Biology subjects

Saitta, E. T.

Publications and source records attributed to Saitta, E. T..

6 recordsLinked to original sources

bioRxiv2025

THE FUNCTION AND EVOLUTION OF STEGOSAUR OSTEODERMS AND HYPOTHESIZED SEXUAL DIMORPHISM IN HESPEROSAURUS

Stegosaurus is one of the most iconic organisms in Earths history due to its impressive dermal osteoderms. These consisted of throat ossicles, two pairs of posterior tail spikes, and an estimated 18 large plates arranged in two staggered rows along the neck, back, and tail. The function and evolution of these structures in stegosaurs is a quintessential question in dinosaur paleontology. Although sociosexual display has become a popular explanation of plate function, sexual dimorphism in dinosaurs has been a contentious topic. Spikes are meanwhile often described as defensive structures, but we consider an additional intra-sex combat function worthy of further study. To investigate the questions surrounding stegosaur osteoderms, we reevaluate variation in North American stegosaur osteoderms, with attention to hypothesized sexual dimorphism in Hesperosaurus, a close relative of Stegosaurus with similar osteoderms. We correct errors in previous analyses, address challenges of the dimorphism hypothesis, and use outline analysis and effect size statistics to provide further statistical support that one sex possibly had larger, wide/broad plates (hypothesized male), while the other sex had smaller, tall/narrow plates (hypothesized female). Stegosaur plates are a difficult case study for sexual variation because the multiple plates borne by a single individual can result in datasets that violate assumptions of independence. Despite appreciable variation from head to tail along an individual, the variation in size and shape seen in Hesperosaurus plates is still consistent with the presence of sexual variation as evidenced by both size-independent, two-dimensional outline analysis as well as size-versus-shape regression analysis of principal component data, with high confidence in the latter. We note pathologies on stegosaur caudal vertebrae, particularly in old adults of wide-morph Hesperosaurus, as well as in Stegosaurus tail spikes. These pathologies are consistent with not just predator-prey interactions but also intraspecific combat. There currently is no unambiguous indication of dimorphism in stegosaur thagomizers because alternative hypotheses, such as ontogenetic or interspecific variation, are not yet fully tested. Finally, we present a hypothetical model of stegosaur osteoderm evolution and function based on fossil and extant evidence. Osteoderms adapted for defense in earlier thyreophorans likely became exapted for sexual display (i.e., plates) and intra-sex combat (e.g., tail spikes in stegosaurs, clubs in ankylosaurs). Therefore, sexual selection may have initiated the differentiation of plates from the terminal thagomizer and induced the unique asymmetry of staggered plates in Stegosaurus and Hesperosaurus. Osteoderm development in hypothesized males could have been driven to physiological upper limits by sexual selection, while the intensity of predation pressure dictated osteoderm development in hypothesized females. Increasing predation pressure might correlate with large body size, open habitats, spine-like plate processes, long tail spikes, and gular ossicles. Within this hypothetical evolutionary model, Hesperosaurus would represent a stegosaur taxon with a high magnitude of sexual dimorphism, consistent with its smaller body size, rounder plates, presumed lack of gular ossicles, and niche occupation within a wet, forested, vegetation-rich habitat in the northern Morrison foreland basin. The question of stegosaur plate function requires us to look more broadly at thyreophoran osteoderm variation and evolution and to consider complex, multifactor models that incorporate many previously proposed hypotheses. We must sort between primary versus secondary functions of plates, spikes, and ossicles and how these functions changed over time.

paleontology↗

bioRxiv2024

DO BIRDS SHOW UNIQUE MACROEVOLUTIONARY PATTERNS OF SEXUAL SIZE DIMORPHISM COMPARED TO OTHER AMNIOTES?

Withdrawal StatementThe authors have withdrawn their manuscript because it was overlooked that the data downloaded from the CD-ROMs accompanying Dunning (2007) had been previously filtered through statistical testing, which affected subsequent interpretation of the dataset. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author. The manuscript is undergoing major revisions that substantially impact the overall conclusions. The authors thank an anonymous peer reviewer for bringing this issue to attention.

evolutionary biology↗

bioRxiv2023

RELAXING SELECTIVE PRESSURES ON DEVELOPMENTALLY COMPLEX INTEGUMENTARY STRUCTURES: FEATHER VANE SYMMETRY EVOLVES IN ADDITION TO BODY MASS AND WING LENGTH AFTER FLIGHT LOSS IN RECENT BIRDS

