In the aftermath of the end‐Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian‐negative carbonate ...carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ13Ccarb variations obtained here reproduce the known Smithian δ13Ccarb‐negative excursion. However, the δ13C signal of the bulk organic matter is invariant across the SSB and variations in the δ34S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ13Ccarb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.
The Early Triassic is generally portrayed as a time of various, high ecological stresses leading to a delayed biotic recovery after the devastating end-Permian mass extinction. This interval is ...notably characterized by repeated biotic crises (e.g., during the late Smithian), large-scale fluctuations of the global carbon, nitrogen and sulfur cycles as well as harsh marine conditions including a combination of ocean acidification, anoxia, extreme seawater temperatures and shifting productivity. Observations from different paleolatitudes suggest that sulfidic (H2S-rich) conditions may have developed widely during the Early Triassic, possibly reaching up to ultra-shallow environments in some places. However, the existence and the spatio-temporal extent of such redox swings remain poorly constrained. In order to explore Early Triassic paleoceanographic redox changes and their potential influences on the biotic recovery, we analyzed multiple sulfur isotopes (32S, 33S, 34S, and 36S) of sedimentary pyrite and carbonate associated sulfate (δ34SCAS) from the Mineral Mountains section, Utah. Sediments from this section were mainly deposited in shallow waters and span the Smithian and lower Spathian. We report a 68‰ range of variations in δ34Spy associated with Δ33Spy varying from −0.01‰ to +0.12‰, whereas the δ34SCAS varies between +19.5‰ and + 34.8‰. We interpret the observed signal of multiple sulfur isotopes as reflecting the operation of pore-water synsedimentary microbial sulfate reduction in open system with respect to sulfates before the late Smithian, evolving to a closed system, sulfate limited, Rayleigh-type distillation across the Smithian/Spathian boundary (SSB) and immediately after the SSB. We argue that this marked change is driven by the effectiveness of the connection between the sedimentary pore waters and the overlying water column, which is, in this case, controlled by the local sedimentological conditions such as the bioturbation intensity and the sedimentation rate. Therefore, our results suggest that changes in the sulfur cycle before and across the SSB at Mineral Mountains is probably a local consequence of the loss of the mixed sedimentary layer during the late Smithian extinction event, as opposed to reflecting the development of a lethal anoxic ocean at the global scale.
The fossil record of the Cretaceous is critical for understanding the evolution of modern tetrapods. Using a measure of relative completeness of the fossil record--the Simple Completeness Metric ...(SCM)--quality of the fossil record and diversity during the Cretaceous appear to be closely related, suggesting an artifactual component. The SCM calculations also show that knowledge of the fossil record has improved in the last ten years. Recent proposals that modern orders of birds and mammals originated early in the Cretaceous are rendered unlikely by four arguments: (1) the SCM calculations indicate that the fossil record of Cretaceous birds and mammals is relatively good; (2) it is unlikely that all modern orders, independently, would have remained cryptic throughout the Cretaceous; (3) control samples of exquisitely preserved tiny Cretaceous tetrapods lack any specimens of modern groups of birds and mammals; and (4) the suggestion that the undiscovered ancestors of modern groups are to be found in unsampled parts of the Earth is not supported by cladistic evidence.
The diversity of tetrapod families increased through the Cretaceous, punctuated by three majorextinction events at the Jurassic/Cretaceous, Cenomanian/Turonian, and Cretaceous/Tertiary boundaries. ...Extinction and survivorship at these times are analysed with regard to taxonomy and ecological categories of diet, habitat and size. Whereas no constant selectivity pattern is found for diet and size through the period, freshwater tetrapods seem to have been less extinction-prone than terrestrial and marine ones. The Cretaceous/Tertiary extinction had less effect on small, omnivorous, and freshwater families than other ecological categories. This strong selectivity signal is consistent with other work made at higher taxonomic and stratigraphic levels of resolution, thus legitimising the use of fossil families in global studies of diversity and palaeoecology.
La diversité des familles de tétrapodes a augmenté au cours du Crétacé mais trois épisodes d'extinctionsla ponctuent aux limites Jurassique/Crétacé, Cénomanien/Turonien et Crétacé/Tertiaire. Extinction et survie lors de ces événements sont étudiées pour différentes catégories de régime alimentaire, d'habitat et de taille. Régime alimentaire et taille ne font preuve d'aucun motif constant de sélectivité au cours de la période mais les tétrapodes vivant en eaux douces affichent une survie préférentielle par rapport aux tétrapodes terrestres et marins. La crise Crétacé-Tertiaire a peu affecté les familles de petite taille, omnivores et vivant en eaux douces. Le fort signal de sélectivité écologique corrobore les travaux réalisés à des niveaux de résolution taxinomique et stratigraphique plus élevés, légitimant ainsi l'emploi du niveau familial dans les études globales de diversité et de paléoécologie.
Estimates of past diversity using palaeontological evidence can be achieved within two main analytical frameworks. The traditional ‘taxic approach’ makes a straightforward use of stratigraphic ranges ...at a given taxonomic level, whereas the ‘phylogenetic approach’ derives palaeodiversity estimates from both phylogenetic topologies and stratigraphic occurrences. At lower taxonomic levels, the taxic and phylogenetic approaches have limitations for several groups because of the incompleteness of the fossil record and the frequent absence of stable, comprehensive phylogenetic hypotheses. In particular, fossil specimens unidentifiable at lower taxonomic levels are frequently discarded from both kinds of diversity analysis. However, these specifically or generically indeterminate occurrences may prove crucial for recovering minimal lineage-level diversity patterns, especially for groups with poor fossil records. Diversity counts can be achieved by a simple, intuitive protocol that incorporates all fossil occurrences and minimal principles of evolutionary continuity. This method, intermediate between the taxic and the phylogenetic approaches, has received little attention among palaeobiologists so far although it holds great promises for diversity studies. Here this alternative method is extended, generalised, and tested. When empirical occurrence data are artificially degraded, the technique captures an equal or a better part of the original diversity signal than the taxic approach.
