Margaret M. Yacobucci

Ammonoid Intraspecific Variability

Two main types of intraspecific variation can be distinguished in ammonoids, which are not mutually exclusive: continuous and discontinuous variation. Although many authors acknowledge or implicitly assume a large intraspecific variability is possible in shell shape, ornamentation and suture line, it has only been rarely studied quantitatively. Several potential biases need to be taken into account when studying intraspecific variation of fossil populations including paleoecological, taphonomic and collection biases. Intraspecific variation might be controlled both by genetic and environmental parameters, although both are difficult to separate in fossil samples. In ammonoids, a large part of intraspecific variation in morphology and size has been attributed to differences in growth rates and development. Taking intraspecific variation properly into account is not only of prime importance for taxonomy, but also for studies on biostratigraphy, paleobiogeography, ecology, paleobiology and evolution of ammonoids.

Buckman's Paradox: variability and constraints on ammonoid ornament and shell shape

Buckmans Law of Covariation states that ammonoid shell shape and ornamentation are typically correlated, such that compressed, involute forms have light ornament while more inflated, evolute forms have heavier ornament. Such covariation has been observed in many ammonoid groups, and implies a link between the morphogenesis of shell shape and ornamentation. However, other evidence suggests that while ornament growth is controlled by the genetic-developmental program of the ammonoid, shell shape is strongly influenced by environmental factors. These differing viewpoints lead to Buckmans Paradox – are ornamentation and shell shape tightly linked, as implied by Buckmans covariation, or is the morphogenesis of ornament controlled genetically, while shell shape is controlled environmentally? To address this issue, the variability of shell shape and rib morphology has been compared for a group of endemic acanthoceratid ammonites from the Cretaceous Western Interior Seaway of North America. If Buckmans Law holds due to a morphogenetic connection between shell shape and ornamentation, we would expect taxa with more variable shell shapes to also show more variable rib features and growth. Morphometric analysis of seven shell shape and two rib characters for the Western Interior acanthoceratids finds no such correlation, suggesting that shell shape and rib growth are controlled by different processes. Indeed, rib growth appears to be more constrained than shell shape, consistent with the view that ornamentation is more tightly controlled by the developmental-genetic growth program of the ammonoid. These results emphasize the complexity of ammonoid morphogenesis and highlight our limited understanding of the causes underlying Buckmans Law.

Ammonite extinction and nautilid survival at the end of the Cretaceous

One of the puzzles about the end-Cretaceous extinctions is why some organisms disappeared and others survived. A notable example is the differential extinction of ammonites and survival of nautilids, the two groups of co-occurring, externally shelled cephalopods at the end of the Cretaceous. To investigate the role of geographic distribution in explaining this outcome, we compiled a database of all the occurrences of ammonites and the nautilid genus Eutrephoceras in the last 0.5 m.y. of the Maastrichtian. We also included recently published data on ammonite genera that appear to have briefly survived into the Paleocene. Using two metrics to evaluate the geographic range of each genus (first, a convex hull encompassing all of the occurrences of each genus, and second, the maximum distance between occurrences for each genus), we documented that most ammonite genera at the end of the Maastrichtian were restricted in their geographic distribution, possibly making them more vulnerable to extinction. The geographic distribution of those genera that may have briefly survived into the Paleocene is significantly greater than that of non-surviving genera, implying that more broadly distributed genera were more resistant to extinction. This pattern is further emphasized by the broad distribution of Eutrephoceras , which matches that of the most widely distributed ammonites at the end of the Maastrichtian. However, even the most widely distributed ammonites eventually succumbed to extinction, whereas Eutrephoceras survived. Evidently, a broad geographic distribution may have initially protected some ammonites against extinction, but it did not guarantee their survival.

