Palaeontological Evidence Research Articles

Determining the age and origin of a Tertiary karstic system by in situ U-Pb geochronology on speleothems

Abstract Dating the onset of ancient (>1 Ma) karstification is a challenge. One approach is to date the earliest calcite cements in speleothems. We show the benefits of in situ U-Pb dating directly on thin sections from ancient (ca. 30 Ma) thin (<1 mm) speleothems in the karstified Lower Oligocene lacustrine-palustrine carbonates of the Paris Basin (France), which cannot be dated using other methods. We dated 32 calcite rafts (a type of speleothem), one geopetal cement, and 10 calcite cements precipitated along the karstic walls. The ages of the calcite rafts and cements at 29 ± 1 Ma (Lower Oligocene) fall within the age range of the host deposits (≈29 Ma) previously deduced from palaeontological evidence. We demonstrate that cementation of the carbonate host rock, its dissolution, and the speleothem precipitation occurred within 2 m.y. after deposition. Ostracods and intraclasts trapped within Rupelian calcite rafts clearly indicate that the karst developed deep underground shortly before a phase of lacustrine-palustrine sedimentation at the surface. This very early dissolution episode is attributed to the uplift of the Paris Basin as a result of the far-field intraplate deformation induced by the alpine orogenesis. This study shows that in situ U-Pb geochronology on ancient calcite rafts is a promising technique for the indirect dating of karstification and, more broadly, for dating geodynamic events and diagenetic evolution of sedimentary basins.

The oldest known larvae of Megaloptera (Insecta) from the Triassic of Ukraine

The oldest unquestionable fishfly, Izyumochauliodes aristovi gen. et sp. nov. (Corydalidae, Chauliodinae), is described based on several larval fossils from the Upper Triassic Garazhovka locality in eastern Ukraine (Seversky Donets River basin, Kharkiv Region). The new genus with a typical Chauliodinae structure of the abdomen is distinguished from all other genera of the subfamily by having the occipital suture ventrally directed forward along the gula and reaching the submentum, and by the shape of mandibula with evenly curved inner margin, both characters similar to Sialidae. Another, poorly preserved larva, found in the same locality, is attributed to Sialidae incertae sedis and also represents the oldest record of the family. The co-occurrence of fossil larvae of both known families in the same locality is the palaeontological evidence for the Triassic divergence of the extant megalopteran lineages.

Late Cenozoic evolution of the latitudinal diversity gradient

AbstractAimThe Late Cenozoic flourishing of polar marine ecosystems, just when temperatures were reaching their lowest levels, has always seemed anomalous. Such an observation is coupled with an increasing volume of molecular phylogenetic evidence to indicate that some polar taxa radiated at exceptionally high evolutionary rates. The canonical latitudinal diversity gradient (LDG) may not be underpinned by a parallel gradient in evolutionary rates. This study provides a review of this critical question.LocationA variety of polar – tropical comparisons are made, with some emphasis on the Southern Ocean.TaxaBoth marine and terrestrial taxa are used, with some emphasis on marine Mollusca.MethodsHigh – low latitude comparisons of evolutionary rates over the last 66 Myr are made using two approaches: rates derived from molecular phylogenies, and rates derived from the fossil record. For the former of these data are taken from various marine and terrestrial taxa, and for the latter use is made of the excellent fossil record of marine Mollusca.ResultsFor approximately the first half of the Cenozoic era (i.e., 66 Ma onwards) there is strong palaeontological evidence to suggest that the global LDG developed in a conventional way, with higher rates of origination in the tropics than at the poles. However, there then appears to have been a marked change in the latter half of the Cenozoic when origination rates increased in the high‐latitude and polar regions. The modern latitudinal diversity gradient may be a fossil feature that formed in the Early Cenozoic.Main ConclusionsAs the extra‐tropical regions expanded through the Cenozoic they provided increasingly more ecological opportunities for a wide variety of marine and terrestrial taxa. Substantially lower temperatures in the polar regions were clearly not an obstacle to spectacular evolutionary radiations such as that based on the krill ecosystem.

