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bioRxiv2025

Reefal regions were biodiversity hotspots throughout the Phanerozoic

Reefs are important hotspots of marine biodiversity today, and acted as cradles of diversification in the geological past. However, we know little about how the diversity of reef-supporting regions varied through deep time, and how this differed from other regions. We quantified regional diversity patterns in reef-supporting and non-reef-supporting regions in the fossil record of Phanerozoic marine invertebrates. Diversity in reef-supporting regions is on average two- to three-fold higher than in non-reef-supporting regions, and has been remarkably stable over timescales of tens to hundreds of millions of years. This signal is present in both reefal and non-reefal facies within reef-supporting regions, suggesting that reefs enriched diversity in surrounding environments. Sepkoskis Modern Fauna, an assemblage of higher taxa that includes gastropods, bivalves and echinoids, has been a key component of reef-supporting regions since the Paleozoic, contrasting with its later rise to dominance in non-reef-supporting regions during the later Mesozoic-Cenozoic. One-Sentence SummaryRegions of the globe that supported reefal environments have been key hotspots of marine animal diversity for over 400 million years.

paleontology↗
bioRxiv2025

Decoding Demosponge Diversity: Bayesian Analysis of Biodiversity, Extinction Events and Environmental Influences throughout the Phanerozoic

Sponges (phylum Porifera) have been essential to marine ecosystems for over 600 million years, contributing to nutrient cycling, reef building, and ecological stability. Despite their evolutionary importance, the drivers of their diversity dynamics remain poorly understood. This study investigates the diversification patterns of Demospongiae, the largest class of sponges with a well-documented fossil record throughout the Phanerozoic Eon (541-0 Ma). Using fossil occurrence data and a Bayesian framework, we modeled origination and extinction rates to understand their evolutionary history. Our findings reveal key extinction events, including three previously unrecognized events in the Cambrian, Late Silurian, and Late Jurassic, alongside known events such as the Permian-Triassic and Triassic-Jurassic extinctions. Notably, there was no statistical evidence for mass extinction during the Late Ordovician or Late Devonian. Additionally, Demosponges underwent a significant decline prior to the Cretaceous-Paleogene extinction. To explore abiotic influences, we applied the Multivariate Birth-Death (MBD) model across all extinction events. This analysis identified correlations between major speciation and extinction events and key variables, including temperature, continental fragmentation, oxygen levels, and sea level, as well as geochemical proxies such as sulfur and strontium, which may reflect underlying drivers like anoxia, weathering, or tectonic activity. Temperature and oxygen levels, in particular, correlated with speciation and extinction during the Permian-Triassic extinction. These findings underscore the role of environmental fluctuations in shaping demosponge diversity and highlight the complex interplay between changes in abiotic factors and sponge evolution. Our work provides critical insights into the factors driving sponge biodiversity and offers perspectives on how modern sponges might respond to ongoing climate change. Significance StatementSponges are ancient and ecologically pivotal marine organisms that have persisted through Earths major environmental upheavals. Despite their resilience, the mechanisms driving their evolutionary responses to past climate shifts remain unclear. By leveraging the fossil record of Demospongiae, this study identifies previously unrecognized extinction events and elucidates how abiotic factors, including temperature and oxygen levels, shaped their diversity during these crises. These findings enhance our understanding of sponge evolutionary dynamics and resilience, offering critical insights into the vulnerability of marine ecosystems under current and future climate change scenarios.

paleontology↗
bioRxiv2025

The lower jaw of Devonian ray-finned fishes (Actinopterygii): anatomy, relationships, and functional morphology

Actinopterygii is a major extant vertebrate group, but limited data are available for its earliest members. Here we investigate the morphology of Devonian actinopterygians, focusing on the lower jaw. We use X-Ray Computed Tomography (XCT) to provide comprehensive descriptions of the mandibles of 19 species, which span the whole of the Devonian and represent roughly two thirds of all taxa known from more than isolated or fragmentary material. Our findings corroborate previous reports in part but reveal considerable new anatomical data and represent the first detailed description for roughly half of these taxa. The mandibles display substantial variation in size, spanning more than an order of magnitude. Although most conform to a generalized pattern of a large dentary and one or two smaller infradentaries, XCT data reveal significant differences in the structure of the jaw and arrangement of teeth that may be of functional relevance. We report the presence of a rudimentary coronoid process in several taxa, contributed to by the dentary and/or infradentaries, as well a raised articular region, resulting in a mandible with an offset bite and that functions as a bent level arm. Among the most striking variation is that of tooth morphology: several taxa have heterodont dentary teeth that vary in size and orientation, and multiple variations on enlarged, whorl-like and posteriorly-oriented anterior coronoid dentition are observed. We use these new data to revise morphological characters that may be of phylogenetic significance and consider the possible functional implications of these traits. The observed variation in mandible form and structure suggests previously unappreciated functional diversity among otherwise morphologically homogenous Devonian ray-finned fishes.

