Neil H. Landman

Mode and Rate of Growth in Ammonoids

In this chapter we discuss the mode and rate of growth in ammonoids, focusing primarily on postembryonic growth. We first discuss the general mode of growth and then describe the ontogenetic sequence of growth stages. These stages are recognized on the basis of changes in morphology. For example, a graph of the increase in size of whorl width versus shell diameter in an individual reveals changes through ontogeny that pinpoint the end of one growth stage and the beginning of another. We next discuss the overall rate of growth through ontogeny and establish a generalized growth curve. In this discussion, we refer to other cephalopods whose rate of growth is known. Fluctuations in the rate of growth that are superimposed on this growth curve are indicated in ammonoids by the presence of such shell features as varices and constrictions.

The role of ammonites in the Mesozoic marine food web revealed by jaw preservation.

Analysis of x-ray microtomographic reconstructions of ammonite fossils reveal their feeding habits. Ammonites are prominent in macroevolutionary studies because of their abundance and diversity in the fossil record, but their paleobiology and position in the marine food web are not well understood due to the lack of preserved soft tissue. We present three-dimensional reconstructions of the buccal apparatus in the Mesozoic ammonite Baculites with the use of synchrotron x-ray microtomography. Buccal mass morphology, combined with the coexistence of food remains found in the buccal mass, suggests that these ammonites fed on plankton. This diet may have extended to all aptychophoran ammonites, which share the same buccal mass morphology. Understanding the role of these ammonites in the Mesozoic food web provides insights into their radiation in the Early Jurassic, as well as their extinction at the end of the Cretaceous/early Paleogene.

Mature Modifications and Dimorphism in Ammonoid Cephalopods

The shell of an ammonoid is a kind of autobiography of the animal that once occupied it. Different parts of the shell tell different parts of the life history. The growth lines and the tiny intervals in between, along with the shape of the shell itself, record what was happening at the anterior end of the body. The septa and their sutures relate the tale of the other extremity.

Growth rate and habitat of Nautilus pompilius inferred from radioactive and stable isotope studies

The growth rate of Nautilus pompilius in its natural environment has been determined from radioactive disequilibrium between 210 Pb (half-life 22.3 yr) and its granddaughter 210 Po (half-life 138 d) in septa of two juvenile specimens. 210 Pb and 210 Po data from the most recently formed shell material of both specimens indicate that 210 Pb from sea water is incorporated into septa during septal formation and 210 Po is excluded. Therefore the 210 Po/ 210 Pb activity ratio serves as a chronometer to estimate the age of each septum and the time between formation of septa. In the specimens studied the average time between sucessive points in septal deposition is 75 d for the nine most recent septa of one specimen and 23 d for the six most recent septa of the other specimen. These different growth rates, if representative of the ontogeny of each animal, suggest that the timing of septal deposition probably is dependent on the rate of shell and tissue growth coupled with buoyancy requirements and is not a unique period for all Nautilus. The habitat and ontogeny of Nautilus may be inferred from the pattern of stable isotopes of oxygen and carbon in the septa. Both specimens show a pronounced break in δ 18 O from nearly uniform light values in the first seven septa to heavier values (∼1%) after the seventh septum. We interpret this break to correspond to the hatching of Nautilus. A temperature (i.e. water depth) interpretation of the δ 18 O data for septa after the eighth is complicated by a positive correlation between δ 18 O and δ 13 C. This may reflect horizontal migration of the animal or a kinetically controlled fractionation of carbon and oxygen isotopes during septal formation.

Ammonoid Embryonic Development

A great number of new studies have been carried out on ammonoid embryonic development in the last two decades. We focus here on novel developments and interpretations in the description of the embryonic shell (including terminology, shape, size, ornamentation, microstructure, septa, siphuncle and muscle scars), the sequence of embryonic development, reproductive strategy and post-hatching mode of life, followed by conclusions and possible future areas of research.