The geological and historical milieu of an ornamental cephalopod limestone (‘orthoceratite limestone’, Ordovician, Sweden) used in the Clerk Mausoleum (1684), St Mungo's Kirkyard, Penicuik, Scotland

A slab of cephalopod limestone bears a dedicatory Latin inscription on the mausoleum built around 1684 by Sir John Clerk of Penicuik (1649–1722) for his wife Elizabeth Henderson (1658–83) at St Mungo's Church, Penicuik, near Edinburgh, Scotland. The stone is identified on sedimentological and palaeontological evidence and historical context as Ordovician ‘orthoceratite limestone’ from Sweden, probably the island of Öland, rather than Carboniferous cephalopod limestone from the much nearer Closeburn area of Dumfriesshire. ‘Orthoceratite limestone’ was little used in Great Britain, and mainly as paving, so its use in a funerary monument is unusual. It is, however, paralleled by contemporary examples at Winchester Cathedral. The Penicuik slab was probably imported either directly from Sweden, or through Rotterdam or another Netherlands entrepôt. It is the only surviving historical example of this stone known in Edinburgh and the Lothians, probably because of changing fashions, building demolition and renewal of worn paving. The inscription shows errors of composition, carving and installation, ascribed to inexperience or haste. The employment of ‘orthoceratite limestone’ is interpreted as seeking to emulate Roman use of marbles and similar ornamental stones. It contributes to the Penicuik mausoleum's significance as a pioneering example of classical or Antique architecture.

Ostracod and Foraminifer Responses to Late Pleistocene–Holocene Volcanic Activity in Northern Victoria Land as Recorded in Ross Sea (Antarctica) Marine Sediments

The impacts on ostracods and foraminifers caused by three Late Quaternary ashfalls of different intensities and recovered in the ANTA02-NW2 core sediments (Drygalski Basin, western Ross Sea) were analysed for the first time. Albeit with different timing, both associations demonstrated similar response patterns associated with the deposition of material from volcanic eruptions. In particular, based on the palaeontological evidence, it was possible to divide the cores into four intervals/phases recording the evolution of the ecosystem before and after the deposition events: (1) Pre-extinction phase (high abundance and high diversity values). (2) Extinction phase, characterised by the complete disappearance of ostracod fauna; the foraminiferal assemblage, although not entirely absent, records extremely low values of abundance and diversity (survivor assemblage). (3) Recovery phase (increasing abundance and diversity values), characterised by the recolonisation of some opportunistic taxa; species such as Australicythere devexa and Australicythere polylyca dominate the ostracod assemblage. (4) Post-extinction phase (high abundance and high diversity values), with the return to an environmental equilibrium characterised by the colonisation of specialised taxa such as Argilloecia sp., Cytheropteron sp., Echinocythereis sp., and Hemicytherura spp. Our results may aid in the understanding of how communities (i.e., ostracods and foraminifers) recovered after the impact of direct deposits of volcanic ash into ocean waters. The mechanisms by which disappearance and/or mortality was induced are still not clear. The release of toxic metals during the reaction of the volcanic ash with seawater, the resulting chemical alteration in the seawater, and the change in pH, together with the possible suppression of planktonic organisms, may have caused the two main extinction phases recorded by the ANTA02-NW2 core sediments.

Polyplacophoran Feeding Traces on Mediterranean Pliocene Sirenian Bones: Insights on the Role of Grazing Bioeroders in Shallow-Marine Vertebrate Falls