paleontology↗
bioRxiv2025

Reassessing Deep Learning (and Meta-Learning) Computer Vision as an efficient method to determine taphonomic agency in bone surface modifications

Recent critiques of the reliability of deep learning (DL) for taphonomic analysis of bone surface modifications (BSM), such as that presented by Courtenay et al. (2024) based on a selection of earlier published studies, have raised concerns about the efficacy of the method. Their critique, however, overlooked fundamental principles regarding the use of small and unbalanced datasets in DL. By reducing the size of the training and validation sets--resulting in a training set only 20% larger than the testing set, and some class validation sets that were under 10 images--these authors may inadvertently have generated underfit models in their attempt to replicate and test the original studies. Moreover, errors in coding during the preprocessing of images have resulted in the development of fundamentally biased models, which fail to effectively evaluate and replicate the reliability of the original studies. In this study, we do not aim to directly refute their critique, but instead use it as an opportunity to reassess the efficiency and resolution of DL in taphonomic research. We revisited the original DL models applied to three targeted datasets, by replicating them as new baseline models for comparison against optimized models designed to address potential biases. Specifically, we accounted for issues stemming from poor-quality image datasets and possible overfitting on validation sets. To ensure the robustness of our findings, we implemented additional methods, including enhanced image data augmentation, k-fold cross-validation of the original training-validation sets, and a few-shot learning approach using model-agnostic meta-learning (MAML). The latter method facilitated the unbiased use of separate training, validation, and testing sets. The results across all approaches were consistent, with comparable--if not almost identical--outcomes to the original baseline models. As a final validation step, we used images of recently generated BSM to act as testing sets with the baseline models. The results also remained virtually invariant. This reinforces the conclusion that the original models were not subject to methodological overfitting and highlights their nuanced efficacy in differentiating BSM. However, it is important to recognize that these models represent pilot studies, constrained by the limitations of the original datasets in terms of image quality and sample size. Future work leveraging larger datasets with higher-quality images has the potential to enhance model generalization, thereby improving the applicability and reliability of deep learning approaches in taphonomic research.

paleontology↗
bioRxiv2025

Microstructural and geochemical evidence offers a solution to the cephalopod cameral deposits riddle

Orthoceratoid cephalopods are common in the Palaeozoic rock record but became extinct in the Late Triassic. Many orthoceratoids contain cameral deposits, which are enigmatic calcareous structures within their chambered shell that presumably balanced their straight conchs in a horizontal position. Since the mid-19th century, palaeontologists have attempted to understand the cameral deposit formation process. The various hypotheses include growth from cameral fluids, precipitation by a cameral mantle or even their dismissal as post-mortem structures. All of these previous interpretations have in common that they are complicated with contradictory evidence. Here, we present evidence from well-preserved Trematoceras elegans specimens from the Late Triassic St. Cassian Formation (Dolomites, northern Italy). We studied the specimens by using optical and electron beam microanalysis techniques and argue that the cameral deposits consist of primary aragonite and calcite fabrics. A fibrous microstructure, which is bilaterally symmetrically arranged with irregularities, is documented. Thin organic sheets originally delimited radial growth sectors. Based on these observations, we propose a new growth model that explicitly involves the cameral sheets. These sheets acted as an extension of the pellicle and held a thin film of supersaturated liquid in the otherwise emptied chambers by the cameral sheets via the capillary effect. Ions were supplied through the siphuncle, such as in living Nautilus, and enabled the precipitation of aragonite and calcite fabrics. This model goes beyond previous interpretations, resolves contradictory observations and has functional implications, suggesting that cameral sheets and deposits were an adaptation to increased growth rates.

paleontology↗
bioRxiv2025

Phytoliths to apPROXYmate Maya landscapes over early Anthropocene : case of Naachtun tropical wetland.

