DUROPHAGOUS PREDATION ON MIDDLE JURASSIC MOLLUSCS, AS EVIDENCED FROM SHELL FRAGMENTATION

Abstract:  Durophagous (shell-crushing) predation is known from the beginning of the Phanerozoic, but it has been suggested that modern intensity was not reached until the Late Cretaceous and Early Cenozoic, when specialized marine durophagous taxa increased in diversity. In this paper, evidence of durophagous predation on Middle Jurassic communities of molluscan prey is presented on the basis of distinct accumulations of fossil remains in the Polish Jura (south-central Poland) that contain characteristic, angular shell fragments with sharp, non-abraded margins. The diverse fossil content of the accumulations studied, consisting of either benthic or nektic/nekto-benthic taxa, indicates that the potential predatory taxon was an opportunistic generalist, most probably fish. On the basis of taphonomic observations, the faunal accumulations are interpreted to represent regurgitated remains (pellets). The common occurrence of such accumulations in the Middle Jurassic clays of the Polish Jura indicates that durophagous predation has been intense since the mid-Mesozoic, at least locally.     

Ammonoid taphonomy, palaeoenvironments and sequence stratigraphy at the Bajocian/Bathonian boundary on the Bas Auran area (Subalpine Basin, south-eastern France)

Bas Auran, in south‐eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno‐calcaires àCancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/Bathonian transition, hemipelagic, bed‐scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half‐cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian.     

Taphonomy and palaeoecology of Late Triassic (Carnian) ammonoid concentrations from the Taurus Mountains,Turkey

The deposits of the Carnian Kas?mlar Formation within the Taurus Platform Units of south‐western Turkey represent an important archive of a Late Triassic ecosystem. New palaeontological information was obtained by analysing the Kasimlarceltites mass occurrence, located within the Kas?mlar Formation and named after the Lower Carnian (Julian) ammonoid genus Kasimlarceltites. This is the dominant taxon (> 94%) within the mass occurrence: nearly 775 million ammonoids and 50 million gastropods were extrapolated for the whole extension (at least 5 km²) of the Kasimlarceltites beds. This calculation is one of the main findings within this study, as it is the first time that such a fossil mass occurrence was quantified. Additionally, orientation measurements of the planispiral ammonoids and the helical gastropods enabled reconstructing the history of the mass occurrence and interpreting the underlying transport mechanisms. Further taphonomic aspects (e.g. biofabric, preservation, bioerosion or genetic classification) as well as comparisons with samples of the same acme zone from different localities near A?a?iyaylabel (AS IV, KA I‐II) point to a two‐phased genetic history. Accordingly, local mass mortality within the Kasimlarceltites fauna due to oxygen fluctuations or methane degassing may have initially led to a primary accumulation. These deposits were then reworked and redeposited basinward by gravity flows to create the present‐day secondary allochthonous concentrations.     

Buoyancy mechanisms limit preservation of coleoid cephalopod soft tissues in Mesozoic Lagerstätten

Coleoid cephalopods are characterized by internalization of their shell, and are divided into the ten‐armed Decabrachia (squids and cuttlefish) and the eight‐armed Vampyropoda (octopuses and vampire squid). They have a rich fossil record predominantly of the limited biomineralized skeletal elements they possess: arm hooks, statoliths, mouthparts (the buccal mass) and internal shell (gladius or pen), although exquisitely preserved soft tissue coleoids are known from several Lagerstätten worldwide. Recent studies have shown that although morphological similarities between extant decabrachian gladii and fossil examples exist, no known examples of fossil decabrachians are currently known. However, molecular clock data and phylogenetic bracketing suggest that they should be present in Lagerstätten that are rich in vampyropod soft tissue fossils (i.e. Hâkel and Hâdjoula Lagerstätten, Cretaceous, Lebanon). We propose that a hitherto unknown taphonomic bias pertaining to the differing methods of buoyancy control within coleoid groups limits preservation potential. Both negatively and neutrally buoyant decabrachians use chemical buoyancy control (ammonia) whereas vampyropods do not. In the event of rapid burial in an environment conducive to exceptional preservation, ammonia dramatically decreases the ability of the decabrachian carcass to generate the required pH for authigenic calcium phosphate replacement, limiting its preservation potential. Moreover, the greater surface area and comparatively fragile dermis further decrease the potential for fossilization. This taphonomic bias may have contributed to the lack of preserved labile soft‐tissues in other cephalopods groups such as the ammonoids.     