Chitons (Polyplacophora) include some of the most conspicuous bioeroders of the present-day shallow seas. Abundant palaeontological evidence for the feeding activity of ancient chitons is preserved in the form of radular traces that are usually found on invertebrate shells and hardgrounds. We report on widespread grazing traces occurring on partial skeletons of the extinct sirenian Metaxytherium subapenninum from the Lower Pliocene (Zanclean) of Arcille (Grosseto Province, Tuscany, Italy). These distinctive ichnofossils are described under the ichnotaxonomic name Osteocallis leonardii isp. nov. and interpreted as reflecting substrate scraping by polyplacophorans. A scrutiny of palaeontological literature reveals that similar traces occur on fossil vertebrates as old as the Upper Cretaceous, suggesting that bone has served as a substrate for chiton feeding for more than 66 million years. Whether these bone modifications reflect algal grazing, carrion scavenging or bone consumption remains unsure, but the first hypothesis appears to be the most parsimonious, as well as the most likely in light of the available actualistic data. As the role of bioerosion in controlling fossilization can hardly be overestimated, further research investigating how grazing organisms contribute to the biostratinomic processes affecting bone promises to disclose new information on how some marine vertebrates manage to become fossils.

Palaeontological evidence for community-level decrease in mesopelagic fish size during Pleistocene climate warming in the eastern Mediterranean.

Mesopelagic fishes are an important element of marine food webs, a huge, still mostly untapped food resource and great contributors to the biological carbon pump, whose future under climate change scenarios is unknown. The shrinking of commercial fishes within decades has been an alarming observation, but its causes remain contended. Here, we investigate the effect of warming climate on mesopelagic fish size in the eastern Mediterranean Sea during a glacial-interglacial-glacial transition of the Middle Pleistocene (marine isotope stages 20-18; 814-712 kyr B.P.), which included a 4°C increase in global seawater temperature. Our results based on fossil otoliths show that the median size of lanternfishes, one of the most abundant groups of mesopelagic fishes in fossil and modern assemblages, declined by approximately 35% with climate warming at the community level. However, individual mesopelagic species showed different and often opposing trends in size across the studied time interval, suggesting that climate warming in the interglacial resulted in an ecological shift toward increased relative abundance of smaller sized mesopelagic fishes due to geographical and/or bathymetric distribution range shifts, and the size-dependent effects of warming.

The Neo-Gouldian Argument for Evolutionary Contingency: Mass Extinctions

The Gouldian argument for evolutionary contingency found in Wonderful Life can be dissected into three premises: palaeontological, macro-evolutionary, and developmental. Discussions of evolutionary contingency have revolved primarily around the developmental. However, a shift in methodological practice and new palaeontological evidence subsequent to the book’s publication appears to threaten the palaeontological premise that asserts high Cambrian disparity, or, roughly, that morphological differences between the Cambrian species were high. This presents a prima facie problem: Did the Cambrian consist of enough anatomical variety to fuel a different outcome upon a replay of the tape of life? I investigate the role that disparity plays in the Gouldian argument for evolutionary contingency and conclude that the reduced disparity view does not undermine the argument, per se. As for the macro-evolutionary premise, Gould never offers any explicit justification for the idea that species sorting was indiscriminate or ‘lottery-like’ (though he certainly gestures at it). Putting aside the exegetical, I consider certain empirical avenues for underwriting the macro-evolutionary premise and argue that the most plausible route is to appeal to the special status of mass extinctions, a move that is not entirely unfamiliar to macro-evolutionists. That is, mass extinctions can be good sources of contingency due to their being ‘chancy’ in at least three respects. In sum, I update the palaeontological and macro-evolutionary premises as to reconstrue the Gouldian argument for evolutionary contingency in its strongest form.

Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization.