Reconstitutions of past anthromes in the tropical karstic environments of the Central Maya lowlands (CMLs) are generally restrained by bioindicators such as pollen in the rare lakes and perennial wetland systems and charcoals. This original work mobilizes phytoliths and, to a lesser extent, other bioindicators such as molluscs and diatoms. It contributes to demonstrate of these fossils potential to participate in the reconstruction of the Maya society-environment interactions, in the CMLs Elevated Interior Region. We studied swamp and lake sediment archives of a karst polje (nammed bajo El Infierno), located north of the regional Maya city of Naachtun. These bioindicators were studied on three sediment series covering the last 3500 years, during which this region was settled by the Maya. This multiproxy and multi-local approache allowed to propose an environnemental history for this wetland. It shows multi-millennial anthropogenic impacts in the bajo. The local opening of the swamp forest suggests the occupation of this bajo during the early Preclassic. The bioindicators also provide information on the variability of lake conditions, marked in particular by perennial shallow lake dynamics during the middle Preclassic, which provided a resource for the Maya. The direct trace of maize agriculture and pervasive herbaceous formations through the bajo are finally noted during the late Preclassic. Demographical and political disruptions occurred to Naachtun during the Preclassic-Classic transition, simultaneous to regional droughts. The Classic is marked by the presence of an environmental mosaic which may indicate a change in bajo land use. This landscape diversity demonstrates a new time the need to carry out multi-local paleoecological approaches to understand the society-environment interactions. Phytoliths, by their siliceous mineralization and local expression, seem appropriate to become an essential tool for these approaches in the CMLs.

paleontology↗
bioRxiv2025

Spatial patterns of trematode-induced pits on bivalve skeletons: Challenges and prospects for research on parasite-host dynamics

Interactions between the parasitic larvae of digenean trematodes (mainly gymnophallids) and bivalves often result in characteristic shell malformations, i.e., pit-like traces. Tracking these traces through the Holocene and modern marine death assemblages has made studying parasite-host responses to natural and anthropogenic environmental change possible. Despite major breakthroughs, empirical explorations of parasite-host dynamics in the geological record are primarily based on trace occurrence data, overlooking that trace spatial patterns on the host skeleton could carry ecological information and potentially document different aspects of the parasite-host interactions (e.g., infective behavior, association with specific host anatomy, spatial relationships of traces with different qualitative properties such as size class, etc.). The Spatial Point Pattern Analysis of Traces (SPPAT) has been increasingly employed to overcome similar challenges in studying predatory traces on bivalve prey. Although this approach holds considerable promise for research on trematode-host dynamics, several assumptions and caveats need to be considered (e.g., the number of traces required to capture the parasite-host dynamics accurately, the reliability of point patterns constructed from multiple host skeletons in describing parasite interactions). Here, we introduce a spatially explicit framework for extracting information from spatial patterns of trematode-induced pits on bivalve shells using SPPAT, address methodological questions involved in assembling a point pattern of traces from multiple host specimens, and discuss critical issues related to drawing inferences from pooled point data. We illustrate our approach using a case study on late Holocene samples of the commercially relevant bivalve Chamelea gallina from the northern Adriatic of Italy. This species holds high value in the seafood industry and is increasingly used in climate change research. Our results reveal that trematode-induced malformations on bivalve shells are not random; they show an aggregated pattern for metacercaria traces of the same size classes, while an independent pattern arises when examining traces of two distinct size classes. This study highlights the potential value of spatial information from parasite-induced traces in enhancing our understanding of parasite-host dynamics over time.

paleontology↗
bioRxiv2025

Three-dimensional fossils of a Cretaceous collared carpet shark (Parascylliidae, Orectolobiformes) shed light on skeletal evolution in galeomorphs

A rich fossil record of teeth shows that many living shark families origins lie deep in the Mesozoic. Skeletal fossils of the sharks to whom these teeth belonged are far rarer and when they are preserved are often flattened, hindering understanding of the evolutionary radiation of living shark groups. Here we use computed tomography to describe two articulated Upper Cretaceous shark skeletons from the Chalk of the UK preserving three- dimensional neurocrania, visceral cartilages, pectoral skeletons and vertebrae. These fossils display skeletal anatomies characteristic of the Parascylliidae, a family of Orectolobiformes now endemic to Australia and the Indo Pacific. However, they differ in having a more heavily mineralised braincase and a tri-basal pectoral fin endoskeleton, while their teeth can be attributed to a new species of the problematic taxon Pararhincodon. Phylogenetic analysis of these new fossils confirms that Pararhincodon is a stem-group parascylliid, providing insight into the evolution of parascylliids distinctive anatomy during the late Mesozoic-Cenozoic shift in orectolobiform biodiversity from the Northern Atlantic to the Indo Pacific. Meanwhile both Pararhincodon and extant parascylliids have a distinctive vertebral morphology previously described only in Carcharhiniformes, contributing a skeletal perspective to the picture emerging from macroevolutionary analyses of coastal, small-bodied origins for galeomorphs.

