Most downloaded biology preprints, all time
in category paleontology
256 results found. For more information, click each entry to expand.
21,383 downloads bioRxiv paleontology
Our knowledge of the reproductive biology of dinosaurs covers a range of aspects, from brooding behaviour to nesting style and the timing of sexual maturity. Yet, the basic anatomy and function of the cloaca in non-avialan dinosaurs remains unknown. Here, we describe the outer morphology of the only known non-avialan dinosaur cloaca, preserved in an exceptional specimen of the early-diverging ceratopsian dinosaur Psittacosaurus . We clarify the position of the cloaca with respect to the ischia and caudal vertebrae and document the scales immediately adjacent to the abdomen and tail. We find that the cloaca is from a near-sexually mature subadult individual and is most similar to the cloaca of crocodylians, to the exclusion of lepidosaurians and birds. However, the sex of SMF R 4970 could not be determined as the cloaca and the rest of the specimen does not yield any sexually dimorphic information. This study highlights the ongoing role of exceptional specimens in providing rare soft tissues that help to bridge longstanding gaps in our knowledge of the basic biology of dinosaurs and other extinct reptiles. ### Competing Interest Statement The authors have declared no competing interest.
5,598 downloads bioRxiv paleontology
A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur, Spinosaurus aegyptiacus, when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture, that S. aegyptiacus was the first fully aquatic dinosaur. The "aquatic hypothesis" posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow surface swimmer (<1m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail in living reptiles are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.
5,378 downloads bioRxiv paleontology
Over millennia, steppe nomadic tribes raided and sometimes overran settled Eurasian civilizations. Most polities formed by steppe nomads were ephemeral, making it difficult to ascertain their genetic roots or what present-day populations, if any, have descended from them. Exceptionally, the Khazar Khaganate controlled the trade artery between the Black and Caspian Seas in VIII-IX centuries, acting as one of the major conduits between East and West. However, the genetic identity of the ruling elite within the polyglot and polyethnic Khaganate has been a much-debated mystery; a controversial hypothesis posits that post-conversion to Judaism the Khazars gave rise to modern Ashkenazim. We analyzed whole-genome sequences of eight men and one woman buried within the distinctive kurgans of the Khazar upper (warrior) class. After comparing them with reference panels of present-day Eurasian and Iron Age populations, we found that the Khazar political organization relied on a polyethnic elite. It was predominantly descended from Central Asian tribes but incorporated genetic admixture from populations conquered by Khazars. Thus, the Khazar ruling class was likely relatively small and able to maintain a genetic identity distinct from their subjugated populations over the course of centuries. Yet, men of mixed ancestry could also rise into the warrior class, possibly providing troop numbers necessary to maintain control of their large territory. However, when the Khaganate collapsed it left few persistent genetic traces in Europe. Our data confirm the Turkic roots of the Khazars, but also highlight their ethnic diversity and some integration of conquered populations.
3,854 downloads bioRxiv paleontology
Oculudentavis khaungraae was described based on a tiny skull trapped in amber. The slender tapering rostrum with retracted osseous nares, large eyes, and short vaulted braincase led to its identification as the smallest avian dinosaur on record, comparable to the smallest living hummingbirds. Despite its bird-like appearance, Oculudentavis showed several features inconsistent with its original phylogenetic placement. Here we describe a more complete, specimen that demonstrates Oculudentavis is actually a bizarre lizard of uncertain position. The new interpretation and phylogenetic placement highlights a rare case of convergent evolution rarely seen among reptiles. Our results re-affirm the importance of Myanmar amber in yielding unusual taxa from a forest ecosystem rarely represented in the fossil record. ### Competing Interest Statement The authors have declared no competing interest.
3,848 downloads bioRxiv paleontology
The domestication of dogs probably occurred in Eurasia by 16,000 years ago, with the initial peopling of the Americas potentially happening around the same time. Dogs were long thought to have accompanied the first migrations into the Americas, but conclusive evidence for Paleoindian dogs is lacking. The direct dating of two dogs from the Koster site (Greene Co., Illinois) and a newly-described dog from the Stilwell II site (Pike Co., Illinois) to between 10,190-9,630 cal BP represents the earliest evidence of domestic dogs in the Americas and individual dog burials in worldwide archaeological record. The over 4,500 year discrepancy between the timing of initial human migration into the Americas and the earliest evidence for domesticated dogs suggests either earlier dogs are going unseen or unidentified or dogs arrived later with a subsequent human migration.
