Bernard Laurin

Evolution of ammonoid morphospace during the Early Jurassic radiation

The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.

Phyletic gradualism in a European Plio-Pleistocene Mimomys lineage (Arvicolidae, Rodentia)

The European Plio-Pleistocene Mimomys ( Hintonia ) occitanus-ostramosensis lineage of arvicolid rodents appears to present a genuine case of gradual evolution. Analysis of numerous dental samples recorded from 3 to 1.5 ma by several biometrical methods led to the following conclusions: samples from the whole geographical area order into a morphological continuum (chronomorphocline); the successive populations in the trend show important overlap of their variation; and the absence of stasis or any interruption in the trend, combined with its unidirectionality, suggests genuine phyletic gradualism. The chronomorphocline is characterized by an important increase in tooth hypsodonty, by the progressive and continuous development of lateral enamel tracks. Four phases of evolution can be recognized in the lineage, the rates of morphological changes being variable and related to climatic fluctuations (Reuverian, Pretiglian, Tiglian, Eburonian). Morphological changes are initiated by environmental fluctuations, although the polarity of climatic change is not a determining factor. Environmental changes act as stimulus and release drift processes. Phyletic gradualism occurring over such a wide area implies extensive gene flow, and suggests that only general, not local, environmental components act to produce the observed morphological changes.

MORPHOMETRICS AND CLADISTICS: MEASURING PHYLOGENY IN THE SEA URCHIN ECHINOCARDIUM

A phylogenetic approach to the study of evolutionary patterns is based on taxic homologies (synapomorphies). In contrast, the recognition of evolutionary processes (namely heterochronies) involves analysis of the entire morphology. Recent developments in geometric morphometry permit analysis of morphological similarities grounded in operational homologies. Such morphometric techniques are explored (1) at the level of evolutionary processes, and (2) as a complement in exploration of phylogenetic relationships. To examplify this, we perform a two‐part study of the ontogeny and phylogeny of the spatangoid sea urchin Echinocardium. First, a phylogenetic analysis of ten Recent species in the genus is performed on 18 informative characters of the test. Second, morphological divergences among the species are analyzed using procrustean (superimposition) methods based on 49 homologous points. An additive distance tree is built from a matrix of morphometric distances among adult specimens. This tree is fully congruent with the phyletic results. Ontogenetic processes are explored by inserting ontogenetic series into the analysis. A distance tree including the juvenile stages shows that the general evolutionary trend of the genus is peramorphic, but species‐to‐species comparisons attest that no general clinal trend exists. Our analysis emphasizes the importance of morphometric approaches in evolutionary studies (1) for the understanding of heterochronies; (2) to trace the morphological implications of phylogenetic patterns; and (3) to estimate the impact of homoplasies.

The recovery and radiation of Early Jurassic ammonoids: morphologic versus palaeobiogeographical patterns

Abstract The recovery and radiation of the Early Jurassic ammonoid morphospace, as represented by nine morphologic groups identified in an earlier study, are traced through the first 36 subzones of the Hettangian to Domerian time interval. A quantitative survey of the dispersion of 436 species over 15 palaeogeographical areas reveals seven palaeobiogeographical patterns, each corresponding to an exclusive set of species exhibiting similar characteristics in terms of distribution and abundance. This study combines morphologic, chronostratigraphical, and palaeobiogeographical data in an attempt to investigate possible connections between morphologic recovery and radiation patterns and the history of ammonoid distribution on a global scale. Two out of nine morphologic groups are found to be prevalent in a single palaeobiogeographical pattern through the major part of the studied time interval, and a single morphologic group prevails in all the marine areas considered during Domerian times. All the other relationships identified are less tightly constrained in space and time and indicate loose ties over more limited periods and narrower distributions. The present results suggest that such complex relationships can be deciphered more successfully by using analytical palaeobiogeographical patterns as proposed here rather than classical palaeobiogeographical units (biochores).

Miniaturization and heterochrony in Homoeorhynchia meridionalis and H. cynocephala (Brachiopoda, Rhynchonellidae) from the Jurassic of the Iberian Range, Spain

The miniaturization of shells, often observed in Mesozoic brachiopods, is analyzed in two species of the well-known genus Homoeorhynchia. We investigated shell development in the context of recent works that consider growth as a function of size, shape, and age. The miniaturization results from heterochronic disturbances during development. Although these shells exhibit a large morphological plasticity, architectural constraints limit their morphogenesis, so that small adult specimens are not exactly reduced replicas of large ones. Adult morphology depends largely on the pattern of rib formation, with miniaturization arising by two different processes affecting this pattern: predisplacement of the appearance of ribs in H. meridionalis and acceleration in H. cynoceph- ala. The first case seems to be a reversible process without evolutionary consequences, but the second occurs within an evolutionary trend. In both cases, the miniaturization seems to be related to environmental influences. Study of the growth pathway of these species allows recognition of basic characters in the morphological variation of populations.