Simple SummaryThe transition of life from the aquatic realm onto land represented one of the fundamental episodes in the evolution of the Earth that laid down the foundations for modern ecosystems as we know them today. This key transition in the history of life is poorly known, owing to the scarcity of ancient terrestrial fossil deposits; complex terrestrial ecosystems with plants and animals appear in the fossil record during the Silurian and Devonian. However, recent molecular clock studies and new lines of palaeontological evidence point to a possibly much earlier origin of life on land, dating back as far as the Cambrian. Here, we review this controversy, using the arthropods as a case study of the possible cryptic Cambrian explosion on land. In particular, we highlight approaches for reconciling the disagreement between molecular clock estimates and the fossil record for the arthropod colonization of land.Arthropods, the most diverse form of macroscopic life in the history of the Earth, originated in the sea. Since the early Cambrian, at least ~518 million years ago, these animals have dominated the oceans of the world. By the Silurian–Devonian, the fossil record attests to arthropods becoming the first animals to colonize land, However, a growing body of molecular dating and palaeontological evidence suggests that the three major terrestrial arthropod groups (myriapods, hexapods, and arachnids), as well as vascular plants, may have invaded land as early as the Cambrian–Ordovician. These dates precede the oldest fossil evidence of those groups and suggest an unrecorded continental “Cambrian explosion” a hundred million years prior to the formation of early complex terrestrial ecosystems in the Silurian–Devonian. We review the palaeontological, phylogenomic, and molecular clock evidence pertaining to the proposed Cambrian terrestrialization of the arthropods. We argue that despite the challenges posed by incomplete preservation and the scarcity of early Palaeozoic terrestrial deposits, the discrepancy between molecular clock estimates and the fossil record is narrower than is often claimed. We discuss strategies for closing the gap between molecular clock estimates and fossil data in the evolution of early ecosystems on land

Discovery of Early Cretaceous Sinuiju Biota in the Democratic People's Republic of Korea and comparison with Jehol Biota in China

In recent years, some important fossils of the Jehol Biota of Early Cretaceous age in north-eastern China have been recorded from the Lower Cretaceous Sinuiju Formation in the Sinuiju area (north-western part of the Democratic People's Republic of Korea. This discovery indicates that the biota from the Sinuiju Formation can be related to those from the Yixian Formation in western Liaoning Province, China. The palaeontological evidence in the Sinuiju Basin shows that the Jehol Biota is more widely distributed than previously thought and the fossils composing the Jehol Biota include a greater variety of animal and plant taxa than earlier known. In previous studies, the age of the Sinuiju Formation was considered to be Late Jurassic–earliest Cretaceous on the basis of limited fossil evidence. In contrast, it is now considered to be earliest Cretaceous based on the new fossil finds. During a recent field survey, abundant fossils were collected from the lacustrine deposits of this formation. The discovery of the Sinuiju Biota adds significant new data to the study of the geographical distribution and diversity of the Jehol Biota in north-eastern Asia.

The origin and early evolution of arthropods.

The rise of arthropods is a decisive event in the history of life. Likely the first animals to have established themselves on land and in the air, arthropods have pervaded nearly all ecosystems and have become pillars of the planet's ecological networks. Forerunners of this saga, exceptionally well-preserved Palaeozoic fossils recently discovered or re-discovered using new approaches and techniques have elucidated the precocious appearance of extant lineages at the onset of the Cambrian explosion, and pointed to the critical role of the plankton and hard integuments in early arthropod diversification. The notion put forward at the beginning of the century that the acquisition of extant arthropod characters was stepwise and represented by the majority of Cambrian fossil taxa is being rewritten. Although some key traits leading to Euarthropoda are indeed well documented along a diversified phylogenetic stem, this stem led to several speciose and ecologically diverse radiations leaving descendants late into the Palaeozoic, and a large part, if not all of the Cambrian euarthropods can now be placed on either of the two extant lineages: Mandibulata and Chelicerata. These new observations and discoveries have altered our view on the nature and timing of the Cambrian explosion and clarified diagnostic characters at the origin of extant arthropods, but also raised new questions, especially with respect to cephalic plasticity. There is now strong evidence that early arthropods shared a homologous frontalmost appendage, coined here the cheira, which likely evolved into antennules and chelicerae, but other aspects, such as brain and labrum evolution, are still subject to active debate. The early evolution of panarthropods was generally driven by increased mastication and predation efficiency and sophistication, but a wealth of recent studies have also highlighted the prevalent role of suspension-feeding, for which early panarthropods developed their own adaptive feedback through both specialized appendages and the diversification of small, morphologically differentiated larvae. In a context of general integumental differentiation and hardening across Cambrian metazoans, arthrodization of body and limbs notably prompted two diverging strategies of basipod differentiation, which arguably became founding criteria in the divergence of total-groups Mandibulata and Chelicerata. The kinship of trilobites and their relatives remains a source of disagreement, but a recent topological solution, termed the 'deep split', could embed Artiopoda as sister taxa to chelicerates and constitute definitive support for Arachnomorpha. Although Cambrian fossils have been critical to all these findings, data of exceptional quality have also been accumulating from other Palaeozoic Konservat-Lagerstätten, and a better integration of this information promises a much more complete and elaborate picture of early arthropod evolution in the near future. From the broader perspective of a total-evidence approach to the understanding of life's history, and despite persisting systematic debates and new interpretative challenges, various advances based on palaeontological evidence open the prospect of finally using the full potential of the most diverse animal phylum to investigate macroevolutionary patterns and processes.

