Catherine Crônier

Larval morphology and ontogeny of a Late Devonian phacopid with reduced sight from Thuringia, Germany

Exceptionally rich and well-preserved silicified exuviae comprising numerous immature growth stages allow the tracing of detailed morphological and morphogenetical features of the reduced-eyed phacopid species Cryphops ? ensae (Richter and Richter, 1926) from the latest Famennian of Thuringia (Central Germany). Ontogenetic features of this species are more similar to those of Phacops than to Trimerocephalus.

Variation in the eye of Acuticryphops (Phacopina, Trilobita) and its evolutionary significance: a biometric and morphometric approach

Abstract Distributional patterns of eye lens variation in different morphs of the phacopine trilobite Acuticryphops acuticeps (Kayser 1889) are investigated. The specimens were collected from the latest Frasnian preceding the Upper Kellwasser (UKW) global extinction event in the Frasnian– Famennian (Late Devonian) stratotype section at Coumiac, southern France. In six successive populations a gradual reduction in the mean number of lenses occurs within the short time span of a single conodont Zone. This morphological change cannot be imputed either to the size of individual specimens or to variation in cephalic morphology. Thus morphs with different numbers of eye lenses are considered intraspecific. However, the intrapopulational percentage relation between morphs does not remain constant, as the coefficient of variation in lens number continuously increases from one population to the next. Cases of individuals with asymmetric eyes appear in the two latest assemblages prior to the UKW level. The fact that the total variation of the shape of the cephalon is not affected in these assemblages suggests that the morphological changes observed in the visual complex may account for the relaxation of selective pressures on this trait. Such a change in the regime of selection would have been accompanied by a lessening of the processes that control the development of this trait. As the phenomenon of eye reduction is not restrained by local conditions at Coumiac (Montagne Noire) but occurs contemporaneously to various extents in other crustal blocks such as Rhenish Slate Mountains (Avalonia), Thuringia (Armorica), and Morocco (Northern Gondwana), it is considered as constituting an adaptation to global eustatic deepening that occurred in the terminal Frasnian just before the global extinction event.

Evolution et systématique du groupeCryphops (Phacopinae, Trilobita) du Dévonien supérieur

RésuméLes phacopinés du Dévonien supérieur à œil réduit étaient jusqu’ici intégrés dans le seul genreCryphops, bien que sa répartition dans le temps soit discontinue. En tenant compte des structures ventrales sous-céphaliques très variables au sein du groupeCryphops, une nouvelle interprétation des relations phylétiques est proposée. Les différents morphotypes dériveraient, à plusieurs reprises et de façon indépendante, successivement de la souche ancestrale du groupePhacops, par évolution itérative. Par conséquent, de nouvelles entités taxinomiques sont crées:Acuticryphops n. g.,Weyerites n. g.,Struveops n. g. etSpinicryphops n. g. avec la nouvelle espèceSpinicryphops lemkei n. sp.AbstractThe Upper Devonian Phacopinae with reduced eyes have hitherto been placed in the genusCryphops despite their differing time distributions. The phylogenetic relationships within theCryphops group are re-interpreted, based on the very variable nature of the cephalic structures. The different morphotypes probably derived iteratively from their ancestralPhacops lineage, several times and indepedently from one another. Thus, the following new taxonomic units can be introduced:Acuticryphops n. g.,Weyerites n. g.,Struveops n. g. andSpinicryphops n. g. with the new speciesSpinicryphops lemkei n. sp.KurzfassungDie oberdevonischen Phacopinae mit reduziertem Auge wurden bisher dem einzigen TaxonCryphops zugeordnet trotz der Diskontinuität in ihrem zeitlichen Auftreten. Nach Berücksichtigung der Ventralstrukturen des Cephalons, die innerhalb derCryphops-Gmppe sehr variabel sind, werden die phylogenetischen Beziehungen innerhalb der Gruppe neu interpretiert. Die unterschiedlichen Morphotypen dürften mehrmals und unabhängig voneinander direkt von der StammliniePhacops iterativ entsprungen sein. Demnach werden die folgenden neuen taxonomischen Einheiten aufgestellt:Acuticryphops n. g.,Weyerites n. g.,Struveops n. g. undSpinicryphops n. g. mit der neuen ArtSpinicryphops lemkei n. sp.