This method is applied here to an original locality-level compilation of fossil lissamphibians occurring in the Late Jurassic–Eocene interval. This empirical study provides the first global lineage-level palaeodiversity estimate ever made for this group, and it shows that specifically indeterminate occurrences and evolutionary continuity can increase species diversity estimates by 400%. The major implications of this work are that (1) palaeontological reports and datasets should record
all fossil occurrences, regardless of the taxonomic level at which they are diagnostic; (2) it seems preferable to abandon the old commitment of using only taxa assigned to the same rank in diversity analyses; (3) the clade Lissamphibia has experienced a virtually extinction-free gradual rise during the Late Jurassic–Eocene interval; (4) long-term climatic disruptions for the K/T bio-event and hypothetical megabiases in the Late Cretaceous fossil record of small terrestrial vertebrates are unlikely.
The early Pliensbachian (Early Jurassic) is known as a time of marked provincialism in the marine realm, notably between the Mediterranean Tethys and North–West Europe. In order to test this ...observation quantitatively, we compiled 104 locality-level species lists from those areas based on a comprehensive revision of early Pliensbachian ammonites. With this dataset, we also explore the relationship between ammonite richness and biogeography at the scale of the sub-chronozone during the early Pliensbachian. Using various multivariate statistics and rarefaction techniques, we show that: (i) there is a sharp contrast between the NW European (NWE) and the Mediterranean (MED) provinces, although there is some mixing in Austroalpine and Pontic ammonite faunas; (ii) species richness in the MED province is about twice that in the NWE province for each chronozone; (iii) ammonite species richness tends to decrease during the early Pliensbachian, especially at the Ibex–Davoei transition; and (iv) the NWE and MED
sensu stricto provinces both record the same pattern of variations in richness despite the fact that their taxonomic compositions have virtually nothing in common at the species level. We suggest that the low ammonite richness of the Davoei chronozone may be related to a coeval warming of seawaters, but that this was insufficient to affect the sharp palaeobiogeographic contrast between the two provinces. This persistent compartmentalisation probably reflects a major palaeogeographical structure, such as an emerged or near-emerged barrier running from the Betic range to the Briançonnais ridge. Overall, it seems that the diversity and distribution of early Pliensbachian ammonite species were simultaneously controlled by climate, palaeogeography and eustasy.
The Romualdo Member of the Santana Formation (Araripe Basin, northeastern Brazil) is famous for the abundance and the exceptional preservation of the fossils found in its early diagenetic carbonate ...concretions. However, a vast majority of these Early Cretaceous fossils lack precise geographical and stratigraphic data. The absence of such contextual proxies hinders our understanding of the apparent variations in faunal composition and abundance patterns across the Araripe Basin.
We conducted controlled excavations in the Romualdo Member in order to provide a detailed account of its main stratigraphic, sedimentological and palaeontological features near Santana do Cariri, Ceará State.
We provide the first fine-scale stratigraphic sequence ever established for the Romualdo Member and we distinguish at least seven concretion-bearing horizons. Notably, a 60-cm-thick group of layers (“Matracão”), located in the middle part of the member, is virtually barren of fossiliferous concretions.
Moreover, a sample of 233 concretions shows that (i) the stratigraphic distribution of the concretions is very heterogeneous and their density varies from 0.8 to 15 concretions/m
3; (ii) concretions have a preferential, bimodal orientation pattern (major NW–SE axis and secondary ≈N–S axis) throughout the section, suggestive of permanent palaeocurrents of low energy; (iii) few concretions yield the well-preserved vertebrates that have made the Romualdo Member so famous, and those are mainly restricted to four stratigraphic horizons; (iv) only six fish taxa were recovered, the most common being
Vinctifer and
Tharrhias, followed by
Cladocyclus, whereas
Brannerion,
Calamopleurus (=
Enneles) and
Notelops are rare. No tetrapod was found in the sample; (v) there is a strong stratigraphic control in the distribution of these taxa and one can distinguish at least three major assemblages at the same locus. These are, from older to younger, a
Tharrhias-dominated assemblage, an assemblage dominated by
Tharrhias and by
Cladocyclus, and a
Vinctifer-dominated assemblage. The stratigraphic sequence of these assemblages also corresponds to their ranking in terms of diversity (richness and evenness); (vi) previous accounts on the taxonomic composition and relative abundance of fossils from the Romualdo Member were severely biased toward well-preserved and exotic fossils. They are therefore inappropriate for drawing palaeoecological inferences.
The factors responsible for the variations in faunal composition and abundance patterns across the Araripe Basin remain largely unknown, and we hypothesize that climate and/or palaeogeography might be the major forcing agents. Only fine-scale stratigraphic and palaeontological investigations have the potential to solve this issue. In turn, this work marks the first step of an expanded research program that aims at explaining the spatio-temporal relationships between palaeocommunities and their palaeoenvironment in the Araripe Basin during Aptian/Albian times.
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