Paleoceanography of the Late Cretaceous (Maastrichtian) Western Interior Seaway of North America: evidence from Sr and O isotopes

Abstract Well-preserved fossils of the Late Cretaceous Western Interior Seaway (WIS) of North America have been analyzed for Sr concentration and Sr and O isotopes in order to decipher paleosalinities and paleotemperatures. The samples are from four biofacies within the Seaway (late Maastrichtian): offshore Interior (Pierre Shale), nearshore Interior (Fox Hills Formation), brackish (reduced salinity; Fox Hills Formation) and freshwater (Hell Creek Formation). Samples were also obtained from the Severn Formation of Maryland (considered to be representative of the open ocean). All biofacies (except the freshwater) are demonstrably within the Jeletzkytes nebrascensis ammonite zone ( 87 Sr/ 86 Sr ratios show significant and systematic decreases from marine (mean±1 S.D.=0.707839±0.000024) to brackish facies (mean±1 S.D.=0.707677±0.000036), consistent with dilution by freshwater with a lower 87 Sr/ 86 Sr ratio than seawater. Such variation disallows using the 87 Sr/ 86 Sr ratios of fossil shell material to assign ages to fossils from the Late Cretaceous WIS without knowledge of the salinity in which the organism grew. The Sr isotope ratios for scaphitid ammonites within a single biofacies are similar to each other and different from those for scaphites in other biofacies, implying that these organisms are restricted in their distribution during life. The 87 Sr/ 86 Sr values of freshwater unionid mussels range widely and are not compatible with the freshwater endmember 87 Sr/ 86 Sr ratio required by the trend in 87 Sr/ 86 Sr vs. biofacies established from the other samples. Paleosalinities for the biofacies are estimated to range from 35‰ in the open marine to a minimum of 20‰ in the brackish, based on the presence of cephalopods in all four facies and the known salinity tolerance of modern cephalopods. Producing reasonable 87 Sr/ 86 Sr values for the freshwater endmember of a 87 Sr/ 86 Sr vs. 1/[Sr] plot requires a Sr concentration 0.2–0.5 that of seawater for the dominant freshwater input to the WIS. Such high Sr concentrations (relative to seawater) are not observed in modern rivers, and we suggest that the brackish environment in the WIS arose through the mixing of freshwater and seawater in a nearshore aquifer system. Reactions of the solution with aquifer solids in this ‘subterranean estuary’ [Moore, Mar. Chem. 65 (1999) 111–125] produced brackish water with the Sr concentration and isotopic composition recorded in the brackish biofacies. δ 18 O values of the fossils show decreases from the marine to brackish biofacies consistent with increasing temperatures (from ∼13 to 23°C) or, if temperatures were relatively constant, to a decrease in the δ 18 O of the water in which the shell formed. The latter interpretation is consistent with less-than-fully marine salinities in the nearshore biofacies, but both changes in temperature and the isotopic composition of the water may have occurred in this environment.

Early life history of Nautilus: evidence from isotopic analyses of aquarium-reared specimens

Specimens of Nautilus species caught in the wild show a marked increase in oxygen isotopic composition between embryonic and postembryonic septa. The significance of this increase in terms of the early life history of Nautilus has been unclear. To help explain this pattern, we analyzed the isotopic composition of the septa of three specimens of Nautilus belauensis raised in aquariums under controlled temperature conditions. Our results indicate that both embryonic and postembryonic septa are secreted with the same temperature-dependent fractionation of aragonite relative to water as that of other aragonite-secreting molluscs (Grossman and Ku 1986). The δ 18 O values of the septa thus provide a reliable means of determining the water temperature in which the septa form. Calculated temperatures based on oxygen isotopic data from specimens caught in the wild reveal that embryonic development occurs at 22°-24° corresponding to a depth of 100-200 m depending on the location. The increase in δ 18 O in postembryonic septa reflects a migration into colder, deeper water after hatching. In Cretaceous nautilids, a systematic shift in δ 18 O is not present, indicating that these animals probably did not change their habitat after hatching. This is consistent with the likelihood that they lived in shallower environments than that of modern Nautilus.