Hiatal shell concentrations, sequence analysis, and sealevel history of a Pleistocene coastal alluvial fan, Punta Chueca, Sonora

Richly fossiliferous marine sediments exposed along the Sonoran coastline of the Gulf of California near Punta Chueca provide an excellent setting in which to test (a) the strength of the association of skeletal concentrations with sedimentary hiatuses, (b) the utility of taphonomic evidence for reconstructing detailed histories of those non-depositional episodes, and thus (c) the largely unexploited potential of skeletal concentrations in the identification and interpretation of lithologically obscure unconformities and condensed sequences in shallow marine deposits. Sequence analysis based on discontinuity surfaces is possible in the complex, alluvial fan-to-shallow marine transition at Punta Chueca despite rapid facies changes. Progradation of depositional sequences that contain cobbles reworked from older terrace deposits indicates accumulation during a fall in eustatic sea level. The supratidal to subtidal conglomerates and sands contain a variety of predominantly molluscan shell concentrations that, on the basis of postmortem histories of shells, formed during periods of low net sedimentation (i.e. depositional hiatuses); the majority of these shell beds lie along discontinuity surfaces identified by independent physical stratigraphic evidence. Although not all discontinuity surfaces in the terrace are paved by shell material. and not all relative concentrations of shells indicate distinct discontinuities, the strength of the association between skeletal concentrations and stratigraphic hiatuses reveals the high degree of control on fossil occurrence by sedimentation rates, and indicates that skeletal concentrations can provide good clues to stratigraphically significant surfaces. Moreover, the detailed dynamics of non-depositional episodes are reliably revealed by taphonomic analysis of the associated fossil assemblages, improving interpretations of non-depositional episodes in local sedimentary history.     

Proposition d’un référentiel taphonomique fossile de faunes issues d’avens-pièges

Pitfalls are formations characterized by a vertical opening that connects the karstic system to the outside. The presence of faunas trapped without the intervention of man or carnivores, offers the opportunity to establish a fossil referential of modifications generated by identified taphonomic agents. Three natural deposits containing large faunal accumulations feed this work. Their characteristics differ. The Igue des Rameaux, to Saint-Antonin-Noble-Val in Tarn-et-Garonne, is a pitfall which contains two accumulations of fauna independent from the Middle Pleistocene. Scavenging by man and carnivores (hyenas and wolves) are observed. The Igue du Gral, to Sauliac-sur-Célé in Lot, is a second pitfall. His principal phase of filling took place 14,000 years ago. Man did not exploit this assemblage. The activity of wolves is locally important, some animals are scavenged. The Grotte de l’Escale, in Bouches-du-Rhône, contains a very important filling of lower and middle Pleistocene. For the levels presented here, only a very weak presence of carnivores is attested. The thar (Hemitragus bonali) is the majority specie. We describe here the main forms of climato-edaphic modifications: fragmentation, dissolution, weathering and deposits observed on the osseous material. A differential analysis enables us to evaluate the variability of the situations according to the stratigraphic units and the different species. In a second time, we estimate the impact of the taphonomic agents on the differential conservation of bones and skeletal representations. Many phenomena are producing a significant correlation between anatomical representations and osseous densities. However, we find for all the species a certain homogeneity in the skeletal profiles where no precise element dominates. In a last part, we give two examples of the use of the taphonomic phenomena in the individualization of filling episodes within sometimes disrupted stratigraphies.     

