Evan T Saitta

Body size is undoubtedly one of the most useful measures of sexual dimorphism and, by proxy, sexual selection. Here, I examine large, published datasets of average sexual size dimorphism (SSD) in four clades of amniotes: birds, mammals, squamates, and turtles. Most sexual variation is of subtle magnitude; attempts to discretely categorize species as monomorphic may overlook genuine and common sexual variations of small magnitude (e.g., <10-20% difference). Mammals, squamates, and turtles have unimodal SSD distributions centered close to zero that vary in skew. Mammals skew towards a preponderance of taxa with larger males than females, and mammals with the most extreme SSD have larger males than females. Turtles, however, skew strongly towards a preponderance of taxa with larger females than males, and turtles with the most extreme SSD have larger females than males. Squamates are intermediate to these two clades. Birds are unique in that they 1) are noticeably deficient in taxa near monomorphism, 2) have a bimodal distribution with peaks closely and roughly equidistantly straddling either side of monomorphism, and 3) have a high preponderance of taxa with larger males than females. This suggests stronger disruptive selection or constraints against monomorphism in birds compared to other amniotes. Bird data from Dunning (2007) yields bimodality, while other datasets do not, possibly due to data artefacts/errors. Although Rensch's rule (RR) is difficult to apply to broad clades, scaling patterns were nevertheless examined here. While turtles and squamates show full adherence to RR, mammals show weaker adherence. Mammal scaling is comparatively less male-biased with increased size than scaling in squamates and turtles, and sex-role reversed mammals instead approach isometry between male and female size. Although bird taxa with larger males than females follow RR, sex-role reversed birds show the converse RR pattern. In birds, increasing size leads to increased dimorphism magnitude regardless of the direction of dimorphism, even though regression of the entire clade deceptively suggests they scale isometrically. This paradoxical scaling explains their unusual bimodal SSD distribution, as shown here through simulation. Equidistant bimodality from monomorphism might suggest disruptive selection where both mating systems have mirrored sexual selection dynamics of comparable effect. Scaling patterns between dimorphism magnitude and overall taxon size in non-reversed and reversed systems might not be readily apparent when examining the whole clade. Large mammals have disproportionately male-biased and more extreme SSD magnitudes. In comparison, large birds have relatively numerous sex-role reversed taxa as well as more extreme SSD magnitudes. These results deserve further testing with tighter phylogenetic controls and comparison of data sources. Additional ecological, physiological, and behavioral variables should also be examined in relation to SSD (e.g., altriciality vs. precociality, oviparity vs. viviparity, clutch size, neonate mass).

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 loss 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.

Tyrannosaurs, giant predatory dinosaurs from the end of the Cretaceous, are among the most intensively researched and best-known groups of dinosaurs. Despite this, their relationships and systematics are highly controversial, and the number of tyrannosaur species occurring in the latest Cretaceous in North America is debated. An ongoing debate concerns the status of Nanotyrannus lancensis, which has variously been interpreted as a distinct taxon of small-bodied tyrannosaur or a juvenile of the coeval Tyrannosaurus rex. Here, we review multiple lines of evidence and show that the totality of evidence strongly supports recognition of Nanotyrannus as a distinct species: 1. The high diversity of Late Cretaceous tyrannosaurs and predatory dinosaurs in general is consistent with the idea that more than one species lived in the late Maastrichtian of Western North America; 2. Nanotyrannus shows few if any diagnostic characters allowing referral specifically to Tyrannosaurus or even Tyrannosaurinae, but is differentiated from T. rex by at least 77 morphological characters, while intermediate forms, combining characteristics of Nanotyrannus and T. rex, remain unknown; 3. Histological evidence shows that individuals previously referred to Nanotyrannus lancensis show (i) skeletal fusions consistent with maturity, (ii) skull bone textures consistent with maturity, (iii) slow growth rates relative to T. rex, (iv) decelerating growth in their final years of life, and (v) growth curves predicting adult body sizes of ~1500 kg or less, implying that these animals are young adults, not juveniles of Tyrannosaurus; 4. Juveniles of other tyrannosaurids, including Gorgosaurus and Tarbosaurus, do not show the kinds of changes proposed for a Nanotyrannus-Tyrannosaurus growth series, suggesting the Nanotyrannus morphology cannot simply be explained as the result of immaturity; 5. Small T. rex exist, comparable in size to Nanotyrannus, which exhibit diagnostic features of Tyrannosaurus, and which differ from Nanotyrannus; 6. Phylogenetic analysis suggests that Nanotyrannus is not a tyrannosaurid. Taken together, the totality of the evidence rejects referral of Nanotyrannus to T. rex. Phylogenetic analysis tentatively supports placement of Nanotyrannus outside of Tyrannosauridae as a nontyrannosaurid member of Tyrannosauroidea. Tyrannosaur diversity appears to have been higher than previously appreciated in the latest Cretaceous before the K-Pg extinction. The difficulties in recognizing species based on fossils alone mean that paleontologists may be systematically biased towards underestimating the species diversity of ancient ecosystems.

The Society of Vertebrate Paleontology has urged scientific journals to reject studies that use data from privately owned fossil collections. Here, I argue that the Society’s perspective on reproducibility in science is overly simplistic. Their suggested publication policy, at best, slows the progress of science and, at worst, permits scientific misconduct through a form of data falsification and provides a potential mechanism to bully and censor researchers. The best way to ensure the long-term survival of fossil data is to collect and publish the data while the specimens are available.

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.