CEPHALOPODS FROM THE CRETACEOUS/TERTIARY BOUNDARY INTERVAL ON THE ATLANTIC COASTAL PLAIN, WITH A DESCRIPTION OF THE HIGHEST AMMONITE ZONES IN NORTH AMERICA. PART 2. NORTHEASTERN MONMOUTH COUNTY, NEW JERSEY

Abstract Geological investigations in the upper Manasquan River Basin, central Monmouth County, New Jersey, reveal a Cretaceous/Tertiary ( =  Cretaceous/Paleogene) succession consisting of approximately 2 m of the Tinton Formation overlain by 2 m of the Hornerstown Formation. The top of the Tinton Formation consists of a very fossiliferous unit, approximately 20 cm thick, which we refer to as the Pinna Layer. It is laterally extensive and consists mostly of glauconitic minerals and some angular quartz grains. The Pinna Layer is truncated at the top and is overlain by the Hornerstown Formation, which consists of nearly equal amounts of glauconitic minerals and siderite. The base of the Hornerstown Formation is marked by a concentration of siderite nodules containing reworked fossils. This layer also contains a few fossils of organisms that were living in the environment during the time of reworking. At some downdip sites, there is an additional layer (the Burrowed Unit), which is sandwiched between the top of the Pinna Layer and the concentrated bed of nodules. This unit is very thin and is characterized by large burrows piping down material from above. The Pinna Layer is abundantly fossiliferous and represents a diverse, nearshore marine community. It contains approximately 110 species of bivalves, gastropods, cephalopods, echinoids, sponges, annelids, bryozoans, crustaceans, and dinoflagellates. The cephalopods include Eutrephoceras dekayi (Morton, 1834), Pachydiscus (Neodesmoceras) mokotibensis Collignon, 1952, Sphenodiscus lobatus (Tuomey, 1856), Eubaculites carinatus (Morton, 1834), Eubaculites latecarinatus (Brunnschweiler, 1966), Discoscaphites iris (Conrad, 1858), Discoscaphites sphaeroidalis Kennedy and Cobban, 2000, Discoscaphites minardi Landman et al., 2004b, Discoscaphites gulosus (Morton, 1834), and Discoscaphites jerseyensis, n.sp. The dinoflagellates include Palynodinium grallator Gocht, 1970, Thalassiphora pelagica (Eisenack, 1954) Eisenack & Gocht, 1960, Deflandrea galeata (Lejeune-Carpentier, 1942) Lentin & Williams, 1973, and Disphaerogena carposphaeropsis Wetzel, 1933. These ammonites and dinoflagellates are indicative of the uppermost Maastrichtian, corresponding to the upper part of calcareous nannofossil Subzone CC26b. The mode of occurrence of the fossils in the Pinna Layer suggests an autochthonous accumulation with little or no postmortem transport. Many of the benthic organisms are preserved in life position. For example, specimens of Pinna laqueata Conrad, 1858, are oriented in a vertical position, similar to that of modern members of this genus. The echinoids also occur in aggregations of hundreds of individuals, suggesting gregarious feeding behavior. In addition, there are monospecific clusters of baculites and scaphites. These clusters are biological in origin and could not have been produced by hydraulic means. Scaphite jaws are also present, representing the first reports of these structures in the Upper Cretaceous of the Atlantic Coastal Plain. They occur both as isolated specimens and inside the body chamber, and indicate little or no postmortem transport. The Pinna Layer represents a geologically short interval of time. The fact that most of the animals are mature suggests that the community persisted for at least 5–10 years. If multiple generations of animals are present, perhaps reflecting multiple episodes of colonization and burial, then this unit probably represents more time, amounting to several tens of years. The fact that the Pinna Layer is truncated at the top implies a still longer period of time, amounting to hundreds of years. These age estimates are consistent with observed rates of sedimentation in nearshore environments. Iridium analyses of 37 samples of sediment from three sites in the Manasquan River Basin reveal an elevated concentration of iridium of 520 pg/g, on average, at the base of the Pinna Layer. The iridium profile is aymmetric with an abrupt drop off above the base of this unit and a gradual decline below the base. The elevated concentration of iridium is not as high as that recorded from some other Cretaceous/Tertiary boundary sections. However, it is sufficiently above background level to suggest that it is related to the global Ir anomaly documented at many other localities, and attributed to a bolide impact. The position of the iridium anomaly at the base of the Pinna Layer is inconsistent with the biostratigraphic data, because this anomaly occurs below the unit containing fossils indicative of the uppermost Maastrichtian. We present two alternative hypotheses: (1) If the enriched concentration of iridium is in place, it marks the Cretaceous/Tertiary boundary by reference to the global stratotype section and point at El Kef, Tunisia. The position of the iridium anomaly further implies that the Pinna community was living at the moment of impact and may even have flourished in its immediate wake. Subsequently, the community may have been buried by pulses of mud-rich sediment, possibly associated with enhanced riverine discharge following the impact. The Burrowed Unit may represent a subsequent pulse of riverine discharge that scoured the top of the Pinna Layer. (2) The iridium anomaly was originally located at the top of the Pinna Layer and was displaced downward due to bioturbation and/or chemical diffusion. This hypothesis implies that the Pinna Layer was deposited prior to the deposition of the iridium. The Pinna community may have died before or at the moment of impact. Erosion of the top of the Pinna Layer and deposition of the Burrowed Unit may have been associated with events immediately following the impact. In both hypotheses, the sea floor experienced an extended period of erosion and reworking in the early Danian, which may have lasted for several hundred thousand years, producing a concentrated lag of siderite nodules containing reworked fossils in the basal part of the Hornerstown Formation. This lag deposit is equivalent to the Main Fossiliferous Layer at the base of the Hornerstown Formation elsewhere in New Jersey. This period of erosion and reworking was probably associated with a transgression in the early Danian. The post-impact community was greatly reduced in diversity, with most of the species representing Cretaceous survivors.