New palaeontological evidence suggests an early Middle Pleistocene age for the lower levels of Sun Hole Cave, Cheddar, Somerset, UK

Sun Hole, a small fissure cave on the north side of Cheddar Gorge, Somerset, is best known as the site where late Upper Palaeolithic artefacts are associated with human and other mammal remains that immediately pre-date the Lateglacial interstadial. These remains from the upper levels (Unit I) overlie sediments (Unit II) that are thought to have accumulated during a full glacial period, below which are sediments (Unit III) attributed to an interglacial stage. The extinct land snail Retinella (Lyrodiscus) sp., which in Britain was only known from the Hoxnian Stage, had previously been recovered from Unit III. The interglacial at the base of the sequence was therefore assigned to the Hoxnian Stage (Marine Isotope Stage (MIS) 11, 424–374 kyr). Here we provide additional analyses of samples from Unit III, which as well as containing further Retinella (Lyrodiscus), also yielded a specimen of Monachoides incarnatus, its first record from the British Pleistocene. An associated assemblage of small mammals included a few that are rare in the British Pleistocene, such as birch mouse (Sicista cf. betulina) and garden dormouse (Eliomys quercinus). The vole Lasiopodomys gregaloides and the shrew Sorex (Drepanosorex) savini do not occur as late as the Hoxnian and therefore suggest an older age in the early Middle Pleistocene. The record from Sun Hole therefore has parallels with the well-known sequence from the upper Calcareous Member in the nearby cavern system at Westbury-sub-Mendip.

New dustywings (Insecta: Neuroptera: Coniopterygidae) from the Miocene Zhangpu amber

We describe the first two dustywing species (Neuroptera: Coniopterygidae) from the Miocene Zhangpu amber. One of them is confirmed to be a new species of the genus Heteroconis Enderlein, 1905 (Aleuropteryginae), namely Heteroconis zhangpuensis n. sp., while the other belongs to the genus Coniopteryx Curtis, 1834 (Coniopteryginae) but could not be identified to species. The new Heteroconis species resembles some extant congeners from China and Thailand. These East Asian Miocene dustywings provide valuable palaeontological evidence for understanding the evolution of Coniopterygidae as well as the history of the diversification of the Asian lacewings.

Calibrating phylogenies assuming bifurcation or budding alters inferred macroevolutionary dynamics in a densely sampled phylogeny of bivalve families.

Analyses of evolutionary dynamics depend on how phylogenetic data are time-scaled. Most analyses of extant taxa assume a purely bifurcating model, where nodes are calibrated using the daughter lineage with the older first occurrence in the fossil record. This contrasts with budding, where nodes are calibrated using the younger first occurrence. Here, we use the extensive fossil record of bivalve molluscs for a large-scale evaluation of how branching models affect macroevolutionary analyses. We time-calibrated 91% of nodes, ranging in age from 2.59 to 485 Ma, in a phylogeny of 97 extant bivalve families. Allowing budding-based calibrations minimizes conflict between the tree and observed fossil record, and reduces the summed duration of inferred 'ghost lineages' from 6.76 billion years (Gyr; bifurcating model) to 1.00 Gyr (budding). Adding 31 extinct paraphyletic families raises ghost lineage totals to 7.86 Gyr (bifurcating) and 1.92 Gyr (budding), but incorporates more information to date divergences between lineages. Macroevolutionary analyses under a bifurcating model conflict with other palaeontological evidence on the magnitude of the end-Palaeozoic extinction, and strongly reduce Cenozoic diversification. Consideration of different branching models is essential when node-calibrating phylogenies, and for a major clade with a robust fossil record, a budding model appears more appropriate.