Preservational bias controls the fossil record of pterosaurs

Pterosaurs, a Mesozoic group of flying archosaurs, have become a focal point for debates pertaining to the impact of sampling biases on our reading of the fossil record, as well as the utility of sampling proxies in palaeo‐diversity reconstructions. The completeness of the pterosaur fossil specimens themselves potentially provides additional information that is not captured in existing sampling proxies, and might shed new light on the group's evolutionary history. Here we assess the quality of the pterosaur fossil record via a character completeness metric based on the number of phylogenetic characters that can be scored for all known skeletons of 172 valid species, with averaged completeness values calculated for each geological stage. The fossil record of pterosaurs is observed to be strongly influenced by the occurrence and distribution of Lagerstätten. Peaks in completeness correlate with Lagerstätten deposits, and a recovered correlation between completeness and observed diversity is rendered non‐significant when Lagerstätten species are excluded. Intervals previously regarded as potential extinction events are shown to lack Lagerstätten and exhibit low completeness values: as such, the apparent low diversity in these intervals might be at least partly the result of poor fossil record quality. A positive correlation between temporal patterns in completeness of Cretaceous pterosaurs and birds further demonstrates the prominent role that Lagerstätten deposits have on the preservation of smaller bodied organisms, contrasting with a lack of correlation with the completeness of large‐bodied sauropodomorphs. However, we unexpectedly find a strong correlation between sauropodomorph and pterosaur completeness within the Triassic–Jurassic, but not the Cretaceous, potentially relating to a shared shift in environmental preference and thus preservation style through time. This study highlights the importance of understanding the relationship between various taphonomic controls when correcting for sampling bias, and provides additional evidence for the prominent role of sampling on observed patterns in pterosaur macroevolution.     

New bethylid and chrysidid wasps (Hymenoptera: Chrysidoidea) from Canadian Late Cretaceous amber

A new bethylid species, Celonophamia granama, and two new chrysidid species, Procleptes eoliami, and P. hopejohnsonae, are described from Late Cretaceous (Campanian) amber collected at the Grassy Lake locality in Alberta, Canada. Within the deposit these taxa constitute the first bethylid, and the second and third chrysidid species to be described, respectively. The new taxa expand the sparse fossil record of Chrysidoidea, particularly that of Chrysididae—a group that was previously represented by only three described species in the Mesozoic. The presence of Celonophamia species in both Canadian amber and Siberian (Taimyr) amber further emphasizes faunal similarities between these two northern Late Cretaceous amber deposits. Given the prevalence of metallic coloration in Chrysididae, the specimens described here also provide evidence for the taphonomic alteration of perceived insect colors in Cretaceous amber inclusions.     

Taphonomy and ichnology: tools for interpreting a maximum flooding interval in the Berriasian of Tlemcen Domain (Western Tellian Atlas,Algeria)

During the Middle-Late Berriasian, a long-term climatic and eustatic change occurred, documented in the literature. However, data from the northern Gondwana paleomargin are scarce. This research analyzes the Lamoricière Clay Formation at the Ouled Mimoun section, focusing on fossil assemblages and using taphonomic and ichnological aspects to interpret a transgressive–regressive cycle. The section starts with mudstones and oolitic grainstones representing shallow-water environments in the top part of the Ouled Mimoun Marly Limestones Formation (Upper Tithonian p.p. to lowermost Berriasian). The base of the Lamoricière Clay Formation is characterized by a high clay content but was still deposited in shallow water, as indicated by the record of the ostracod Asciocythere, dasyclad green algae, and the sponge Cladocoropsis. The subsequent record of fossil-rich calcareous beds at the beginning of the Upper Berriasian (Boissieri Zone) with ammonoids and calpionellids is congruent with an increase in water depth. The sedimentation rate in the Late Berriasian was reduced, as indicated by the increment of fossil remains and trace fossils. Ammonoid moulds show taphonomic features pointing to long-lasting exposure on the sea floor prior to burial with corrasion and encrustation by sessile organisms such as serpulids, thecideidinids, and bryozoans. During calm periods, crustaceans and worms intensely burrowed the sea floor. The record of Thalassinoides and Rhizocorallium indicates bottom conditions ranging from soft to firm. The low sedimentation rate and sediment by-passing probably favored early lithification. The increasing carbonate content as well as decreasing sedimentation rate is compatible with the maximum distance to emerged areas during maximum flooding. High-energy events, probably related to storms, favored the exhumation and extreme corrasion of ammonite moulds and trace fossils. In the resulting substrate, limonitic films developed and encrusting organisms proliferated (serpulids, bryozoans, and thecideidinids), colonizing both the bottom surface (hardground) and exhumed moulds of ammonoids and Thalassinoides. The uppermost 0.7 m of the section represents the return to shallow conditions, with increasing sedimentation rate and terrigenous detrital content, along with the disappearance of hemipelagic forms (ammonoids); hence it is interpreted as having developed at the beginning of a regressive context.     