3,185 downloads bioRxiv paleontology
Fossil age data and molecular sequences are increasingly combined to establish a timescale for the Tree of Life. Arthropods, as the most species-rich and morphologically disparate animal phylum, have received substantial attention, particularly with regard to questions such as the timing of habitat shifts (e.g. terrestrialisation), genome evolution (e.g. gene family duplication and functional evolution), origins of novel characters and behaviours (e.g. wings and flight, venom, silk), biogeography, rate of diversification (e.g. Cambrian explosion, insect coevolution with angiosperms, evolution of crab body plans), and the evolution of arthropod microbiomes. We present herein a series of rigorously vetted calibration fossils for arthropod evolutionary history, taking into account recently published guidelines for best practice in fossil calibration. These are restricted to Palaeozoic and Mesozoic fossils, no deeper than ordinal taxonomic level, nonetheless resulting in 80 fossil calibrations for 102 clades. This work is especially timely owing to the rapid growth of molecular sequence data and the fact that many included fossils have been described within the last five years. This contribution provides a resource for systematists and other biologists interested in deep-time questions in arthropod evolution.
3,036 downloads bioRxiv paleontology
Here we present the first full anatomical description of the 3.67 million-year-old Australopithecus skull StW 573 that was recovered with its skeleton from the Sterkfontein Member 2 breccia in the Silberberg Grotto. Analysis demonstrates that it is most similar in multiple key morphological characters to a group of fossils from Sterkfontein Member 4 and Makapansgat that are here distinguished morphologically as A. prometheus. This taxon contrasts with another group of fossils from those sites assigned to A. africanus. The anatomical reasons for why these groupings should not be lumped together (as is frequently done for the South African fossils) are discussed in detail. In support of this classification, we also present for the first time a palate (StW 576 from Sterkfontein Member 4) newly reconstructed by RJC, which has a uniquely complete adult dentition of an A. africanus. The StW 573 skull also has certain similarities with other earlier Australopithecus fossils in East Africa, A. afarensis and A. anamensis, which are discussed. One of its most interesting features is a pattern of very heavy anterior dental wear unlike that found in A. africanus but resembling that found in A. anamensis at 4.17 Ma. While StW 573 is the only hominid fossil in Sterkfontein Member 2, we conclude that competitive exclusion probably accounts for the synchronous and sympatric presence of two species of Australopithecus in the younger deposits at Makapansgat and Sterkfontein Member 4. Because the StW 573 skull is associated with a near-complete skeleton that is also described for the first time in this special issue, we are now able to use this individual to improve our understanding of more fragmentary finds in the South African fossil record of Australopithecus.
3,005 downloads bioRxiv paleontology
Robin Huw Crompton, Juliet McClymont, Susannah Thorpe, William Sellers, Jason Heaton, Travis Rayne Pickering, Todd Pataky, Dominic Stratford, Kristian Carlson, Tea Jashashvili, Amélie Beaudet, Laurent Bruxelles, Colleen Goh, Kathleen Kuman, Ronald Clarke
StW 573, from Sterkfontein Member 2, dated ca 3.67 Ma, is by far the most complete skeleton of an australopith to date. Joint morphology is in many cases closely matched in available elements of Australopithecus anamensis (eg. proximal and distal tibial and humeral joint-surfaces) and there are also close similarities to features of the scapula, in particular, of KSD-VP-1/1 A. afarensis from Woranso-Mille. The closest similarities are, however, to the partial skeleton of StW 431 from Sterkfontein Member 4. When considered together, both StW 573 and StW 431 express an hip joint morphology quite distinct from that of A. africanus Sts14, and a proximal femur of a presumed A. africanus from Jacovec Cavern at Sterkfontein, StW 598. This, and other evidence presented herein, suggests there are two pelvic girdle morphs at Sterkfontein, supporting Clarke (2013) in his recognition of a second species, A. prometheus, containing StW 573 and StW 431. StW 573 is the first hominid skeleton where limb proportions are known unequivocally. It demonstrates that some early hominins, at the time of formation of the Laetoli footprints (3.6 Ma), were large-bodied. with hindlimbs longer than forelimbs. Modelling studies on extant primates indicate that the intermembral index (IMI) of StW 573, low for a non-human great ape, would have substantially enhanced economy of bipedal walking over medium-to-long distances, but that it was still too high for effective walking while load-carrying. It would, however, have somewhat reduced the economy of horizontal climbing, but made Gorilla-like embracing of large tree-trunks less possible. Consideration of both ethnographic evidence from modern indigenous arboreal foragers and modern degeneracy theory cautions against prescriptive interpretations of hand- and foot-function, by confirming that both human-like upright bipedalism and functional capabilities of the hand and foot can be effective in short-distance arboreal locomotion.
