Fossil Studies

The fossil record of Cretaceous avian dinosaurs in Mexico remains limited compared with that of other vertebrate groups, but provides important evidence on the distribution, taxonomy, and paleoecology of birds in southern North America during the Late Cretaceous. Here, we critically review the currently known Mexican record, integrating osteological remains from Baja California and Coahuila with avian and avian-like ichnofossils from northeastern Mexico. Confirmed skeletal records include the enantiornithine Alexornis antecedens from the Campanian La Bocana Roja Formation and Ichthyornis sp. from the Coniacian–Campanian Austin Group. The ichnological record comprises several Campanian and Maastrichtian occurrences from the Cerro del Pueblo and Las Encinas formations, including the formally recognized ichnotaxon Leptoptilostipus isp. and additional footprint morphotypes. Critical reassessment indicates that footprint morphotypes cannot be equated directly with biological taxa because substrate properties, preservation, locomotor behavior, and other extramorphological factors may influence track morphology. Nevertheless, the combined osteological and ichnological evidence documents the recurrent presence of avian trackmakers across non-marine, marine, deltaic, wetland, and marginal environments in Mexico. The Mexican record therefore provides an important southern North American perspective on Cretaceous avian distribution while also highlighting substantial preservational and sampling biases. Future progress will depend on the renewed study of existing collections, targeted recovery of small vertebrate remains, and systematic reassessment of historically important tracksites. Full article

Kozarnika Cave revealed a lengthy archaeological sequence covering the Lower, Middle, and Upper Paleolithic periods. The site is a significant milestone in understanding the various stages of early human colonization and migration into Europe. The available paleomagnetic data indicate that the lower boundary of the deposits corresponds to the Brunhes–Matuyama reversal. During the archaeological excavations, an exceptionally rich assemblage of small mammal remains was recovered. Based on small mammals, two biozones have been delineated—lower (Biozone B) and upper (Biozone A)—which correspond to the Late Biharian and Toringian, respectively. In the present study, a total of 75 species of terrestrial small mammals (Eulipotyphla, Lagomorpha, Rodentia) are analyzed in detail. One new species, Nannospalax sirakovi sp. n., is described. Based on geometric analysis of the mass material of voles (Microtina and Lagurus), the controversial taxonomic positions of some species have been clarified. It has been proven that Microtus arvalinus is a distinct species. Rapid evolution within the species Lagurus transylvanicus towards the modern species L. lagurus has been demonstrated. In all layers of Biozone B, two important guide fossils, Mimomys savini and M. blanci, occur. They are indicative of the second stage of the Early Biharian, which is considered earlier than the Brunhes/Matuyama reversal. Besides this, multiple species characteristic of the Early Pleistocene (Villanyian and the Early Biharian) have been identified (Borsodia newtoni, B. arankoides, Prolagurus pannonicus, Ungaromys nanus, Villanyia cf. exilis, Allophaiomys deucalion, A. cf. pliocaenicus, A. cf. burgondiae, Mimomys pitymyoides, M. cf. tornensis, Lagurus arankae). Although very scarce, they occur regularly in the stratigraphic assemblages of Biozone B, which is dominated by typical Late Biharian species, such as Allophaiomys gregaloides, Microtus arvalidens, M. nivalinus, M. nivaloides, and L. transylvanicus. The occurrence of Mesocricetus newtoni and Spermophilus citellus in Biozone B marks the earliest known occurrence of these species. The faunal turnover within the lower part of the deposits has been analyzed in detail. A reorganization of the fauna has resulted from the gradual disappearance of the Early Pleistocene species. Paleoecological reconstructions based on stratigraphic assemblages of small mammals suggest that the faunal turnover recorded in Biozone B is associated with paleoclimatic changes during the MIS19–MIS17 interval and reflects a reorganization of the biota related to the mid-Pleistocene transition. This period was marked by significant changes in Earth’s climate, shifting from low-amplitude 41,000-year obliquity-driven cycles to high-amplitude, roughly 100,000-year fluctuations that define the later Pleistocene and Holocene. Full article