A review of the fossil record of monocotyledons

The fossil record of monocotyledons is reviewed in order to determine the families actually represented by fossils and their stratigraphic ranges. In providing this information, attention is called to deficiencies and strengths in the record. Most notable is the uneven representation of the four subclasses of monocotyledons. Three distinct phases in the history of monocots can be delineated when the record and ranges of modern families are compared with paleotemperature data. First is a phase of initial radiation and diversification primarily restricted to the Cretaceous and represented by the appearance of a variety of monocotyledonous leaf forms. Second is a phase of modernization of the monocot flora resulting from further development of preexisting Cretaceous forms during favorable climatic conditions during the latest Cretaceous and Paleogene. The third phase is one marked by changes in distributions and expansion of other groups corresponding with climatic deteriorations of the late Paleogene. Some suggestions concerning the limitations of and potential for further research into the fossil record of monocots are presented.     

A new mid-Cretaceous Neomeris (dasycladacean alga) from the Potiguar Basin, Brazil

Neomeris (Lamouroux, 1816) is an extant taxon, the origin of which can be tracked back into Early Cretaceous times. The introduction of a new mid-Cretaceous species from Brazil, i.e., Neomeris srivastavai n. sp., offers the opportunity to review the subdivision of the genus into three subgenera, to complete the catalogue of the fossil calcareous algae of Brazil, and to point out the huge stratigraphic gap and lack of documentation between the first occurrence of the dasycladacean model of reproduction, i.e., choristospory, and the oldest record so far known of an undescribed fossil Neomeris (from Portugal).     

Ammonite taphonomy and stratigraphy of the Bajocian at Maizet,south of Caen (Calvados,NW France)

A study of the ammonite assemblages from the “Oolithe ferrugineuse de Bayeux” Formation of Bajocian age is presented herein. The section at the locality of Maizet shows a high level of stratigraphic condensation, and taphonomic reworking is common within the sequence. All the ammonites being studied are classified herein as having been reworked, transported or displaced on the sea-floor prior to burial, and as such, are determined taphonomically as being resedimented or reelaborated fossil elements. Seven evidences of reelaboration within the sequence under investigation are detailed here. The palaeontological units, so-called taphorecords, characterized by distinctive taphonomic features, are used to directly or indirectly assign beds to biochronostratigraphic units. In addition, identification of taphorecord relationships regarding successive or contemporaneous deposition allows their registratic succession and order of depositional events to be inferred. The latter may be deduced on the basis of bed succession or by reference to stratigraphical intervals that now are only represented in the stratigraphic column by fossils. Deposition by tractional currents and winnowing is indicated by sharp bed-base and by reworked fossil elements. Biochronostratigraphic correlation with other sections of the inland Bajocian successions in the Calvados area (Bretteville, Feuguerolles) highlights a common depositional evolution that may be related to an eastward-deepening carbonate ramp.     

Die Datenbanken PaleoTax und Goniat - Vorstufen eines paläontologischen Informationssystems

The database systems PaleoTax and GONIAT provide detailed information on literature, taxonomy, morphology and occurrence of fossil invertebrate groups. PaleoTax is designed for Mesozoic corals, GONIAT for Paleozoic ammonoids, but both can be modified to cover other fossil groups. Both systems use dBASE format, but with different database structures. PaleoTax aims at the complete storage of all available objective data, GONIAT provides determinations based upon morphologic criteria, and includes information on geographic distribution and stratigraphic range of every taxon. A combination of both systems could lead to the establishment of a comprehensive paleontological information system useful for research and practical stratigraphic applications.     

Fossil angiosperm wood: its role in the reconstruction of biodiversity and palaeoenvironment

Fossil wood is subject to different taphonomic, sampling and recognition biases in the palaeobotanical record when compared with leaves and palynomorphs. Wood therefore provides a systematically independent source of information that can increase our knowledge of past biodiversity and environments. Increase in fossil wood records from Cretaceous and Tertiary sediments helps further the understanding of trends in anatomical specialization through geological time. These data can then be used to distinguish such specialization from anatomical response to environmental change. Two case studies, a Late Cretaceous early Tertian' wood flora from Antarctica and a lower Tertiary w ood flora from southern England, have been used to exemplify the importance of studying the fossil wood component of palaeofloras.     

