Osteology and reassessment of Dineobellator notohesperus, a southern eudromaeosaur (Theropoda: Dromaeosauridae: Eudromaeosauria) from the latest Cretaceous of New Mexico.

Dromaeosaurids (Theropoda: Dromaeosauridae), a group of dynamic, swift predators, have a sparse fossil record, particularly at the end of the Cretaceous Period. The recently described Dineobellator notohesperus, consisting of a partial skeleton from the Upper Cretaceous (Maastrichtian) of New Mexico, is the only diagnostic dromaeosaurid to be recovered from the latest Cretaceous of the southwestern United States. Reinterpreted and newly described material include several caudal vertebrae, portions of the right radius and pubis, and an additional ungual, tentatively inferred to be from manual digit III. Unique features, particularly those of the humerus, unguals, and caudal vertebrae, distinguish D. notohesperus from other known dromaeosaurids. This material indicates different physical attributes among dromaeosaurids, such as use of the forearms, strength in the hands and feet, and mobility of the tail. Several bones in the holotype exhibit abnormal growth and are inferred to be pathologic features resulting from an injury or disease. Similar lengths of the humerus imply Dineobellator and Deinonychus were of similar size, at least regarding length and/or height, although the more gracile nature of the humerus implies Dineobellator was a more lightly built predator. A new phylogenetic analysis recovers D. notohesperus as a dromaeosaurid outside other previously known and named clades. Theropod composition of the Naashoibito Member theropod fauna is like those found in the more northern Late Cretaceous North American ecosystems. Differences in tooth morphologies among recovered theropod teeth from the Naashoibito Member also implies D. notohesperus was not the only dromaeosaurid present in its environment.

Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture.

The internal architecture of chambered ammonoid conchs profoundly increased in complexity through geologic time, but the adaptive value of these structures is disputed. Specifically, these cephalopods developed fractal-like folds along the edges of their internal divider walls (septa). Traditionally, functional explanations for septal complexity have largely focused on biomechanical stress resistance. However, the impact of these structures on buoyancy manipulation deserves fresh scrutiny. We propose increased septal complexity conveyed comparable shifts in fluid retention capacity within each chamber. We test this interpretation by measuring the liquid retained by septa, and within entire chambers, in several 3D-printed cephalopod shell archetypes, treated with (and without) biomimetic hydrophilic coatings. Results show that surface tension regulates water retention capacity in the chambers, which positively scales with septal complexity and membrane capillarity, and negatively scales with size. A greater capacity for liquid retention in ammonoids may have improved buoyancy regulation, or compensated for mass changes during life. Increased liquid retention in our experiments demonstrate an increase in areas of greater surface tension potential, supporting improved chamber refilling. These findings support interpretations that ammonoids with complex sutures may have had more active buoyancy regulation compared to other groups of ectocochleate cephalopods. Overall, the relationship between septal complexity and liquid retention capacity through surface tension presents a robust yet simple functional explanation for the mechanisms driving this global biotic pattern.

Breaking down shell strength: inferences from experimental compression and future directions enabled by 3D printing.

Mollusc and brachiopod shells have served as biological armour for hundreds of millions of years. Studying shell strength in compression experiments can provide insights into macroevolution, predator-prey dynamics, and anthropogenic impacts on aquatic ecosystems. These studies have been conducted across fields including palaeontology, ecology, conservation biology and engineering using a range of techniques for a variety of purposes. Using this approach, studies have demonstrated that predators can cause changes in prey shell morphology in the laboratory over both short timescales and over longer evolutionary timescales. Similarly, environmental factors such as nutrient concentration and ocean acidification have been shown to influence shell strength. Experimental compression tests have been used to study the functional morphology of shell-crushing predators and to test how the taphonomic state of shells (e.g. presence of drill holes, degree of shell degradation) may influence their likelihood of being preserved in the fossil record. This review covers the basic principles and experimental design of compression tests used to infer shell strength. Although many investigations have used this methodology, few provide a detailed explanation of how meaningfully to interpret data generated using compression experiments for those unfamiliar with this method. Furthermore, this review provides a compilation of the findings of studies that have employed these experimental methods to address specific themes: taphonomy, morphology, predation, environmental variables, and climate change. Many authors have used experimental compression tests, however, disparities among methodologies (e.g. in experimental design, taxa, specimen preservation, etc.) limit the applicability of findings from taxon-specific studies to broader eco-evolutionary questions. The review highlights confounding factors, such as shell thickness, size, damage, microstructure, and taphonomic state, and address how they can be mitigated using three-dimensional (3D)-printed model shells. 3D prints have been demonstrated as valuable proxies for understanding aspects of shell morphology that cannot otherwise be experimentally isolated. Using 3D printed models allows simplification of complex biological systems for idealized experimental studies. Such studies can isolate specific aspects of shell morphology to establish fundamental relationships between form and function. Establishing standardized methods of testing shell strength in this way will not only permit comparison across studies but also will enable investigators systematically to add complexity to their models.

Experimental tests of bivalve shell shape reveal potential tradeoffs between mechanical and behavioral defenses.

Bivalves protect themselves from predators using both mechanical and behavioral defenses. While their shells serve as mechanical armor, bivalve shells also enable evasive behaviors such as swimming and burrowing. Therefore, bivalve shell shape is a critical determinant of how successfully an organism can defend against attack. Shape is believed to be related to shell strength with bivalve shell shapes converging on a select few morphologies that correlate with life mode and motility. In this study, mathematical modeling and 3D printing were used to analyze the protective function of different shell shapes against vertebrate shell-crushing predators. Considering what life modes different shapes permit and analyzing the strength of these shapes in compression provides insight to evolutionary and ecological tradeoffs with respect to mechanical and behavioral defenses. These empirical tests are the first of their kind to isolate the influence of bivalve shell shape on strength and quantitatively demonstrate that shell strength is derived from multiple shape parameters. The findings of this theoretical study are consistent with examples of shell shapes that allow escape behaviors being mechanically weaker than those which do not. Additionally, shell elongation from the umbo, a metric often overlooked, is shown to have significant effects on shell strength.