2,498 downloads bioRxiv paleontology
In the competition for the opposite sex, sexual selection can favor production of exaggerated features, but the high cost of such features in terms of energy consumption and enemy avoidance makes them go to extinction under the influence of natural selection. However, to our knowledge, fossil on exaggerated traits that are conducive to attracting opposite sex are very rare. Here, we report the exaggerated leaf-like expansion antennae of Magnusantenna wuae Du & Chen gen. et sp. nov. (Hemiptera: Coreidae) with more abundant sensory hairs from a new nymph coreid preserved in a Cretaceous Myanmar amber. The antennae are the largest among species of coreid and one of the largest known insects. Such bizarre antennae indicate that sensitive and delicate sensory system and magnificent appearance in Hemiptera have been already well established in mid-Cretaceous. Our findings provide evidence for Darwin’s view that sensory organs play an important role in sexual selection. This nymph with the leaf-like antennae may also represents a new camouflage pattern for defense. However, the oversized antennae are costly to develop and maintain, increasing the risks from predators. Such unparalleled expanded antennae might be the key factor for the evolutionary fate of this Myanmar amber coreid species. Significance Darwin proposed the importance of sensory organs in sexual selection, but it was greatly ignored compared with weapons and other common ornaments. Here, we report a new type of insect antennae, the multiple segments leaf-like expansion antennae from a new nymph coreid preserved in a Cretaceous Myanmar amber. Our finding provides evidence for the prominent role of sensory organs in sexual selection and thus supports Darwin’s viewpoint. This discovery demonstrates that high-efficiency antennae were present in Coreidae 99 million years ago. In addition, the exaggerated antennae might represent a new evolutionary innovation for defensive behavior. This is a case in which the high benefits and high costs brought by the exaggerated antennae jointly determine the direction of species evolution. ### Competing Interest Statement The authors have declared no competing interest.
2,264 downloads bioRxiv paleontology
The fossil record is a rich source of information about biological diversity in the past. However, the fossil record is not only incomplete but has inherent biases due to geological, physical, chemical and biological factors. Our knowledge of past life is also biased because of differences in academic and amateur interests and sampling efforts. As a result, not all individuals or species that lived in the past are equally likely to be discovered at any point in time or space. To reconstruct temporal dynamics of diversity using the fossil record, biased sampling must be explicitly taken into account. Here, we introduce an approach that utilizes the variation in the number of times each species is observed in the fossil record to estimate both sampling bias and true richness. We term our technique TRiPS (True Richness estimated using a Poisson Sampling model) and explore its robustness to violation of its assumptions via simulations. We then venture to estimate sampling bias and absolute species richness of dinosaurs in the geological stages of the Mesozoic. Using TRiPS, we estimate that 1936 (1543-2468) species of dinosaurs roamed the Earth during the Mesozoic. We also present improved estimates of species richness trajectories of the three major dinosaur clades; the sauropodomorphs, ornithischians and theropods, casting doubt on the Jurassic-Cretaceous extinction event and demonstrating that all dinosaur groups are subject to considerable sampling bias throughout the Mesozoic.