paleontology↗
bioRxiv2025

Prototaxites was an extinct lineage of multicellular terrestrial eukaryotes

Prototaxites was the first giant organism to live on the terrestrial surface, reaching sizes of 8 metres in the Early Devonian. However, its taxonomic assignment has been debated for over 165 years1-7. Tentative assignments to groups of multicellular algae or land plants1,2,8-11 have been repeatedly ruled out based on anatomy and chemistry5,7,11-16 resulting in two major alternatives: Prototaxites was either a fungus5,6,17-22 or a now entirely extinct lineage 7,16,23. Recent studies have converged on a fungal affinity5-7,17-20,22. Here we test this by contrasting the anatomy and molecular composition of Prototaxites with contemporary fungi from the 407-million-year-old Rhynie chert. We report that Prototaxites taiti was the largest organism in the Rhynie ecosystem and its anatomy was fundamentally distinct from all known extant or extinct fungi. Furthermore, our molecular composition analysis indicates that cell walls of P. taiti include aliphatic, aromatic, and phenolic components most similar to fossilisation products of lignin, but no fossilisation products characteristic of chitin or chitosan, which are diagnostic of all groups of extant and extinct fungi, including those preserved in the Rhynie chert. We therefore conclude that Prototaxites was not a fungus, and instead propose it is best assigned to a now entirely extinct terrestrial lineage.

paleontology↗
bioRxiv2025

Sir John Murray's H.M.S. Challenger Sedimentary Deposits Collection at the Natural History Museum, London

The Natural History Museum holds 4,718 bottles, tubes, boxes, and slides containing sediment and derived preparations from 80% of the 504 listed marine collecting localities of H.M.S. Challenger (1873-1876). The sediment collections include manganese nodules, pumice, and various marine vertebrates and invertebrates, predominantly the Foraminifera from all the major oceans of the world. A complete online database of the collection is presented for the first time to include all of the associated collecting and preparation data from the oceanic dredges, trawls, soundings and anchor mud collections alongside a set of previously undocumented shallower water collections that were not included in the Challenger Deep Sea Deposits Report of Murray and Renard (1891). The dataset allows assessment of various other Challenger sediment derived collections dispersed worldwide in museums and private collections. A historical account of the collection follows; collecting and studying onboard, studying in Edinburgh post cruise, donation by Sir John Murrays family to the British Museum (Natural History) and accounts of the collections historical housing at the museum. The historical context and presented dataset are intended to aid future users of the collection to select suitable materials for study or display. A review of previous scientific and exhibition uses of the collection is followed by suggestions for the future in terms of storage and potential for illuminating the debate on anthropologically driven climate, biodiversity and atmospheric changes.

paleontology↗
bioRxiv2025

Non-destructive radiocarbon dating of bone

Since the 1950s, radiocarbon measurements have anchored archaeological chronologies dating back to 50,000 years, with bone collagen being a commonly dated material. Despite advances in collagen extraction protocols, the process remains destructive, requiring sampling by sawing, drilling or crushing of dry bone, that can damage or destroy physical features and archaeological evidence, and often consumes the entire specimen. While non-destructive approaches have recently been applied in ancient genomics and palaeoproteomics, no equivalent approach has been established for radiocarbon dating of bone. Here, we outline a non-destructive collagen extraction workflow that avoids invasive sampling (cutting or drilling) and produces no visible damage to the bone. By heating the bone in hot water for several hours, collagen is solubilized, and the resulting collagenous solution can be purified and AMS dated. Here we show that the amino acid composition, C/N atomic ratios, {delta}13C and {delta}15N values of the hot-water-extracted collagen are comparable to collagen isolated from the same bones using classic decalcification methodologies. The hot water extraction method was tested on various bones, ranging from the Bronze Age to the Middle and Upper Paleolithic periods, which had been dated previously using routine destructive methods that involved acid demineralization. Our results show that non-destructive collagen extraction, coupled with an additional purification step e.g., ultrafiltration, yields identical radiocarbon ages to those obtained via the routine destructive methods.

paleontology↗
bioRxiv2025

Insights on Chamelea gallina growth dynamics from the Holocene climate optimum in the Northern Adriatic Sea (Italy)