Ecological interactions between Rhipidomella (Orthides, Brachiopoda) and its endoskeletobionts and predators from the Middle Devonian Dundee Formation of Ohio, United States

Abstract Only the small (2.0 ± 0.1 cm) orthide brachiopod Rhipidomella in the Middle Devonian Dundee Formation exposed at Whitehouse Quarry, Ohio, preserves evidence of interactions with endoskeletobionts and predators (39.6%, n  =  48), as opposed to the slightly larger atrypides, spiriferides, and stropheodonts (n  =  245). All traces of predation and boring by other organisms are lacking on such larger brachiopods as strophomenides and spiriferides, which are more often encrusted in Devonian localities of North America—the Silica Shale of Ohio, Hamilton Group of New York, and Cedar Valley Limestone of Iowa. Punctate shells of Rhipidomella preserve interactions with endoskeletobionts and predators, phenomena less common for punctate brachiopods. All traces on Rhipidomella were preserved as endoskeletobionts; if calcified encrusters were present, they likely were lost postmortem. Several Rhipidomella individuals bear partially repaired grooves from parasitic interactions with sinuous, boring organisms, attributed to ctenostome bryozoans. These parasites bored into the shell along the commissure, likely benefiting from the inhalant and exhalant currents produced by the brachiopod, and in some cases, expanded away from the commissure following the hosts death. The straight, U-shaped borings in one Rhipidomella specimen with boreholes are similar in morphology to Caulostrepsis traces that occurred with their tubes opening along the margins of a modern brachiopod. Other endoskeletobiont traces likely formed on the postmortem shells of Rhipidomella. Predation repair scars are present on two specimens, indicating the presence of predators and the survival of some Rhipidomella individuals from durophagy. No specimens contain evidence of drilling predators.

Increasing the number of discrete character states for continuous characters generates well-resolved trees that do not reflect phylogeny

Since the introduction of the cladistic method in systematics, continuous characters have been integrated into analyses but no methods for their treatment have received unanimous support. Some methods require a large number of character states to discretise continuous characters in order to keep the maximum level of information about taxa differences within the coding scheme. Our objective was to assess the impact of increasing the character state number on the outcomes of phylogenetic analyses. Analysis of a variety of simulated datasets shows that these methods for coding continuous characters can lead to the generation of well-resolved trees that do not reflect a phylogenetic signal. We call this phenomenon the flattening of the tree-length distribution; it is influenced by both the relative quantity of continuous characters in relation to discrete characters, and the number of characters in relation to the number of taxa. Bootstrap tests provide a method to avoid this potential bias.

Plasticity of Developmental Timing as the Underlying Cause of High Speciation Rates in Ammonoids

Rapid diversification of clades within a restricted geographic area is fairly common, and has been well studied as a model of adaptive radiation. The monophyletic clades produced by this type of event have been called “species flocks.” An investigation of the Cenomanian radiation of acanthoceratid ammonites in the North American Western Interior Seaway reveals that this clade shows all the characteristics of a species flock: the acanthoceratid genera are speciose and diversified rapidly, and many of the species are geographically restricted within the Western Interior, possibly due to environmental barriers. A cladistic analysis shows many true polytomies, a high autapomorphy to synapomor- phy ratio, and much parallelism, all reflecting rapid diversification from a single ancestral lineage. These ammonites also show pronounced plasticity of developmental timing. Most differences in morphology among taxa are due to small changes in developmental timing, and an unusually high number of progenic dwarf spinoff taxa occur in this clade, indicating that larger-scale changes in ontogenetic timing are also common. It seems, then, that the developmental program of this ammonite clade was exceedingly labile during its radiation. This plasticity may have allowed the clade to radiate so rapidly and profusely. As developmental plasticity may be a characteristic of ammonoids as a whole, this internal factor, rather than sensitivity to external environmental conditions, may be the root cause of high speciation rates in ammonoids.

Multifractal and white noise evolutionary dynamics in Jurassic–Cretaceous Ammonoidea

For more than a decade there has been interdisciplinary debate on whether fossil origination and extinction time series show evidence of self-organized criticality (SOC) or some other, more complex hierarchical structure. Analyses of a new data set of Jurassic and Cretaceous ammonoid genera demonstrate that these taxonomic subgroups do not reproduce the patterns seen in larger data sets that pool disparate taxa. Rather, originations and extinctions in these ammonoids show no evidence of SOC, limited evidence for a hierarchical multifractal pattern, and, at least within the suborder Ammonitina, evidence consistent with a white noise signal. Although a white noise pattern has been identified previously in an ammonoid family diversity time series, the significance of such a result for ammonoid genus originations and extinctions is discussed here for the first time. This white noise pattern indicates that processes acting on short time scales (less than a few million years) dominate the Ammonitina origination and extinction records, producing a nonhierarchical diversity dynamic. Because origination events in one interval do not appear to trigger originations in subsequent intervals, ammonite evolutionary radiations were likely to be extremely rapid and driven by physical rather than biological opportunities for diversification.