2,241 downloads bioRxiv paleontology
The somasteroids are Ordovician star-shaped animals widely regarded as ancestors of Asterozoa, the group of extant echinoderms that includes brittle stars and starfish. The phylogenetic position of somasteroids makes them critical for understanding the origin and early evolution of crown group Echinodermata. However, the early evolution of asterozoans, the origin of their distinctive body organization and their relationships with other Cambrian and Ordovician echinoderms, such as edrioasteroids, blastozoans, crinoids, and other asterozoans, remain problematic due to the difficulties of comparing the calcitic endoskeleton of these disparate groups. Here we describe the new somasteroid Cantabrigiaster fezouataensis from the Early Ordovician (Tremadocian) Fezouata Lagerstatte in Morocco. Cantabrigiaster shares with other somasteroids the presence of rod-like virgal ossicles that articulate with the ambulacrals, but differs from all other known asterozoans in the absence of adambulacral ossicles defining the arm margins. The unique arm construction evokes parallels with non-asterozoan echinoderms. Developmentally informed Bayesian and parsimony based phylogenetic analyses, which reflect the homology of the biserial ambulacral ossicles in Paleozoic echinoderms according to the Extraxial-Axial Theory, recover Cantabrigiaster as basal within stem group Asterozoa. Our results indicate that Cantabrigiaster is the earliest diverging stem group asterozoan, revealing the ancestral morphology of this major clade and clarifying the affinities of problematic Ordovician taxa. Somasteroids are resolved as a paraphyletic grade within stem and crown group Asterozoa (starfishes), whereas stenuroids are paraphyletic within stem group Ophiuroidea (brittle stars). Cantabrigiaster also illuminates the relationship between Ordovician crown group Echinodermata and its Cambrian stem lineage, which includes sessile forms with incipient radial symmetry such as edrioasteroids and blastozoans. The contentious Pelmatozoa hypothesis (i.e. monophyly of blastozoans and crinoids) is not supported; instead, blastozoans represent the most likely sister-taxon of crown group Echinodermata.
2,224 downloads bioRxiv paleontology
Fabbri et al. claim that the huge sail-backed dinosaur Spinosaurus aegyptiacus was a "subaqueous forager," diving underwater in pursuit of prey, based on their measure of bone "compactness." Using thin-sections and computed tomographic (CT) scans of thigh bone (femur) and trunk rib from various living and extinct vertebrates, they claim to be able to distinguish taxa with "aquatic habits" from others. Their conclusions are undermined by selective bone sampling, inaccuracies concerning spinosaurid bone structure, faulty statistical inferences, and novel redefinition of the term "aquatic."
2,157 downloads bioRxiv paleontology
The database regarding the earliest occupation of Europe has increased significantly in quantity and quality of data points over the last two decades, mainly through the addition of new sites as a result of long-term systematic excavations and large-scale prospections of Early and early Middle Pleistocene exposures. The site distribution pattern suggests an ephemeral presence of hominins in the south of Europe from around one million years ago, with occasional short northward expansions along the western coastal areas when temperate conditions permitted. From around 600,000-700,000 years ago Acheulean artefacts appear in Europe and somewhat later hominin presence seems to pick up, with more sites and now some also present in colder climatic settings. It is again only later, around 350,000 years ago, that the first sites show up in more continental, central parts of Europe, east of the Rhine. A series of recent papers on the Early Pleistocene palaeontological site of Untermassfeld (Germany) makes claims that are of great interest for studies of earliest Europe and are at odds with the described pattern: the papers suggest that Untermassfeld has yielded stone tools and humanly modified faunal remains, evidence for a one million years old hominin presence in European continental mid-latitudes, and additional evidence that hominins were well-established in Europe already around that time period. Here we evaluate these claims and demonstrate that these studies are severely flawed in terms of data on provenance of the materials studied and in the interpretation of faunal remains and lithics as testifying to a hominin presence at the site. In actual fact any reference to the Untermassfeld site as an archaeological one is unwarranted. Furthermore, it is not the only European Early Pleistocene site where inferred evidence for hominin presence is problematic. The strength of the spatiotemporal patterns of hominin presence and absence depend on the quality of the data points we work with, and data base maintenance, including critical evaluation of new sites, is crucial to advance our knowledge of the expansions and contractions of hominin ranges during the Pleistocene.
