magnetic studies were used to determine that the principal magnetic carrier is magnetite. The paleomagnetic measurements reveal the existence of an interval of reversed polarity that is consistently associated with one of the zones, namely, the Baculites chicoensis Zone. Given the biostratigraphic control, the reversed interval can be unambiguously correlated with the first reversed interval following the Cretaceous long normal interval. On the Magnetic Polarity Time Scale, this reversed interval corresponds to marine magnetic anomaly 33R. The correlation of the re-versed interval implies that the base of the Chicoensis Zone is nearly coincident with the Santonian/Campanian boundary in California and that it is not significantly time-transgressive between the shallow-water facies on the east side of the Sacramento Valley and the deep-water facies on the west side. Furthermore, the magnetostratigraphy supports the conclusion that all of the zones underlying the Chicoensis Zone are pre-Campanian in age, a result which is consistent with other lines of evidence. The biostratigraphy of these sections in California can now be compared directly to that of the paleomagnetic stratotype section in Gubbio, Italy.

Chamber refilling in Nautilus

Freshly captured Nautilus macromphalus were observed to place new cameral liquid into emptied or partially emptied chambers, both at the surface and at 250 m, in response to sudden buoyancy increase. The sudden addition of buoyancy was accomplished either through removal of cameral liquid from chambers, removal of shell material from the apertural region of the body chamber, or by cementing buoyant corks on the sides of the shell. A maximal refilling rate of 100 μ l/h was observed. The osmolality of refilled cameral liquid was observed to match closely the osmolality of the original cameral liquid. The refilling mechanism allows Nautilus to regain neutral buoyancy after sudden buoyancy gain, such as could be expected to occur from shell loss due to predatory attack on the Nautilus , or from shell breakage.

Functional morphology of Cretaceous helically-coiled ammonite shells

Torticonic (helically-coiled) ammonoids have been most commonly interpreted as vagile, benthonic forms. Their mode of coiling, however, places the siphuncle in a functionally dorsolateral, rather than ventral position on the shell whorl due to: 1) their probable apex-upward shell orientation during life, and 2) size asymmetry of the lateral sutural saddles due to helical coiling, associated with upward siphuncle displacement. The resultant positioning of the siphuncle produces cameral liquid de-coupling soon after initiation of cameral liquid removal (emptying of liquid from a newly formed chamber within the phragmocone). Since in Recent chambered cephalopods cameral liquid de-coupling increases efficiency of vertical migration, a similar mode of life was indicated for the torticonic ammonites.

Stable isotopic investigations of early development in extant and fossil chambered cephalopods I. Oxygen isotopic composition of eggwater and carbon isotopic composition of siphuncle organic matter in Nautilus

Eggwaters from the chambered cephalopod Nautilus are depleted in both 18O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive δ18O values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the δ18O values of shell carbonates for some but not all extinct and extant chambered cephalopods. The δ13C values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in δ13C values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested.

Marine magnetic anomaly 33–34 identified in the Upper Cretaceous of the Great Valley Sequence of California

An interval of reversed magnetic polarity has been located in Upper Cretaceous rocks of the Great Valley Sequence of California. The interval has been found at four sites, two on each side of the Sacramento Valley. In each case, the reversed interval is associated with the same ammonite local range zone. Firm biostratigraphic correlation and the structure of the magnetic polarity time scale allow us to determine that the reversed interval corresponds to marine magnetic anomaly 33–34 and represents the lower part of the Campanian Stage. The reversed interval serves as a marker horizon linking the different faunal assemblages and sedimentary facies of the Great Valley Sequence. It also provides the first direct correlation of provincial biostratigraphic zones of the North Pacific province with the Cretaceous stages in Europe.

Sr, Mg, and Ca chemistry of the skeleton of Nautilus

Concentrations of Sr, Mg, and Ca were quantified for the shell and septa formed during the final 3 to 4 yr of growth for four species of the extant cephalopod genus Nautilus [ N. belauensis Saunders 1981, N. macromphalus Sowerby 1849, N. scrobiculatus (Lightfoot 1786), and N. pompilius Linnaeus 1758]. These data are used to describe several aspects of the physiochemistry of Nautilus in relation to biomineralization of shell and septal aragonite. Species of Nautilus discriminate against the concentrations of Sr and Mg relative to Ca in seawater by 79% and 99.9%, respectively. The power of this discrimination against Sr and Mg varies for shell and septal aragonite within and among species. Although the among-species variation in the Sr and Mg content of shell and septal aragonite is greater than the within-species variation, differences in these concentrations are not unique among all species. However, when the covariance of Sr and Mg is computed as a transformed variable (discriminant function), species can be distinguished with significance ( P <0.001) by the combined Sr and Mg chemistry of septal aragonite. Similarly, all species with the exception of N. macromphalus and N. pompilius can be distinguished by the combined Sr and Mg chemistry of shell aragonite.