L'ontogenèse complexe du spatangue Echinocardium cordatum: Un test des standards des trajectoires hétérochroniques

The growth of the spatangoid echinoid Echinocardium cordatum is described by using the three standards (size,shape and age) that are classicaly involved in analyses of heterochronies. A population from Normandy, including all the growth stages, allows an ontogenetic reference to be established, while comparisons are performed with adult samples from other localities (Brittany, Aquitaine, and New-Zealand). The morphology of E. cordatum is characterized by 23 measured parameters and 13 calculated indexes. The ontogenetic trajectory of the species is depicted from the measurements of the size increments, from the analysis of the shape changes, and from an estimate of age. An estimate of the age-standard is deduced from the recognition of size-classes in relation to the time of sampling. The close equivalence of size-standard and total length of the test is discussed. Statistical analysis of ontogenetic trajectories are carried out for the 13 indexes. A multivariate analysis provides a synthetic picture of these trajectories and leads to define the shape-standard. These standards offer a frame to describe the succeeding events of the ontogeny of E. cordatum (shape changes, growth crisis) and to describe this ontogeny as the succession of four main stages. Comparisons with samples from other localities are performed: they show the originality of the population of New-Zealand and suggest that the species is polytypic. The palaeontological definition and use of heterochronic standards are discussed according to the described growth model.

Morphological and genetic analyses reveal a cryptic species complex in the echinoid Echinocardium cordatum and rule out a stabilizing selection explanation

Preliminary analyses revealed the presence of at least five mitochondrial clades within the widespread sea urchin Echinocardium cordatum (Spatangoida). In this study, we analyzed the genetic (two mitochondrial and two nuclear sequence loci) and morphological characteristics (20 indices) from worldwide samples of this taxon to establish the species limits, morphological diversity and differentiation. Co-occurring spatangoid species were also analyzed with mitochondrial DNA. The nuclear sequences confirm that mitochondrial lineages correspond to true genetic entities and reveal that two clades (named A and B1) hybridize in their sympatry area, although a more closely related pair of clades (B1 and B2), whose distributions widely overlap, does not display hybridization. The morphology of all E. cordatum clade pairs was significantly differentiated, but no morphological diagnostic character was evidenced. By contrast, other spatangoid species pairs that diverged more recently than the E. cordatum clades display clear diagnostic characters. Morphological diversity thus appears responsible for the absence of diagnostic characters, ruling out stabilizing selection, a classical explanation for cryptic species. Alternative classical explanations are (i) environmental plasticity or (ii) a high diversity of genes determining morphology, maintained by varying environmental conditions. We suggest a new hypothesis that the observed morphological diversity is selectively neutral and reflects high effective population sizes in the E. cordatum complex. It is supported by the higher abundance of this taxon compared with other taxa, a trend for the genetic and morphological diversity to be correlated in Europe, and the higher genetic and morphological diversities found in clades of E cordatum (except B1) than in other spatangoid samples in Europe. However, the Pacific clades do not confirm these trends.

Succession et remplacements de communautés à brachiopodes en régime de sédimentation discontinue (Jurassique Moyen, Bourgogne, France)

Abstract Sequence stratigraphy throws a new and seminal light on the fossil record and helps in determining the respective role played by external control (community replacement) and internal control (paleoecological succession) in the temporal “train” of marine benthic paleocommunities. Five brachiopod-dominated communities (C1–C5) are recognized and their temporal “train” described, during the uppermost Bathonian-lower Callovian interval in Burgundy. The first series (C1–C3) exhibits a trend interpreted as a paleoecological succession and the next two (C3–C4, C4–C5) as community replacements. Within the scope of sequence stratigraphy, the paleoecological succession comes at the end of a transgressive system tract, during an episode of reduced sedimentation, while the first replacement occurs with a maximum flooding of the platform. Faunal condensations are generally coincident with paleoecological successions but can restrict their retrieval from the fossil record. Conversely community replacement results from an external event, substrate quality related, which represents a triggering ecological factor for benthic community transformations.