Fossil human footprints provide direct evidence of past presence and behaviour, yet the processes governing their preservation remain poorly understood. In particular, it remains unclear whether repeated footprint-bearing surfaces reflect population abundance and occupation continuity, or instead arise through environmental and taphonomic filtering. This study addresses the problem using an agent-based model (ABM) of footprint formation and preservation within a dynamic lake-margin environment. The model simulates interactions between human activity, basin morphology, and time-varying hydrological conditions under contrasting climate forcing regimes and occupation structures. Preservation is treated as a probabilistic process occurring when wetting events coincide with sufficient footprint availability, allowing analysis within an environmental–behavioural state space. Results show that the footprint record is highly sensitive to the temporal organisation of climate variability, with different forcing regimes producing distinct preservation architectures even under comparable mean environmental conditions. Basin geometry exerts a fundamental control, with basins that are not too steep or shallow maximising shoreline mobility and preservation potential, while behavioural organisation shapes how footprints are distributed relative to preservation windows. In contrast, population size alone exerts comparatively limited influence. Comparison with artefact accumulation highlights a key contrast: artefacts integrate behaviour across time, whereas footprints are preserved episodically, capturing short-lived snapshots of activity. Footprint-bearing surfaces should therefore not be interpreted as direct proxies for population size or occupation continuity, but as discrete sampling events generated through the intersection of human activity and transient preservation opportunity. More broadly, the model suggests that fossil footprint assemblages are emergent products of coupled behavioural, geomorphic, and environmental systems, providing a framework for more robust interpretation of the human footprint record. Full article

An isolated left frontal from the Upper Cretaceous (upper Campanian) strata of the San Juan Basin in northwestern New Mexico, USA, was originally identified as a species of the dromaeosaurid (Theropoda: Dromaeosauridae) Saurornitholestes (“S.” robustus) and was subsequently identified as a troodontid (Troodontidae). A reassessment of this holotype specimen shows its distinct nature among troodontids and is here named Dinevenator, gen. nov. Dinevenator robustus differs from other troodontids, including the thick-skulled species Xenovenator espinosai from the upper Campanian Cerro del Pueblo Formation, Coahuila, Mexico. These differences include: dorsoventrally thick and robust frontals; frontals thicker posteriorly; relatively flat frontals, lacking a well-developed parasagittal trough and sagittal ridge; a posteromedial nasal projection between the frontals that is wider than long; a more laterally oriented posterior portion of the lateral endocranial wall; and a mediolaterally wider anteromedial portion of the frontal that houses the orbital fossa. A phylogenetic analysis was also run based on a modified dataset, recovering Dinevenator robustus in a clade of robust troodontins (Troodontini) as the basal-most taxon and as the sister taxon to (Albertavenator curriei + Xenovenator espinosai). The recognition of a distinct, large, robust-skulled troodontid also increases the known diversity of theropod dinosaurs from the southern United States and provides further evidence for dinosaurs that are unique to the Kirtlandian faunas of New Mexico during the Late Cretaceous. Full article

The fossil record of Miocene apes is rich, with taxon groups characteristic of each of its major subdivisions, the early, middle, and late Miocene. However, there are gaps among and within these successive radiations, both temporal and geographic, which have contributed to competing hypotheses of ape and human origins. The gaps are sometimes treated as data, informative of patterns of evolution to be discovered once filled. I review the nature of gaps in the fossil record of African apes and humans, their historic and contemporary contexts, and their relevance to the interpretation of the fossil record. I evaluate the premise that gaps in the fossil record represent missing data of in situ evolution in African hominines (African apes and humans), yet to be discovered, in contrast to direct fossil evidence of vicariance. Full article