DIVERSITY OR DISPARITY IN THE JURASSIC (UPPER CALLOVIAN) GENUS KOSMOCERAS (AMMONITINA): A MORPHOMETRIC APPROACH

Abstract A detailed morphometric approach applied to the diversity (interspecific variation)/disparity (intraspecific range of variation) was developed. It was based on outline analysis, and it was applied to 41 cross sections of Kosmoceras from the Upper Callovian of eastern France (Prusly). The cross sections represent an excellent basis for evaluation of the global shape of the shell: the main morphologic descriptors include the proportion of umbilical diameter/conch diameter and the thickness of the whorl; they also indirectly describe the ornament that influences the aspect of the whorl section. This analysis was conducted to quantify the size and shape variations and to determine the relations among some of the ‘morphological species’ defined by Tintant: K. bizeti, K. fibuliferum, K. phaienum? and K. interpositum? Numerous analyzed individuals may not belong to these categories. The results suggest that despite a wide range of variability four main morphotypes can be ascribed a probable taxonomic status, whose mean representatives can be properly assigned to Tintants four ‘species.’ Moreover, three of them could constitute a morphological series, continuously related through ontogeny. The fourth (K. interpositum) may be an independent species, rarely recovered in the studied area.

MORPHOLOGY AND ONTOGENY OF AN EARLY DEVONIAN PHACOPID TRILOBITE WITH REDUCED SIGHT FROM SOUTHERN THAILAND

Abstract Exuviae comprising immature growth stages allow the tracing of morphological and morphogenetical features of the reduced-eyed phacopid species Plagiolaria poothaii Kobayashi and Hamada, 1968, from the Early Devonian of Satun Province, southern Thailand. Biometric and morphometric approaches have been used to characterize the shape changes. This is the fourth phacopid genus for which ontogenetic information is known. Ontogenetic features of this species are generally similar to those of Phacops and Weyerites. However, the ankylosis of the facial sutures occurs at the end of the meraspid period, later than in Phacops, and this is probably a derived character.

LARVAL MORPHOLOGY AND ONTOGENY OF AN UPPER DEVONIAN PHACOPID: NEPHRANOPS FROM THURINGIA, GERMANY

Abstract The recovery of well-preserved silicified immature exuviae allows the first description of the early ontogeny of the phacopid Nephranops incisus incisus from the lower Famennian of Thuringia (central Germany). Evidence is presented that both planktonic and benthic protaspid larval stages were present in phacopid trilobites. Ontogenetic meraspid changes of this species are similar to those of Weyerites and Phacops, supporting the close phyletic relationships between these genera and the ontogenetic homogeneity of the Phacopidae.

Ontogeny of an Ordovician Trinucleid (Trilobita) from Armorica, France: A Morphometric Approach

Abstract A large number of well-preserved immature exoskeletons enable the first detailed quantitative description of the ontogeny of the trinucleid Marrolithus bureaui (Oehlert, 1895) from the Lower Caradoc of the Armorican Massif (Armorica, northwest France). For the accurate description of cephalic features, a landmark-based approach was adopted in order to recognize ontogenetic instars and to establish the patterns of size increase and shape change during ontogenetic development. The quantitative analysis permitted the demonstration of a progressive shape change in agreement with ontogenetic ordination and a comparison of the timing of size and shape changes. Changes that took place during the meraspid period included, in particular, the development of a more pronounced sub-quadrangular cephalic outline, a compression (sag.) of the glabella and a size decrease until disappearance of the alae in late stages. This quantitative ontogenetic reconstitution of Marrolithus bureaui might be considered as a reference for the better understanding of phylogenetic affinities between other trinucleid species.