GIS-based morphometrics

Ammonoids are known for their intraspecific and interspecific morphological variation through ontogeny, particularly in shell shape and ornamentation. Many features covary and individual elements are difficult to homologize, which make qualitative descriptions and widely-used morphometric tools inappropriate for quantifying these complex morphologies. However, spatial analyses conducted in a geographic information systems (GIS) environment allow for quantification and visualization of global shell form. Here, we present a GIS-based methodology in which the variation of shell features is assessed to evaluate evolutionary patterns in an ammonoid clade from the Late Cretaceous Western Interior Seaway of North America. We investigate the ancestral and more variable scaphitid heteromorph ammonoid Hoploscaphites spedeni and its less variable descendant H. nebrascensis. We created three-dimensional (3D) digital models of the fossil shells’ lateral surfaces using photogrammetric software and imported the reconstructions into a GIS environment. We used the number of discrete aspect patches and the surface-to-planimetric (3D to 2D) area ratios of the lateral surface as terrain roughness indices. These 3D spatial analyses demonstrate the overlapping morphological ranges of H. spedeni and H. nebrascensis in ornamentation despite differences in shell compression. Additionally, the target for evolutionary change in this clade resides in the macroconch body chamber, which becomes less variable through development from H. spedeni to subsequent H. nebrascensis in macroconchs; however, microconchs retain the ancestral variation through ontogeny. These geospatial analyses not only successfully quantified variation in complex morphologies, but also demonstrated the versatility of this method to address questions related to ontogeny and phylogeny. Mathew J. Knauss. Department of Earth Sciences, University of California, Riverside, 1242 Geology Building, Riverside, California, 92521, USA. mknau001@ucr.edu Margaret M. Yacobucci. Department of Geology, Bowling Green State University, 190 Overman Hall, Bowling Green, Ohio, 43403-0218, USA. mmyacob@bgsu.edu

Buckman’s Rules of Covariation

Most planispirally and regularly coiled ammonoid species show to varying degrees continuous morphological intraspecific variation of their shell. In many species, individuals range from slender, involute, compressed, and weakly ornamented forms to “robust” forms, which are more evolute, more depressed, and with coarser ornamentation. Additionally, more compressed shells tend to have more complex sutures. Within a species from a single sample, the frequency of these variants is represented by a continuous unimodal distribution (often normal). These covariation patterns of intraspecific variation have been abundantly documented and were coined “Buckman’s Laws of Covariation”. These rules help to delineate ammonoid morphospecies. This interdependent morphological variation suggests that shell morphogenesis may not be random but constrained either by adaptive constraints or simple constructional scaling rules. The cause(s) remain debated. Hence, although Buckman’s rules of covariation are now widely documented and acknowledged, several aspects of their scope and limits still remain to be investigated.

Macroevolution and Paleobiogeography of Jurassic-Cretaceous Ammonoids

Jurassic-Cretaceous (J-K) ammonoids experienced remarkably rapid rates of evolution and extinction. The processes that fueled this evolutionary volatility are not well understood. Evolutionary relationships among and within the six J-K ammonoid suborders are incompletely reconstructed, in part because the homeomorphy and intraspecific variability complicates phylogenetic analysis. J-K ammonoids appear to have been developmentally flexible; heterochronic shifts in their evolution are common and taxa are often distinguishable by variations in the timing of developmental events. Changes in environmental variables are consistently correlated with both diversification and extinction in J-K ammonoids. The tectonic separation of Pangea, greenhouse warming, and sea level change drove ammonoid dispersal and the development of biogeographic provinces. A synthetic view of J-K ammonoid evolution provides the foundation for a model of ammonoid speciation in which sea level change provides new epeiric sea microhabitats into which ammonoids disperse while developmental flexibility provides the morphological and ecological variation to fuel divergence and speciation. New quantitative and geospatial approaches will allow us to integrate phylogenetic and paleobiogeographic data to better understand the macroevolution of these cephalopods.