Dinosaur footprints are among the most abundant trace fossils, but they are not direct records of anatomy, behaviour or faunal composition. They preserve locomotion, substrate interaction and occurrence data only after those signals have been filtered by foot anatomy, movement, sediment properties and preservation. Classical dinosaur ichnology has relied on two-dimensional outlines, linear and angular measurements, qualitative ichnotaxonomy and influential hypotheses about trackmaker identity, speed, social behaviour and evolutionary timing. Here we review how these hypotheses are being reassessed with three-dimensional digitisation, geometric morphometrics, supervised and unsupervised machine learning, and biomechanical simulation. We first consider how different footprint representations, including interpretive outlines, landmarks, silhouettes, depth maps and three-dimensional models, shape the questions that track data can answer. We then assess analytical approaches ranging from multivariate statistics and landmark-based classifiers to convolutional neural networks and β-variational autoencoders. Against this methodological background, we revisit four linked problem domains: ornithopod–theropod discrimination and the Grallator–Anchisauripus–Eubrontes plexus; speed and gait reconstruction; ecological and behavioural interpretations of track abundance, sauropod gauge and trackway arrangement; and macroevolutionary claims about body-size trends, functional morphotypes and avian-like pedal morphologies. Across these cases, newer methods rarely remove ambiguity. They more often show where classical interpretations are robust, where they depend on representation or prior labels, and where competing explanations remain hard to separate. We argue that footprint-based inference is strongest when tracks are treated as preservationally filtered products of anatomy, motion and substrate mechanics, and when they are integrated with skeletal data, experimental analogues and forward models in explicit, uncertainty-aware frameworks. Full article

Quantitative analysis of footprint shape is central to ichnology, yet the application of geometric morphometrics is often limited by the difficulty of defining homologous landmarks on irregular or variably preserved impressions. Here we present a polygon-based approach to footprint morphometrics in which the footprint margin is digitised as a continuous outline and resampled into a standardised set of boundary points for analysis. Using a combination of simulated perturbation experiments and empirical datasets, we assess the sensitivity of this method to annotation uncertainty and compare it to natural variation within and between trackways. The results demonstrate a clear hierarchy of variance, in which annotation noise is lower than inter-observer variability, intra-trackway variation, and between-trackway differences. These findings suggest that polygon-based representations of footprint shape are robust to realistic levels of digitisation uncertainty. Application of the method to modern and fossil trackways, including the Laetoli footprints, shows that resampled polygon outlines retain meaningful morphological structure and can be used effectively in comparative analyses. Full article

The Sichuan Basin is a pivotal region for understanding the transition and evolution of Jurassic–Cretaceous dinosaur faunas in China. Despite a rich skeletal record, the quadrupedal track record from the Penglaizhen Formation remains scarce. This study reports a new sauropod tracksite discovered near Zhoujiagou in the Shuangliu District of Chengdu, Sichuan Province. The tracks occur within the upper strata of the Lower Cretaceous Penglaizhen Formation. The tracksite preserves at least three tracks constituting a possible trackway. These medium-sized impressions are characterized by a sub-circular morphology, a weak heteropody, and a potential wide-gauge trackway pattern. Based on these morphological attributes, the tracks are preliminarily referred to the Brontopodus morphotype. This discovery fills a critical gap in the sauropod ichnological record of the Penglaizhen Formation and provides evidence suggesting that mamenchisaurid dinosaurs, or their functional equivalents, persisted in the Sichuan Basin into the Early Cretaceous. Full article

The family Otididae (bustards) is one of the oldest groups of extant neoavian birds with a very patchy fossil record and almost unknown evolutionary history. This paper describes an exceptional collection of Early Pleistocene (1.8–1.6 Ma) bustards of two taxa from the Taurida cave in Crimea (Eastern Europe), which enables the first detailed morphological characterization of the poorly known large-sized species Otis lambrechti, confirming its validity and osteological distinction from the modern Great Bustard (Otis tarda). A rich sample of the smaller taxon Tetrax kalmani shows this fossil species to be consistently larger than its modern relative, the Little Bustard (Tetrax tetrax). A comprehensive critical review of Eurasian fossil Otididae, correcting previous misidentifications, is also provided. The new fossil evidence suggests that the Early Pleistocene bustard fauna of Europe was composed of morphologically distinct taxa, which were later replaced by their modern counterparts around the Middle Pleistocene, probably as a result of the mid-Pleistocene climate transition. Full article