Effect of diagenesis on the Sr, O, and C isotope composition of late Cretaceous mollusks from the Western Interior Seaway of North America

Evaluating the effects of diagenesis on the isotopic compositions of Sr, O, and C in marine carbonates is critical to their use as proxies in reconstructing information on the salinity, temperature and dissolved inorganic carbon of ancient oceans. We have analyzed a series of samples of mollusk shells from the Baculites compressus zone (late Campanian) of the Pierre Shale of South Dakota. Samples included outer shell material and septa of cephalopods collected inside and outside concretions. Preservation was evaluated using light microscopy, scanning electron microscopy (SEM), trace element analysis and X-ray diffraction. All of the material consists of aragonite based on X-ray diffraction. An SEM preservation index (PI) was established based on comparison of the microstructure of the fossil material with that of modern Nautilus. Excellent preservation (PI = 5) was characterized by well-defined nacreous plates with discrete, angular boundaries. In contrast, samples showing fused nacreous plates with indistinct boundaries were rated poor (PI = 1). 87Sr/86Sr ratios vary with preservation and average 0.707648 ± .000021 (n = 10) for excellent preservation (PI ≈ 5), 0.707615 ± .000028 (n = 5) for good preservation (PI ≈ 3), 0.707404 ± .000074 (n=7) for fair preservation (PI ≈ 2), and 0.707261 ± .000053 (n=8) for poor preservation (PI ≈ 1). These data suggest that as the quality of the preservation declines, the mean 87Sr/86Sr ratio decreases and the standard error of the mean increases. Oxygen and carbon isotope analyses of the same specimens also show decreases with preservation, and δ18O, δ13C and 87Sr/86Sr are well correlated, suggesting that these tracers are all altered as the PI decreases. The Sr/Ca ratio increases as preservation decreases, indicating that Sr is added to the shell material during diagenesis. In contrast, Mg/Ca shows no trend with preservation. If the increasing Sr concentration (and decreasing 87Sr/86Sr) of the shell material with decreasing preservation represents the addition of Sr to the shell during diagenesis, we calculate that the added Sr had 87Sr/86Sr ranging from 0.707582 to 0.707032. Potential sources of the added Sr include older marine carbonates and weathering of volcanic ash layers present in the shale. The mechanisms of alteration likely include epitaxial growth of strontianite on the original shell aragonite and isotopic exchange of C and O between alteration fluids and shell carbonate. We conclude that SEM preservation criteria are effective in screening shell material that records original isotopic values and that variations in Sr, O and C isotope composition in well-preserved material can be used to assess paleoenvironmental parameters, such as salinity and temperature. Our results also indicate that assessing preservation is a critical prerequisite to the determination of numerical ages of shell material using strontium isotope stratigraphy.

Early ontogeny of Eutrephoceras compared to Recent Nautilus and Mesozoic ammonites: evidence from shell morphology and light stable isotopes

Observations on the morphology of the early whorls of Eutrephoceras dekayi (Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ 18 O and δ 13 C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modern Nautilus , morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa in E. dekayi , δ 18 O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule. Eutrephoceras dekayi hatched at about 9 mm in diameter, one-third the hatching size of modern Nautilus. Like Nautilus, E. dekayi probably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.