Telling time from fossils: a phylogeny-based approach to chronological ordering of paleobiotas

The potential for using fossils for temporal ordering of sedimentary rocks is as old as historical geology itself. In spite of this, however, most current biostratigraphic and biochronologic techniques do not make use of phylogenetic information, but rely instead on some measure of species' presence or absence or their turnover in the fossil record. A common phylogenetic approach to biochronology has been to use “stage of evolution” arguments, whereas more rigorous, cladogram‐based methods have been proposed but have seen little use. Cladistic biochronologic analysis (CBA) is developed here as a new method for determining biochronologic order between paleobiotas based on the phylogenetic relationships of their constituent taxa. CBA is adapted from Brooks' parsimony analysis, and analyzes syntaxon information from clades that transcend a number of paleobiotas to determine relative temporal order among these paleobiotas. Because CBA is based on phylogenetic information, it is suited to problems where a good fossil record is available, but where stratigraphic or chronologic relationships are poorly constrained, such as the terrestrial vertebrate record. A practical example, based on the Cenozoic fossil record of North America, pits CBA against a test case in which the correct temporal order of biotas is known beforehand. The method successfully recovers correct temporal order between paleobiotas with reasonable levels of support, and is also shown to outperform a previously proposed cladistic biochronologic method. In a second example, CBA is used to achieve the first empirical temporal ordination for several Late Cretaceous localities in the Gobi Desert that produce fossils crucial to the understanding of modern amniote clades, but which have poorly resolved temporal relationships. CBA is sensitive to large amounts of extinction and poor sampling of the fossil record, but problems such as gaps in the fossil record (Lazarus taxa) can be dealt with efficiently through a number of a priori and a posteriori scoring techniques. CBA offers a novel approach for biochronologic analysis that is independent of, but complementary to and readily combinable with other chronologic/stratigraphic methods. © The Willi Hennig Society 2007.     

The fossil record and limb disparity of enantiornithines, the dominant flying birds of the Cretaceous

Morphometric and stratigraphic analyses that encompass the known fossil record of enantiornithine birds (Enantiornithes) are presented. These predominantly flighted taxa were the dominant birds of the second half of the Mesozoic; the enantiornithine lineage is known to have lasted for at least 60 million years (Ma), up until the end of the Cretaceous. Analyses of fossil record dynamics show that enantiornithine 'collectorship' since the 1980s approaches an exponential distribution, indicating that an asymptote in proportion of specimens has yet to be achieved. Data demonstrate that the fossil record of enantiornithines is complete enough for the extraction of biological patterns. Comparison of the available fossil specimens with a large data set of modern bird (Neornithes) limb proportions also illustrates that the known forelimb proportions of enantiornithines fall within the range of extant taxa; thus these birds likely encompassed the range of flight styles of extant birds. In contrast, most enantiornithines had hindlimb proportions that differ from any extant taxa. To explore this, ternary diagrams are used to graph enantiornithine limb variation and to identify some morphological oddities ( Otogornis , Gobipteryx ); taxa not directly comparable to modern birds. These exceptions are interesting – although anatomically uniform, and similar to extant avians in their wing proportions, some fossil enantiornithines likely had flight styles not seen among their living counterparts.     

Taphofacies analysis of recent shelly cheniers (beach ridges), northeastern baja california,Mexico

Summary This report presents the results of taphofacies analyses of shelly cheniers (mollusk-dominated lag-concentrations) from the tidal flats of northeastern Baja California, Mexico. The three generations of moderm (formed during last 70 years), submodem (younger than 1,500 BP), and subfossil (5,000–2,000 BP) cheniers can be distinguished by their position relative to the shoreline, their topography, and the radiocarbon-age of their shells. The generations differ in the duration and complexity of their taphonomic history. Sixty-one samples from nine localities were collected to test the utility of the taphofacies approach for studying chenier-type shell deposits. The three chenier generations, although all dominated by the bivalve molluskMulinia coloradoensis, differ significantly in their taxonomic composition due to taphonomic and/or biologic factors. The taphofacies analysis included 4,334 specimens ofM. coloradoensis described by nine taphonomic variables. Univariate analysis of those variables indicated that the shells that accumulated in the cheniers are little-affected by biological processes (bioerosion, encrustation), and moderately affected by physical processes (fragmetation, cracking, peeling, edge preservation). Only the luster features of shells (external luster, internal luster, and internal features) vary substantially and consistently with chenier age —a result of subaerial weathering. Multivariate taphofacies analysis discriminates the three generations of cheniers even when the poorly preservable luster variables are excluded from the analysis. This suggests that taphofacies discrimination is possible for fossil cheniers. The shells collected from the chenier surface have substantially poorer preservation than shells from the subsurface, indicating that taphonomic degradation in the chenier plain environment is a surface phenomenon. Chenier plain shelly assemblages are taphonomically distinct from assemblages formed in other marine environments: they have a very low frequency of macroscopically recognizable bioerosion and encrustation. The existence of preservable taphonomic differences between the cheniers that differ in their age (i.e., duration of preburial history), suggests that fossil lag concentrations may be useful in detecting incompleteness gradients along stratigraphic boundaries. A ‘taphonomic clock’—a correlation between a ‘time-sincedeath’ and shell preservation—was found only for luster features, taphonomic attributes that are unlikely to be preserved in the fossil record.     