2,139 downloads bioRxiv paleontology
Vincent Gaffney, Simon Fitch, Martin Bates, Roselyn L. Ware, Tim Kinnaird, Benjamin Gearey, Tom Hill, Richard Telford, Cathy Batt, Ben Stern, John Whittaker, Sarah Davies, Mohammed Ben Sharada, Rosie Everett, Rebecca Cribdon, Logan Kistler, Sam Harris, Kevin Kearney, James Walker, Merle Muru, Derek Hamilton, Matthew Law, Richard Bates, Robin G Allaby
Doggerland was a land mass occupying an area currently covered by the North Sea until marine inundation took place during the mid-Holocene, ultimately separating the British land mass from the rest of Europe. The Storegga Slide, which triggered a tsunami reflected in sediment deposits in the Northern North Sea, North East coastlines of the British Isles and across the North Atlantic, was a major event during this transgressive phase. The spatial extent of the Storegga tsunami however remains unconfirmed because to date no direct evidence for the event has been recovered from the southern North Sea. We present evidence that Storegga associated deposits occur in the southern North Sea. Palaeo-river systems have been identified using seismic survey in the southwestern North Sea and sedimentary cores extracted to track the Mid Holocene inundation. At the head of one palaeo-river system near the Outer Dowsing Deep, the Southern River, we observed an abrupt and catastrophic inundation stratum. Based on lithostratigraphic, macro and microfossils and sedimentary ancient DNA (sedaDNA) evidence, supported by optical stimulation luminescence (OSL) and radiocarbon dating, we conclude these deposits were a result of the Storegga event. Seismic identification of this stratum to adjacent cores indicated diminished traces of the tsunami, largely removed by subsequent erosional processes. Our results demonstrate the catastrophic impact of Storegga within this area of the Southern North Sea, but indicate that these effects were temporary and likely localized and mitigated by the dense woodland and topography of the area. We conclude clear physical remnants of the wave are likely to be restricted to inland basins and incised river valley systems.
2,127 downloads bioRxiv paleontology
Hereditary hierarchy is one of the major features of complex societies. Without a written record, prehistoric evidence for hereditary hierarchy is rare. Intentional cranial deformation (ICD) is a cross-generational cultural practice that embodies social identity and culture beliefs in adults through the behavior of altering infant head shape. Therefore, ICD is usually regarded as an archeological clue for the occurrence of hereditary hierarchy. With a calibrated radiocarbon age of 11245-11200 years BP, a fossil skull of an adult male displaying ICD discovered in Northeastern China is among the oldest-known ICD practices in the world. Along with the other earliest global occurrences of ICD, this discovery points to the early initiation of complex societies among the non-agricultural local societies in Northeastern Asia in the early Holocene. A population increase among previously more isolated terminal Pleistocene/early Holocene hunter-gatherer groups likely increased their interactions, possibly fueling the formation of the first complex societies.