Animals frequently traverse a range of deformable substrates, leaving tracks. Track morphology is controlled by anatomy and motions of the foot, and substrate consistency, resulting in complex, three-dimensional structures that record dynamic foot-substrate interactions. Understanding such interactions can provide valuable biological and environmental insights, but visualising them remains challenging. Prior physical experiments explored track formation using various indenters impressed into a range of substrate types and consistencies. However, to visualise the deformation below the surface, destructive methods such as physical sectioning are often used, typically resulting in the sample being destroyed. Here, we present the methodology and challenges involved in experimentally generating tracks and introduce micro-CT scanning as a non-destructive approach to visualising sub-surface sediment movement during track formation. Two separate track volumes were produced using different substrate consistencies (‘soft’ and ‘very soft’) whereby sand and clay were alternately layered and then scanned during indentation by a cadaveric pheasant (Phasianus colchicus) foot. Across the two substrate consistencies, the CT reconstructions revealed differences in sub-surface sediment displacement, and in complex sub-surface features. Although there are challenges attributed to experimentally producing and visualising tracks, micro-CT offers a novel approach to viewing sub-surface sediment movement during experimental track formation in the lab. Full article

Numerous ichnotaxa (about 21 ichnogenera and 39 ichnospecies) have been proposed for fossil tridactyl avian footprints. The purpose of this contribution is to analyze type material from Ukraine of the first proposed tridactyl avian ichnogenus, Avipeda, and related ichnogenera from the Carpathians of Hungary and Romania (Aviadactyla, Ornithotarnocia, Passeripedia, and Carpathipeda). The criteria for erection of ichnofamilies, ichnogenera and ichnospecies of avian footprints are discussed. It is proposed that trackway features only be used for ichnospecies. The ichnofamily Avipedidae comprises essentially small leptodactylous avian footprints showing three digits directed forward, with digits united or separate proximally, and lacking web imprint. The type material of Avipeda is represented by small footprints (less than 30 mm in length) that are wider than they are long, showing large total divarication and near symmetry with respect to digit III. Examined material from the type series of Avipeda from the Miocene of Ukraine also contains tetradactyl bird footprints that are better compared with Gruipeda. Aviadactyla and Ornithotarnocia (from the Lower Miocene of Ipolytarnóc, Hungary) are considered potentially available, whereas Passeripedia is deemed unavailable. Carpathipeda (erected for material from the Miocene of Romania) is indistinguishable from Avipeda. Additional comparisons are necessary to evaluate the status of other tridactyl avian ichnotaxa. Full article

Bromalites (trace fossils produced by food processing) from the early Eocene Willwood Formation of Wyoming contain abundant glyptosaurid lizard skeletal elements. They clearly represent bromalites as they are discrete, three-dimensional, disarticulated accumulations of biological materials. Furthermore, they contain abundant skeletal elements, principally osteoderms, that exhibit no indication of digestion, so they can be identified as regurgitalites. The Willwood specimens represent orniothoregurgitalites, specifically strigilites, the regurgitalites of owls. These strigilites are assigned to Sauresus osteodermus igen. et isp. nov. This ichnotaxon is characterized by an elongate, sub-cylindrical or sub-ovoid bromalite pellet with irregular surface texture that is composed of a micritic matrix and contains abundant, well-preserved skeletal elements of glyptosaurids, principally osteoderms. The strigilites were probably produced by Primoptynx poliotauros that used its feet to kill glyptosaurids. The lizard, rather than rodent, content of the strigilite supports the hypothesis that Eocene owls were diurnal in habit and only became nocturnal in the Oligocene. Full article

An isolated fish skull from the Early Eocene Green River Lagerstätte from southwestern Wyoming, USA was tentatively identified as a regurgitalite. It consists of a skull of the perciform moronoid Mioplosus labracoides and articulated vertebrae. After mechanical preparation, the specimen was studied in detail and it could represent a regurgitalite, a pabulite (remnant of predation) or a portion of a decayed fish. Fish decay rarely results in an isolated skull, so this specimen probably pertains to a bromalite (pabulite or regurgitalite). We consider that it most likely is a regurgitalite because of the breakage of bones and displacement of elements. Full article