Recognizing pulses of extinction from clusters of last occurrences

The distribution of last occurrences of fossil taxa in a stratigraphic column are used to infer the pattern, timing and tempo of extinction from the fossil record. Clusters of last occurrences are commonly interpreted as an abrupt pulse of extinction. However, stratigraphic architecture alone can produce clusters of last occurrences that can be misinterpreted as an extinction pulse. These clusters will typically occur in strata that immediately underlie facies changes and sequence‐stratigraphic surfaces. It has been proposed that a basin‐wide analysis of the fossil record within a sequence‐stratigraphic framework can be used to distinguish between clusters of last occurrences caused solely by extinction pulses from those generated by sequence‐stratigraphic architecture. A basin‐wide approach makes it possible to observe lateral facies shifts in response to sea‐level change, mitigating the effects of stratigraphic architecture. Using computer simulations of plausible Late Ordovician mass‐extinction scenarios tuned to an inferred Late Ordovician sea‐level curve, we show that stratigraphically‐generated clusters of last occurrences are observed even in basin‐wide analyses of the simulated fossil records due to the basin‐wide loss of preferred facies for many taxa. Nonetheless, we demonstrate that by coarsening the stratigraphic resolution to the systems‐tract level and identifying facies preferences of simulated taxa, we can filter out taxa whose last occurrences coincide with the basin‐wide loss of their preferred facies. This enables consistent identification of the underlying extinction pattern for a wide variety of extinction scenarios. Applying this approach to empirical field data can help to constrain underlying extinction patterns from the fossil record.     

Les Ammonoïdes (Mollusca,Cephalopoda) : avancées et contributions récentes à la paléobiologie évolutive

Ammonoids ruled the seas for 335 Myr and present themselves as an especially suitable model when analyzing biological evolution. This synthetic paper focuses on (1): the phylogenetic place of ammonoids within cephalopods and the choice of an extant reference model; (2): the establishment of phenotypic spaces supplying relevant insights into biological evolution; (3): the concordances and discordances between phylogenetic reconstructions and the fossil record, and (4): the postcrisis recoveries, as models to study large-scale evolution. It appears from these topics that ammonoids can be used as case studies for many themes in Paleontology (biodiversity dynamics, phylogenetics, analysis of the fossil record) that offered and continues to offer a better understanding of evolutionary patterns and processes, especially in the context of large-scale studies.     

Uncertainty in the age of fossils and the stratigraphic fit to phylogenies

The ages of first appearance of fossil taxa in the stratigraphic record are inherently associated to an interval of error or uncertainty, rather than being precise point estimates. Contrasting this temporal information with topologies of phylogenetic relationships is relevant to many aspects of evolutionary studies. Several indices have been proposed to compare the ages of first appearance of fossil taxa and phylogenies. For computing most of these indices, the ages of first appearance of fossil taxa are currently used as point estimates, ignoring their associated errors or uncertainties. The effect of age uncertainty on measures of stratigraphic fit to phylogenies is explored here for two indices based on the extension of ghost lineages (MSM* and GER). A solution based on randomization of the ages of terminal taxa is implemented, resulting in a range of possible values for measures of stratigraphic fit to phylogenies, rather than in a precise but arbitrary stratigraphic fit value. Sample cases show that ignoring the age uncertainty of fossil taxa can produce misleading results when comparing the stratigraphic fit of competing phylogenetic hypotheses. Empirical test cases of alternative phylogenies of two dinosaur groups are analyzed through the randomization procedure proposed here.