Evidence for Pleistocene gene flow through the ice-free corridor from extinct horses and camels from Natural Trap Cave, Wyoming

Natural Trap Cave (Bighorn Mountains, Wyoming) preserves an abundance of fossil remains from extinct Late Pleistocene fauna and is situated near a past migration route that likely connected populations in Eastern Beringia and the contiguous US—the ice-free corridor between the Cordilleran and Laurentide icesheets. Some palaeontological evidence supports a correspondingly high affinity between fauna recorded in Natural Trap Cave and Eastern Beringia versus elsewhere in the contiguous US, but this hypothesis has not yet been extensively tested using genetic data. In the present study, we analysed 16 horse specimens and one camel specimen from Natural Trap Cave. Of the horse specimens we analysed, we obtained 10 unique and previously unreported mitochondrial haplotypes belonging to two distinct (extinct) genetic clades—two haplotypes corresponded to a caballine horse (Equus sp.) and eight corresponded to the stilt-legged horse (Haringtonhippus francisci). With only one exception, it appears these newly sequenced individuals all shared a common ancestor more recently with Eastern Beringian individuals than with others from the contiguous US. In addition, mitochondrial data from a specimen assigned to Camelops sp. revealed that it shares a closer affinity with specimens from the Yukon Territory than those from Idaho or Nevada, though all appear to belong to a single species (“yesterday's camel”; Camelops cf. hesternus). Together, these results are consistent with a high level of genetic connectivity between horse and camel populations in the Bighorn Mountains and Eastern Beringia during the Pleistocene.

PALYNOLOGY OF THE PERMIAN OF THE MAKHTESH QATAN-2, RAMON-1 AND BOQER-1 BOREHOLES ARQOV FORMATION, NEGEV, ISRAEL

Palynological assemblages from cores 11 to 14 of Makhtesh Qatan-2, core 3 of Ramon-1 and core 3 of Boqer-1 boreholes from the Arqov Formation of the subsurface of the Negev, southern Israel, suggest that at least part of the Arqov Formation can be characterised by Cedripites priscus, Reduviasporonites chalastus and particularly Pretricolpipollenites bharadwajii, while the Saad Formation contains a slightly less diverse assemblage lacking the three taxa above. Palynological evidence is broadly consistent with other palaeontological evidence suggesting that the Saad Formation is in part likely to be Wuchiapingian in age, and the Arqov Formation is at least in part Changhsingian. These conclusions are tentative because core data is restricted to very few well penetrations and a total lack of surface exposure of the Permian.

Through a glass darkly, but with more understanding of arthropod origin

The rise of arthropods is a decisive event in the history of life. Likely the first animals to have established themselves on land and in the air, arthropods have pervaded nearly all ecosystems and have become pillars of the planet’s ecological networks. Forerunners of this saga, exceptionally well-preserved Palaeozoic fossils recently discovered or re-discovered thanks to new approaches and techniques have elucidated the precocious appearance of extant lineages at the onset of the Cambrian explosion, and pointed to the critical role of the plankton and hard integuments in early arthropod diversification. Despite new interpretative challenges, phylogenetic advances based on palaeontological evidence open the prospect of finally using the full potential of the most diverse animal phylum to investigate macroevolutionary patterns and processes.