Thailand preserves one of the most extensive records of Mesozoic vertebrate tracks in Tropical Asia, yet these ichnological data have never been comprehensively synthesized. This review compiles and reassesses all known Triassic to Cretaceous vertebrate tracksites in Thailand to clarify their stratigraphic distribution, taxonomic diversity, and palaeobiogeographical significance. Published records, new field observations, and updated stratigraphic correlations are integrated to evaluate trackmaker attributions and temporal patterns. The Thai record documents diverse assemblages including chirotheriids, early theropods, sauropodomorphs, ornithopods, sauropods, and crocodilians. Late Triassic–Early Jurassic assemblages capture a major faunal transition, revealing the co-occurrence of non-dinosaurian archosaurs and some of the earliest dinosaurs in the region, whereas Lower Cretaceous sites are dominated by theropods, sauropods and diverse ornithopods. Comparison with other Asian ichnofaunas indicates faunal continuity across eastern Asia and supports early dinosaur dispersal into equatorial low latitudes. This synthesis also evaluates site conservation, highlighting the vulnerability of several Triassic localities and a positive trend of community-led discoveries since 2009, underscoring the need for proactive management and standardized digital documentation. Overall, the Thai ichnological succession represents the most complete Mesozoic track record presently known from Tropical Asia and provides key insights into vertebrate evolution, palaeoecology, and regional biogeography. Full article

Trackways preserve sequential records of animal locomotion and provide some of the most direct evidence of locomotor behaviour in the vertebrate fossil record. Alternating short–long pace lengths have historically been used to infer gait irregularities such as limping or lateral limb preference, but these interpretations typically lack statistical validation, treating mean asymmetry as sufficient grounds for behavioural conclusions without first establishing whether observed differences exceed normal locomotor variability. This study introduces a quantitative framework that addresses this gap by applying Welch’s t-test to pace and stride length measurements, establishing statistical confirmation of asymmetry as a prerequisite for behavioural interpretation. The framework is demonstrated on nine dinosaurian trackways drawn from published data. While all had previously been interpreted as asymmetric, seven exhibited statistically significant pace asymmetry (p < 0.05) and two did not exceed the range of normal variation. Stride length showed no significant asymmetry in any trackway, confirming that pace-level metrics are more sensitive to limb bias than stride-based measures. This framework provides an objective, reproducible standard for evaluating asymmetry claims—a necessary and feasible methodological advance for vertebrate ichnology. Full article

Mesozoic tree ferns related to the extant Loxsomopsis, a relict and taxonomically isolated genus endemic to tropical South America, are exceedingly rare, making it difficult to trace its origin and evolution. Here, we describe a mid-Cretaceous fossil attributable to Loxsomopsis, Loxsomopsis minor sp. nov., based on two fertile pinnules preserved in Myanmar amber. The new species is assigned to the extant genus based on a suite of diagnostic features, including pinnate fertile segments with short lobes, marginal paraphysate sori, narrowly cyathiform to urceolate indusia, columnar and exserted receptacles, oblique sporangial annuli, and trilete spores. This discovery extends the fossil record of Loxsomopsis back to the Albian–Cenomanian and provides new evidence that the lineage—now restricted to tropical South America—was once more widespread. The occurrence of Loxsomopsis in the Myanmar amber biota is consistent with a Gondwanan affinity for this mid-Cretaceous forest ecosystem and suggests that the genus may have likely originated prior to the breakup of Pangaea. Full article

This research provides a new comprehensive assessment of Pikimachay Cave fossil remains of extinct fauna from the Late Pleistocene and Early Holocene, curated in the Florida Museum of Natural History’s Environmental Archaeology Program collections. The collection is the result of excavations carried out by Richard MacNeish during the 1960s and 1970s, during which he proposed pre-Clovis dates for human occupation in South America. Archival records housed across three institutions were compiled to reconstruct the spatial distribution of megafauna within the cave. Taphonomic observations of the bone assemblage were conducted to better understand depositional processes and cave conditions. Based on the spatial distribution of fossil evidence mapped and documented by layer and the taphonomic evidence of the bone assemblage, we conclude that the cave functioned primarily as a giant ground sloth burrow, also used by humans and carnivores. Future research, including additional radiocarbon dates and better contextualization of the deposits, will be essential for better understanding the fossil fauna represented in the cave and the ecological relationships among humans, carnivores and extinct species in the highlands of Perú. Full article