2,040 downloads bioRxiv paleontology
Evan T. Saitta, Renxing Liang, Chui Y Lau, Caleb M Brown, Nicholas R Longrich, Thomas G. Kaye, Ben J. Novak, Steven Salzberg, Paul Donohoe, Marc Dickinson, Jakob Vinther, Ian D Bull, Richard A Brooker, Peter Martin, Geoffrey D Abbott, Timothy D. J. Knowles, Kirsty Penkman, Tullis C. Onstott
Fossils were long thought to lack original organic material, but the discovery of organic molecules in fossils and sub-fossils, thousands to millions of years old, has demonstrated the potential of fossil organics to provide radical new insights into the fossil record. How long different organics can persist remains unclear, however. Non-avian dinosaur bone has been hypothesised to preserve endogenous organics including collagen, osteocytes, and blood vessels, but proteins and labile lipids are unstable during diagenesis or over long periods of time. Furthermore, bone is porous and an open system, allowing microbial and organic flux. Some of these organics within fossil bone have therefore been identified as either contamination or microbial biofilm, rather than original organics. Here, we use biological and chemical analyses of Late Cretaceous dinosaur bones and sediment matrix to show that dinosaur bone hosts a diverse microbiome. Fossils and matrix were freshly-excavated, aseptically-acquired, and then analysed using microscopy, spectroscopy, chromatography, spectrometry, DNA extraction, and 16S rRNA amplicon sequencing. The fossil organics differ from modern bone collagen chemically and structurally. A key finding is that 16S rRNA amplicon sequencing reveals that the subterranean fossil bones host a unique, living microbiome distinct from that of the surrounding sediment. Even in the subsurface, dinosaur bone is biologically active and behaves as an open system, attracting microbes that might alter original organics or complicate the identification of original organics. These results suggest caution regarding claims of dinosaur bone soft tissue preservation and illustrate a potential role for microbial communities in post-burial taphonomy.
1,991 downloads bioRxiv paleontology
The origin of extant amphibians has been studied using several sources of data and methods, including phylogenetic analyses of morphological data, molecular dating, stratigraphic data, and integration of ossification sequence data, but a consensus about their affinities with other Paleozoic tetrapods has failed to emerge. We have compiled five datasets to assess the relative support for six competing hypotheses about the origin of extant amphibians: a monophyletic origin among temnospondyls, a monophyletic origin among lepospondyls, a diphyletic origin among both temnospondyls and lepospondyls, a diphyletic origin among temnospondyls alone, and two variants of a triphyletic origin, in which anurans and urodeles come from different temnospondyl taxa while caecilians come from lepospondyls and are either closer to anurans and urodeles or to amniotes. Our datasets comprise ossification sequences of up to 107 terminal taxa and up to eight cranial bones, and up to 65 terminal taxa and up to seven appendicular bones, respectively. Among extinct taxa, only two or three temnospondyl can be analyzed simultaneously for cranial data, but this is not an insuperable problem because each of the six tested hypotheses implies a different position of temnospondyls and caecilians relative to other sampled taxa. For appendicular data, more extinct taxa can be analyzed, including some lepospondyls and the finned tetrapodomorph Eusthenopteron , in addition to temnospondyls. The data are analyzed through maximum likelihood, and the AICc (corrected Akaike Information Criterion) weights of the six hypotheses allow us to assess their relative support. By an unexpectedly large margin, our analyses of the cranial data support a monophyletic origin among lepospondyls; a monophyletic origin among temnospondyls, the current near-consensus, is a distant second. All other hypotheses are exceedingly unlikely according to our data. Surprisingly, analysis of the appendicular data supports triphyly of extant amphibians within a clade that unites lepospondyls and temnospondyls, contrary to all phylogenies based on molecular data and recent trees based on paleontological data, but this conclusion is not very robust.
1,951 downloads bioRxiv paleontology
Ichthyornis has long been recognized as a pivotally important fossil taxon for understanding the latest stages of the dinosaur-bird transition, but little significant new postcranial material has been brought to light since initial descriptions of partial skeletons in the 19th Century. Here, we present new information on the postcranial morphology of Ichthyornis from 40 previously undescribed specimens, providing the most detailed morphological assessment of Ichthyornis to date. The new material includes four partially complete skeletons and numerous well-preserved isolated elements, enabling new anatomical observations such as muscle attachments previously undescribed for Mesozoic euornitheans. Among the elements that were previously unknown or poorly represented for Ichthyornis, the new specimens include an almost-complete axial series, a hypocleideum-bearing furcula, radial carpal bones, fibulae, a complete tarsometatarsus bearing a rudimentary hypotarsus, and one of the first-known nearly complete three-dimensional sterna from a Mesozoic avialan. Several pedal phalanges are preserved, revealing a remarkably enlarged pes presumably related to foot-propelled swimming. Although diagnosable as Ichthyornis, the new specimens exhibit a substantial degree of morphological variation, some of which may relate to ontogenetic changes. Phylogenetic analyses incorporating our new data and employing alternative morphological datasets recover Ichthyornis stemward of Hesperornithes and Iaceornis, in line with some recent hypotheses regarding the topology of the crownward-most portion of the avian stem group, and we establish phylogenetically-defined clade names for relevant avialan subclades to help facilitate consistent discourse in future work. The new information provided by these specimens improves our understanding of morphological evolution among the crownward-most non-neornithine avialans immediately preceding the origin of crown group birds.