The fossil record documents ecological responses to past climate transitions, providing insights into future scenarios of marine ecosystems and taxa under climate change. In this study, we compared shell growth dynamics, specifically linear extension and net calcification rates, of the bivalve Chamelea gallina between Northern Adriatic Sea (NAS) assemblages from the Holocene Climate Optimum (HCO, 9-5 kyr cal. B.P.) and today. This species is a valuable economic resource, currently threatened by climate warming and numerous anthropogenic stressors. During the HCO, regional sea surface temperatures were warmer than today, making it a potential analog for exploring ecological responses to increasing seawater temperatures predicted in the coming decades. By combining standard aging methods with reconstructed sea surface temperatures, we observed a significant reduction in linear extension and net calcification rates in warmer HCO assemblages. During the HCO, immature C. gallina specimens developed a denser shell, especially during their early growth stages, at the expense of a linear extension rate, which was lower than modern specimens. This resulted in an average delay of 3-4 months in reaching sexual maturity, which is currently reached after ~ 13-14 months or at a length of ~18 mm. Several environmental factors are probably responsible for these differences between fossil and modern assemblages. Temperature indirectly impacted the geomorphologic evolution of the NAS over the Early-Mid Holocene, shaping the outlet structure of the Po and other NAS rivers. This, in turn, affected environmental factors such as nutrient load, seawater transparency, salinity, and phytoplankton. In addition, recent anthropogenically-derived decreases in natural NAS predators of infaunal bivalves may have reduced the natural predation pressure on modern C. gallina, thus favoring those populations characterized by faster linear extension rates (at the expense of a higher shell density), especially during the initial stages of life, hence facilitating a quicker attainment of sexual maturity.

paleontology↗
bioRxiv2025

3D Focused Ion Beam Microscopy of Fossilized Albertosaurus sarcophagus reveals Nano to Microscale Structures

Osteohistological investigations of fossilized bone can reveal details about the specimens biological, geological and environmental conditions. Micro-to-nanoscale imaging provides insight into the structural organization of bone and can also reveal indicators of the fossilization process. We examined a petrographic thin section of the left fibula of a [~]71.5 million-year-old Albertosaurus sarcophagus (Canadian Museum of Nature [CMN] catalogue number FV 11315) using nanoscale scanning electron microscopy (SEM) and focused ion beam (FIB)-SEM tomographic imaging to study the arrangement of mineral and organic components of fossil bone in multidimensions. Here, we present evidence of permineralization in Haversian canals by energy dispersive X-ray spectroscopy. Nanoscale 3D FIB-SEM imaging revealed that the characteristic 67 nm banding periodicity of collagen fibrils was remarkably well preserved over 70M years, and 3D imaging allowed for the detection of collagen fibril bundles in parallel fibered and lamellar bone arrangements. A newly discovered structure in modern bone, the ellipsoidal mineral cluster, was tiled throughout the 3D space of fibrolamellar fossil bone. These observations, afforded by the high-resolution and site-specific nature of FIB-SEM, link key fossilized features with the micro-nanoscale structure of modern-day bone. This investigation highlights the persistence of bone formation and organization persisting for over millions of years.

paleontology↗
bioRxiv2025

Diversification dynamics of the Palaeozoic actinopterygian radiation

Ray-finned fish are the most speciose vertebrate group today, but the dynamics of their early diversification are contentious. Their fossil record suggests a first radiation in the Carboniferous following the Late Devonian mass extinction events. Conversely, recent phylogenetic hypotheses imply a radiation originating in the Late Devonian, but lack the taxonomic breadth required to robustly test this. This necessitates phylogeny-free inference of actinopterygian diversification rates from fossil occurrences, itself challenging due to complex systematics, incomplete occurrence databases, and severe spatiotemporal sampling biases. Here, we analyse a comprehensive dataset of Palaeozoic actinopterygian genera and species using approaches that accommodate spatial and temporal sampling variation. We detect elevated actinopterygian diversification in the Late Devonian, with substantially greater lineage survival across the Hangenberg extinction event than indicated by the raw fossil record. Surprisingly, we detect no positive shifts in origination across the event, refuting previous hypotheses of explosive actinopterygian radiation in its wake. Instead, cryptic survival of diversified lineages appears responsible for the robust signal of increased diversity across different geographic scales in the Carboniferous. Nonetheless, these trends are overwhelmingly driven by low palaeolatitude Euramerican fossil assemblages, highlighting the ongoing spatial limitations of the actinopterygian fossil record.

