Brachiopoda and Bivalvia evolved the bivalved condition independently during the early Cambrian, but they have contrasting histories, with bivalves largely replacing brachiopods in warm, shallow, post-Paleozoic seas. Overlapping hypotheses have been proposed to explain this replacement, but it remains unclear how inherent constraints and evolutionary potentials of the bivalved form in the two groups interacted with environmental changes that favoured bivalves. Here I document the origins, fates, limitations, and versatility of 55 morphological and habitat traits, of which 13 are unique to brachiopods, 23 are unique to bivalves, and 19 evolved convergently in both. Only 6.3% of brachiopod traits are post-Paleozoic as compared with 24% of bivalve traits. Extinction affected 44% of brachiopod traits but only 2% of bivalve traits. Characteristics associated with post-Paleozoic bivalve proliferation evolved much earlier, including the early Cambrian origin of the extensible foot, hinge ligament, and morphological within-valve modularity, none of which evolved in brachiopods and all of which provided early versatility in bivalves. Constraints on form were already present in early Cambrian brachiopods and were never overcome. The full potential of bivalves was realized only when more intense biological activity made Mesozoic and Cenozoic seas more productive.
Pleistocene well-sorted bioclastic calcarenites form an aeolian sand-sheet in south-western Maio Island (Cabo Verde Archipelago) that contains distinctive, large, horizontal, sinuous trace fossils attributed to the ichnogenus Cochlichnus Hitchcock, 1858. Owing to its size and morphology, this trace is described herein as a new ichnospecies, Cochlichnus grandis isp. nov. Morphometric parameters, the sedimentary context, and comparison with modern analogues indicate that the trace was most likely produced by a sand-swimming skink lizard, which was partially buried at the surface or fully subsurface within the sand. To date, traces produced by skinks swimming through loose sand have not been documented in the fossil record even if they have been studied thanks to neoichnological experiments. Preservation of C. grandis isp. nov. was facilitated by relatively high substrate moisture in a coastal aeolian setting and by early carbonate cementation – conditions that are rarely met in dry siliciclastic dune systems. This narrow taphonomic window explains the exceptional rarity of fossil traces recording sand-swimming behaviour, despite the wide distribution and long evolutionary history of skinks in arid and semi-arid regions.
Two books (Badgley C., Morgan M. E. & Pilbeam D. (eds) 2025. — At the foot of the Himalayas. Paleontology and ecosystem dynamics of the Siwalik record. Johns Hopkins University Press, Baltimore, xv + 553 p.); and Gheerbrant E. 2025. — The Rise of Mammals in Africa (illustrated by C. Letenneur and A. Lethiers). Johns Hopkins University Press, Baltimore, xvi + 265 p.) are reviewed. They are, each in their own way, fine illustrations of the vitality of an ancient science, palaeontology. They are very different. The first is a collective work on the evolution of Miocene faunas of the Siwaliks, written by specialists for specialists. The second is an introduction to the evolution of placental mammals and the role played by the African continent.
Little is known about the European Pleistocene brown bear Ursus arctos Linnaeus, 1758. To assess its ecological adaptations, the mandibular morphology of fossil U. arctos was analysed using 3D geometric morphometrics and compared to that of closely related Ursini. Landmarks for 3D digitisation of the mandible were chosen to reflect functional morphology relating to the musculus masseter. Extant and extinct Pleistocene Ursini of the species U. deningeri Reichenau, 1904, U. spelaeus Rosenm & uuml;ller, 1794, U. arctos and U. maritimus Phipps, 1774 were digitised or surface scanned. Generalised Procrustes superimposition was performed on the coordinates and allometry corrected for using a regression analysis pooled by species. Principal component analysis (PCA), canonical variate analysis (CVA), and a multivariate analysis of variance (MANOVA) were performed. PCA and CVA differentiate between species and subspecies in extant Ursini. Fossil U. arctos is similar to the extant Eurasian brown bear (U. a. arctos). The mandibular morphology of fossil brown bears from cold periods displays similarities to extant bears living in high latitude and high altitude environments, whereas the mandibular morphology of fossil brown bears from warm periods is significantly different to those from cold periods regardless of geological age. The masticatory differences between warm and cold adapted fossil brown bears, found in the present study, indicate that evolutionary flexibility may have played an important role in the survival of brown bears into the Holocene.
Since the 19th century, vertebrate fossils from the A & ccedil;u Formation (Potiguar Basin, northeastern Brazil) have received limited taphonomic attention, despite recent discoveries of various gnathostomes, including chondrichtyans, actinopterygians, dipnoans, crocodyliforms and dinosaurs. This study presents the first integrated macro-and microscale taphonomic analysis of vertebrate assemblages from selected outcrops of the A & ccedil;u Formation. A total of 134 disarticulated vertebrate specimens, mainly vertebrae and limb bones, were examined, focusing on articulation, fragmentation, weathering, abrasion, bioerosion, root marks, and diagenetic features such as mineral replacement, pore filling, and fracturing. The assemblages record variable degrees of subaerial exposure and episodic transport prior to burial, as indicated by moderate abrasion, the predominance of weathering stage 2, insect bioerosion, tooth marks, and root traces. Petrographic analyses reveal extensive carbonate and iron oxide replacement, osteon infilling, and desiccation-related microfracturing, reflecting fossilization under generally humid conditions punctuated by intervals of reduced precipitation. Integrated sedimentological and taphonomic evidence indicates that the studied deposits accumulated within an estuarine floodplain system, most consistent with the upper, fluvial-dominated sector of an estuary. The predominance of fine-grained facies with pedogenic features, interbedded sandstones, and heterolithic bedding reflects episodic sedimentation under fluctuating hydrological conditions. The fossil assemblages comprise autochthonous to parautochthonous remains, recording localized burial, limited reworking, and temporal mixing during Cretaceous depositional processes.