1,887 downloads bioRxiv paleontology
The Coelurosauria are a group of mostly feathered theropods that gave rise to birds, the only dinosaurs that survived the Cretaceous-Paleogene extinction event and are still found today. Between their first appearance in the Middle Jurassic up to the end Cretaceous, coelurosaurs were party to dramatic geographic changes on the Earth's surface, including the breakup of the supercontinent Pangaea, and the formation of the Atlantic Ocean. These plate tectonic events are thought to have caused vicariance or dispersal of coelurosaurian faunas, influencing their evolution. Unfortunately, few coelurosaurian biogeographic hypotheses are supported by quantitative evidence. Here, we report the first, broadly-sampled quantitative analysis of coelurosaurian biogeography using the likelihood-based package BioGeoBEARS. Mesozoic geographic configurations and changes are reconstructed and employed as constraints in this analysis, including their associated uncertainties. We use a comprehensive time-calibrated coelurosaurian evolutionary tree produced from the Theropod Working Group phylogenetic data matrix. Six biogeographic models in the BioGeoBEARS package with different assumptions about the evolution of spatial distribution are tested against the geographic constraints. Our results statistically favour the DIVALIKE+J and DEC+J models, which allow vicariance and founder events, supporting continental vicariance as an important factor in coelurosaurian evolution. Ancestral range estimation indicates frequent dispersal events via the Apulian Route (connecting Europe and Africa during the Early Cretaceous) and the Bering Land Bridge (connecting North America and Asia during the Late Cretaceous). These quantitative results are consistent with commonly inferred Mesozoic dinosaurian dispersals and continental-fragmentation-induced vicariance events. In addition, we recognise the importance of Europe as a dispersal centre and gateway in the Early Cretaceous, as well as other vicariance events like those triggered by the disappearance of land-bridges.
1,840 downloads bioRxiv paleontology
Image-forming compound eyes are such a valuable adaptation that similar visual systems have evolved independently across crustaceans. But if different compound eye types have evolved independently multiple times, how useful are eye structures and ommatidia morphology for resolving phylogenetic relationships? Crabs are ideal study organisms to explore these questions because they have a good fossil record extending back into the Jurassic, they possess a great variety of optical designs, and details of eye form can be compared between extant and fossil groups. True crabs, or Brachyura, have been traditionally divided into two groups based on the position of the sexual openings in males and females: the so-called Podotremata (females bearing their sexual openings on the legs), and the Eubrachyura, or higher true crabs (females bearing their sexual openings on the thorax). Although Eubrachyura appears to be monophyletic, the monophyly of podotreme crabs remains controversial and therefore requires exploration of new character systems. The earliest podotremous lineages share the plesiomorphic condition of mirror reflecting superposition eyes with most shrimp, lobsters, and anomurans (false crabs and allies). The optical mechanisms of fossil and extant podotreme groups more closely related to Eubrachyura, however, are still poorly investigated. To better judge the phylogenetic utility of compound eye form, we investigated the distribution of eye types in fossil and extant podotreme crabs. Our findings suggest the plesiomorphic mirror eyes\---|seen in most decapod crustaceans including the earliest true crabs\---|has been lost in several podotremes and in eubrachyurans. We conclude that the secondary retention of larval apposition eyes has existed in eubrachyurans and some podotremes since at least the Early Cretaceous, and that the distribution of eye types among true crabs supports a paraphyletic podotreme grade, as suggested by recent molecular and morphological phylogenetic studies. We also review photoreceptor structure and visual pigment evolution, currently known in crabs exclusively from eubrachyuran representatives. These topics are critical for future expansion of research on podotremes to deeply investigate the homology of eye types across crabs.
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