paleontology↗
bioRxiv2025

Cryptovaranoides is not a squamate

Accurate reconstruction of the timescale of organismal evolution requires placement of extinct representatives among living branches. In this way, the fossil record has the capacity to revise hypotheses of organismal evolution by producing representatives of clades that far pre-date the age of the clade inferred using phylogenies built from molecular data and previous fossil calibrations. Recently, one fossil with the potential to drastically change current understanding surrounding the timescale of reptile diversification was described from Triassic fissure-fill deposits in the United Kingdom. This taxon, {dagger}Cryptovaranoides microlanius, was originally placed deep within the squamate crown clade, suggesting that many lineages of living lizards and snakes must have appeared by the Triassic and implying long ghost lineages that paleontologists and molecular phylogeneticists have failed to detect using all other available data. Our team challenged this identification and instead suggested {dagger}Cryptovaranoides had unclear affinities to living reptiles, but a crown-squamate interpretation was later re-iterated by the team that originally described this species. Here, we again challenge the morphological character codings used to support a crown squamate affinity for {dagger}Cryptovaranoides microlanius and illustrate several empirical problems with analyses that find this taxon is a crown squamate. Our analyses emphasize the importance of stringency in constructing hypodigms of fossils, particularly when they may be key for proper time calibration of the Tree of Life.

paleontology↗
bioRxiv2025

The small Canidae from Cuvieri Cave (Quaternary, Pleistocene), Lagoa Santa, eastern Brazil

The Lagoa Santa region is an important karst complex, known for its large number of caves containing abundant fossil and subfossil osteological material. Among the Pleistocene fossils identified in this important area are various remains of paleovertebrates, including members of the order Carnivora. This study describes two fragmented hemimandibles of a small Canidae of the genus Speothos, found in Cuvieri Cave. The specimens likely belonged to the same individual, whose second molar had not yet erupted, indicating that it was a subadult. Although the presence of canids has previously been reported in the region, this is the first identification of Speothos in the Pleistocene deposits of Cuvieri Cave. It is believed that the animal entered the cave in search of prey, such as rodents and other mammals present at the site, and eventually became trapped within the cavity.

paleontology↗
bioRxiv2025

Tongue bite apparatus highlights functional innovation in a 310-million-year-old ray-finned fish

Gill skeleton modifications for processing prey represent a major source of functional innovation in living ray-finned fishes. Here we present the oldest actinopterygian tongue bite, derived from the gill skeleton, in the Early Pennsylvanian ([~]310 Ma) {dagger}Platysomus parvulus. Unrelated to extant tongue biters, this deep-bodied taxon possesses a large, multipartite basibranchial tooth plate opposing an upper tooth field centered on the vomer. This branchial structure occurs in conjunction with toothed jaws, indicating a role for both in feeding. {dagger}P. parvulus illustrates the assembly of the tongue bite in the geologically younger {dagger}Bobasatraniidae: large opposing dorsal (vomerine) and ventral (basibranchial) crushing plates associated with toothless jaws. The origin of tongue bites falls within the Carboniferous actinopterygian radiation, although postdates the first signs of durophagy in other ray-finned lineages by several million years. This lends support to a protracted long-fuse model of actinopterygian diversification in the aftermath of the End-Devonian Mass Extinction.

paleontology↗
bioRxiv2025

Advanced adaptive strategies in an ancestral body plan: insights from a 510-Ma-old leptomitid sponge

Sponges have thrived in diverse environmental conditions since the early Cambrian till today. However, little is known about how their adaptive capability and strategies have been shaped throughout evolutionary history. Here, we explore this question based on a new leptomitid sponge fossil from the Cambrian Stage 4. The family Leptomitidae was an abundant sponge group inhabiting Cambrian soft substrates but significantly declined thereafter. The new species exhibits a sophisticated set of morphological characteristics adaptive to a shallow siliciclastic environment, which are unprecedented among leptomitids. These include (1) a robust body wall woven by spirally twisted monaxonic spicules; (2) a thick stub-like root tuft for anchoring; (3) spicules radiating out from the sponge body to prevent clogging and sinking; and (4) the inferred capability to close the osculum against unfavourable stimuli. Nevertheless, the new fossil species maintains a leptomitid body plan which lacks modularity and morphological plasticity, the two common and critical attributes in extant sponges to enhance flexibility and resilience in changing environmental conditions. This juxtaposition of evolutionary innovation and structural conservatism offers a compelling case for further exploration of the evolutionary mechanisms that shaped early sponge lineages.

paleontology↗