Varied development of trunk segmentation in three related Upper Devonian phacopine trilobites

In many arthropods, the development of the trunk region is a complex part of post-embryonic development. Consideration of fossil ontogeny provides an additional source of data and a broader evolutionary perspective on the evolution of arthropod body patterning. Here, I examine the development of the thoraco-pygidial exoskeleton of three related phacopine trilobites from the Upper Devonian according to the integrated ontogenetic scheme proposed by Hughes et al. Pygidial shields assigned to three ontogenetic series gave us the opportunity to further explore the evolutionary pattern of the trilobite segmentation. The analysis showed a different mode of development in two of three species and thus reveals variability between the related taxa. Comparison of the boundaries of different aspects of ontogenesis ratifies the diversity of the segmentation process among trilobites and even among related phacopine species. Results include (i) in a synarthromeric trunk condition recorded to date, there has consistently been a preceding ‘equilibrium’ phase for the late meraspid pygidium and (ii) two developmental modes, i.e. both hypoprotomeric development and synarthromeric development, occur contemporaneously in closely related taxa. Such developments suggest that aspects of segmental development such as segment accretion and segment articulation were able to vary in a labile manner.

A trilobite faunule from the Lower Devonian of the Saoura Valley, Algeria: biodiversity, morphological variability and palaeobiogeographical affinities

Trilobites are widespread in Lower Devonian deposits of north Gondwana, and some have been collected from two known sections of the Saoura Valley in SW Algeria, from the ‘Chefar el Ahmar’ Formation. This formation is considered to be from late Emsian to Frasnian in age, but only the lower parts of this formation have yielded trilobites. Nevertheless, no detailed studies have focused on their biodiversity and their morphological variability. New occurrences of phacopids including Barrandeops chattertoni sp. nov., Geesops fabrei sp. nov., Austerops legrandi sp. nov. and Phacops boudjemaai sp. nov. are described from this area and comparisons are made with closely allied species. These new occurrences have been integrated into analyses of intra- and inter-specific variability and biodiversity.

The systematics and phylogeny of the Devonian subfamily Asteropyginae (Trilobita: Phacopida)

Traditionally, the evolutionary scenarios of Asteropyginae have been organized in two major clades involving four pygidial patterns. The first cladistic analysis performed on the subfamily maintained the organization in two clades, though the phylogenetic relationships between genera were unexpected compared to the traditional scenarios. Moreover, the previous systematic scheme based on pygidial segmentation was rendered obsolete. In this study, we performed a new phylogenetic analysis from well-known taxa and recent discoveries. Sixty species assigned to 36 genera have been analysed from a dataset of 72 characters. Three more species are used as the outgroup. The 79 most parsimonious trees have a length of 492 steps and a retention index of 0.735. Our analysis confirms the monophyly of Asteropyginae excluding the genus Protacanthina. The phylogenetic pattern is unconventional in that a largely pectinate topology is resolved, rather than two clades corresponding to formerly delimited pygidial morphs. Nevertheless, most relationships between genera suggested in the traditional evolutionary scenarios of the subfamily have been recovered, the main differences being the origin of the group. Whereas Treveropyge was usually considered as the origin of the remaining Asteropyginae, the present results instead identify Destombesina as the sister group to all remaining members of the subfamily. From the new phylogenetic pattern, the diagnoses of genera are rewritten in a standard way and to delimit clades more accurately. This work implies modified generic assignments for some species and the erection of four new genera: Minicryphaeus gen. nov., Gandlops gen. nov., Morzadecops gen. nov. and Pennarbedops gen. nov. The systematic position of Erbenochile is also discussed, the genus no longer being assigned to Asteropyginae. http://zoobank.org/urn:lsid:zoobank.org:pub:C42A54C3-2B8E-439A-8DE0-2F35575F7F40