Feathers are complex integumentary structures with high diversity across species and within plumage and have varied functions (e.g., thermoregulation, flight). Flight is lost in many crown lineages, and frequently occurs in island founding or semiaquatic context. Different extant lineages lost flight across at least three orders of magnitude of time ([~]79.58 Ma-15 Ka). Flight losss effect on sensory capacity, brain size, and skeletomusculature have been studied, but less work exists on relations between flightlessness and feathers. To understand how flight loss affects feather anatomy, we measured 11 feather metrics (e.g., barb length, barb angle) from primaries, tertials, rectrices, and contour feathers on skins of 30 flightless taxa and their phylogenetically closest volant taxa, supplemented with broader sampling of primaries across all orders of volant crown birds. Our sample includes 27 independent losses of flight; the sample contains nearly half the extant flightless species count and matches its [~]3:2 terrestrial:semiaquatic ratio. Vane symmetry increases in flightless lineages, and these patterns are strongest in flight feathers and weakest in coverts. Greatest changes in feathers are in the oldest flightless lineages like penguins, which show robust filaments (rachis, barbs, and barbules) on small feathers, and ratites, which show high interspecific diversity with plumulaceous filaments and/or filament loss. Phylogenetic comparative methods show that some of these microscopic feather traits, such as barb/barbule length and rachis width, are not as dramatically modified upon flight loss as are body mass increase and relative wing and tail fan reduction, whereas the effect on vane symmetry is more easily detected. Upon relaxing selection for flight, feathers do not soon significantly modify many of their flight adaptations, although increased vane symmetry is likely the most detectable shift. Feathers of recently flightless lineages are in many ways like those of their volant relatives. Feather microstructure evolution is often subtle in flightless taxa, except when flight loss is ancient, perhaps because developmental constraints act upon feathers and/or selection for novel feather morphologies is not strong. Changes in skeletomusculature of the flight apparatus are likely more evident in recently flightless taxa and may be a more reliable way to detect flight loss in fossils, with increased vane symmetry as potentially a microscopic signal. Finally, we see an intriguing, reversed pattern in feather evolution after flight loss from the pattern proposed in popular developmental models of feathers, with the later stages of feather development (asymmetric displacement of barb loci) being lost more readily, while early stages of development (e.g., differentiated barb ridges on follicle collar) are only lost after many millions of years of flightlessness.

evolutionary biology↗

bioRxiv2023

Diving dinosaurs? Caveats on the use of bone compactness and pFDA for inferring lifestyle

Measures of bone compactness in amniote tetrapods of varying lifestyle were used to infer that two spinosaurid dinosaurs (Spinosaurus aegyptiacus, Baryonyx walkeri) were diving "subaqueous foragers," whereas a third spinosaurid (Suchomimus tenerensis) and other sampled nonavian dinosaurs were non-diving terrestrial feeders entering water only as waders. We outline shortcomings in this analysis that involve bone compactness sampling and measurement, lifestyle categorization, the inclusion and exclusion of taxa in the dataset, and flawed statistical methods and inferences. These many shortcomings undermine the evidence used to conclude that two spinosaurid taxa were avid divers. Bone compactness indices remain a valuable tool for interpretation of lifestyle in extinct species when based on sound dataset composition, robust statistical analysis, and consilience with evidence from functional, biomechanical, or paleoenvironmental considerations.

paleontology↗

bioRxiv2020

NON-AVIAN DINOSAUR EGGSHELL CALCITE CONTAINS ANCIENT, ENDOGENOUS AMINO ACIDS

Rates of peptide bond hydrolysis and other diagenetic reactions are not favourable for Mesozoic protein survival. Proteins hydrolyse into peptide fragments and free amino acids that, in open systems such as bone, can leach from the specimen and be further degraded. However, closed systems are more likely to retain degradation products derived from endogenous proteins. Amino acid racemisation data in experimental and subfossil material suggests that mollusc shell and avian eggshell calcite crystals can demonstrate closed system behaviour, retaining endogenous amino acids. Here, high-performance liquid chromatography reveals that the intra-crystalline fraction of Late Cretaceous (estimated ~80 Ma) titanosaur sauropod eggshell is enriched in some of the most stable amino acids (Glx, Gly, Ala, and possibly Val) and those that racemise are fully racemic, despite being some of the slowest racemising amino acids. These results are consistent with degradation trends deduced from modern, thermally matured, sub-fossil, and ~3.8 Ma avian eggshell, as well as ~30 Ma calcitic mollusc opercula. Selective preservation of certain fully racemic amino acids, which do not racemise in-chain, along with similar concentrations of free versus total hydrolysable amino acids, likely suggests complete hydrolysis of original peptides. Liquid chromatography-tandem mass spectrometry supports this hypothesis by failing to detect any non-contamination peptide sequences from the Mesozoic eggshell. Pyrolysis-gas chromatography-mass spectrometry reveals pyrolysates consistent with amino acids as well as aliphatic hydrocarbon homologues that are not present in modern eggshell, suggestive of kerogen formation deriving from eggshell lipids. Raman spectroscopy yields bands consistent with various organic molecules, possibly including N-bearing molecules or geopolymers. These closed-system amino acids are possibly the most thoroughly supported non-avian dinosaur endogenous protein-derived constituents, at least those that have not undergone oxidative condensation with other classes of biomolecules. Biocrystal matrices can help preserve mobile organic molecules by trapping them (perhaps with the assistance of resistant organic polymers), but trapped organics are nevertheless prone to diagenetic degradation even if such reactions might be slowed in exceptional circumstances. The evidence for complete hydrolysis and degradation of most amino acids in the eggshell raises concern about the validity of reported polypeptide sequences from open-system non-avian dinosaur bone and other Mesozoic fossils.

paleontology↗

bioRxiv2018

Life Inside A Dinosaur Bone: A Thriving Microbiome

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.

paleontology↗