Dry corridors opened by fire and low CO2 in Amazonian rainforest during the Last Glacial Maximum

The dynamics of Amazonian rainforest over long timescales connect closely to its rich biodiversity. While palaeoecological studies have suggested its stability through the Pleistocene, palaeontological evidence indicates the past existence of major expansions of savannah and grassland. Here we present integrated modelling evidence for a grassier Neotropics during the Last Glacial Maximum, congruent with palaeoecological and biological studies. Vegetation reconstructions were generated using the land processes and exchanges model, driven by model reconstructions of Last Glacial Maximum climate, and compared with palynological data. A factorial experiment was performed to quantify the impacts of fire and low CO2 on vegetation and model–data agreement. Fire and low CO2 both individually and interactively induced widespread expansion of savannah and grassland biomes while improving model–data agreement. The interactive effects of fire and low CO2 induced the greatest ‘savannafication’ of the Neotropics, providing integrated evidence for a number of biogeographically relevant open vegetation formations, including two dry corridors (paths of savannah and grassland through and around Amazonia that facilitated major dispersal and evolutionary diversification events). Our results show a bimodality in tree cover that was driven by fire and further enhanced by ‘CO2 deprivation’, which suggests biome instability in this region of climate space. Lower CO2 and more-frequent fires may have supported grassland expansion in the Amazon during the Last Glacial Maximum, according to vegetation modelling using a range of boundary conditions tested against existing pollen records.

An unpredicted ancient colonization of the West Indies by North American rodents: dental evidence of a geomorph from the early Oligocene of Puerto Rico

AbstractThe West Indies are a natural laboratory for the study of biogeography and evolution, especially for mammals. The modality of their arrival in the Caribbean islands is a highly controversial issue, and palaeontological evidence remains particularly elusive. Lower Oligocene deposits of Puerto Rico (c. 29.5 Ma) have recently yielded the oldest archipelago rodents, with teeth of two chinchilloid caviomorphs of South American origin. Here we report the discovery of additional dental remains from the same level, testifying to the occurrence of another rodent group. Their morphology is singular but otherwise characteristic of North/Central American geomyin geomorphs (including extant kangaroo rats, pocket mice, and pocket gophers), and leads us to propose here a new taxon: Caribeomys merzeraudi gen. et sp. nov. The evidence of geomorphs in the West Indies challenges the view that all extinct and extant Antillean rodents derived from South American sources. The morphological singularity of this Puerto Rican geomorph also suggests an early split of this insular lineage from coeval mainland counterparts, and its dispersal towards the West Indies sometime around the Eocene–Oligocene transition. This unexpected rodent adds to the few cases of Antillean terrestrial vertebrates of North American origins, such as solenodonotan eulipotyphlans. In the absence of subaerial land connections between both landmasses, an overwater dispersal may explain mid‐Palaeogene colonization of the West Indies by geomorph rodents.

The stratigraphy of the Kimmeridge Clay Formation (Jurassic) of the Vale of Pickering, Yorkshire, UK

The Upper Jurassic Kimmeridge Clay Formation (KCF) underlies much of the Vale of Pickering where it is almost wholly concealed by the Cretaceous Speeton Clay Formation and Quaternary deposits. There are few KCF inland or coastal exposures in Yorkshire with the result that the succession was stratigraphically poorly known until the 1970s oil crisis when the British Geological Survey drilled continuously cored boreholes at Marton and Reighton to examine the formation as a possible source of hydrocarbons. These were supplemented in 1987 by continuously cored boreholes drilled at Marton, Reighton, Ebberston and Flixborough by the Institut Français du Pétrole for hydrocarbons research. Taken together, the boreholes have enabled the lithological, palaeontological, geochemical and geophysical characters of the full thickness of the formation to be examined. Comparison of the KCF successions proved in Yorkshire with those in the adjacent North Sea, the East Midlands and the Dorset coast type area, shows marked variations in thickness related to penecontemporaneous faulting. However, there are only minor variations in the lithologies and faunas at any particular stratigraphical level. This appears to be due to a combination of Milankovitch-driven climatic fluctuations and pulsed variations in sea level which combined to produce similar depositional conditions throughout the English KCF at any one time. The chronostratigraphical classification of the KCF developed in southern England has therefore been shown to be applicable to the Yorkshire outcrop and the southern North Sea. The changes in sea level may be eustatic rather than regional events, but there is insufficient palaeontological evidence to enable them to be correlated with confidence with those of the standard Jurassic sea-level curve.