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The late Albian (Early Cretaceous) lineage of Actinoceramus sulcatus (Parkinson 1819) is a remarkable example of macro- and microevolution within the Bivalvia. Immediately following cladogenesis from ancestral A. concentricus, the lineage displays a conspicuous, short-term excursion through morphospace followed by a return to the ancestral form. This excursion is marked by the acquisition, and subsequent loss, of large radial folds affecting some part or all of the shell. The pattern is noteworthy because of the gross scale and rate of morphological evolution, the relatively short life span of an “extreme” morphology, an apparent evolutionary reversal, the presence of phenotypic clines through time, the extent of phenotypic variation within populations and abundance of morphological intermediates between disparate end-member types, the wide geographic distributions of phenotypic clines and variants, and the subtle asymmetry of morphological transitions bounding the evolutionary excursion.
From census and biometric analyses of stratigraphically constrained samples, we conclude that morphological change was not focused at speciation and the pattern of evolution does not conform to the classical paradigm of punctuated equilibrium. Instead, we infer that observed patterns are best explained by phyletic evolution, at widely varying rates, combined with ecophenotypic plasticity. Evolution targeted the potential to form radial folds; the expression of those in any individual was determined, in part at least, by environmental cues. Ecophenotypic plasticity in A. sulcatus was itself probably an evolutionary response favored by the presence of long-lived planktotrophic larvae and wide dispersal of the species. In A. sulcatus, there is a continuum of pattern between intrapopulational, ecophenotypic variation that can be observed on bedding planes, interregional variation, and phyletic change through time. We argue that this continuity of pattern is most easily explained by continuity of process: in the case of the visually striking radial folds in A. sulcatus, there is no reason to invoke distinct hierarchies of macro- and microevolution; instead, these seem to be parts of a continuum.
Mass extinctions can play a role in shaping macroevolutionary trends through time, but the contribution of recoveries to this process has yet to be examined in detail. This study focuses on the effects of three extinction events, the end-Cretaceous (K/T), mid-Eocene (mid-E), and end-Eocene (E/O), on long-term patterns of body size in veneroid bivalves. Systematic data were collected for 719 species and 140 subgenera of veneroids from the Late Cretaceous through Oligocene of North America and Europe. Centroid size measures were calculated for 101 subgenera and global stratigraphic ranges were used to assess extinction selectivity and preferential recovery. Veneroids underwent a substantial extinction at the K/T boundary, although diversity recovered to pre-extinction levels by the early Eocene. The mid-E and E/O events were considerably smaller and their recovery intervals much shorter. None of these events were characterized by significant extinction selectivity according to body size at the subgenus level; however, all three recoveries were strongly size biased. The K/T recovery was biased toward smaller veneroids, whereas both the mid-E and E/O recoveries were biased toward larger ones. The decrease in veneroid size across the K/T recovery actually reinforced a Late Cretaceous trend toward smaller sizes, whereas the increase in size resulting from the Eocene recoveries was relatively short-lived. Early Cenozoic changes in predation, temperature, and/or productivity may explain these shifts.
Kelps and other fleshy macroalgae—dominant reef-inhabiting organisms in cool seas— may have radiated extensively following late Cenozoic polar cooling, thus triggering a chain of evolutionary change in the trophic ecology of nearshore temperate ecosystems. We explore this hypothesis through an analysis of body size in the abalones (Gastropoda; Haliotidae), a widely distributed group in modern oceans that displays a broad range of body sizes and contains fossil representatives from the late Cretaceous (60–75 Ma). Geographic analysis of maximum shell length in living abalones showed that small-bodied species, while most common in the Tropics, have a cosmopolitan distribution, whereas large-bodied species occur exclusively in cold-water ecosystems dominated by kelps and other macroalgae. The phylogeography of body size evolution in extant abalones was assessed by constructing a molecular phylogeny in a mix of large and small species obtained from different regions of the world. This analysis demonstrates that small body size is the plesiomorphic state and largeness has likely arisen at least twice. Finally, we compiled data on shell length from the fossil record to determine how (slowly or suddenly) and when large body size arose in the abalones. These data indicate that large body size appears suddenly at the Miocene/Pliocene boundary. Our findings support the view that fleshy-algal dominated ecosystems radiated rapidly in the coastal oceans with the onset of the most recent glacial age. We conclude with a discussion of the broader implications of this change.
This paper tests whether the most common fossil brachiopod, gastropod, and bivalve genera also have intrinsically more durable shells. Commonness was quantified using occurrence frequency of the 450 most frequently occurring genera of these groups in the Paleobiology Database (PBDB). Durability was scored for each taxon on the basis of shell size, thickness, reinforcement (ribs, folds, spines), mineralogy, and microstructural organic content. Contrary to taphonomic expectation, common genera in the PBDB are as likely to be small, thin-shelled, and unreinforced as large, thick-shelled, ribbed, folded, or spiny. In fact, only six of the 30 tests we performed showed a statistically significant relationship between durability and occurrence frequency, and these six tests were equally divided in supporting or contradicting the taphonomic expectation. Thus, for the most commonly occurring genera in these three important groups, taphonomic effects are either neutral with respect to durability or compensated for by other factors (e.g., less durable taxa were more common in the original communities). These results suggest that biological information is retained in the occurrence frequency patterns of our target groups.
Understanding the patterns, causes, and consequences of biotic interchange—the movement of species between neighboring biotas—is crucial for evaluating the effects of human-introduced species in the modern biosphere. Since at least the early Miocene, the tropical and subtropical western Atlantic has comprised two biogeographic provinces, the Gatunian (including the Caribbean) and the Caloosahatchian (North Carolina to Florida and the Yucatán peninsula). Although these adjacent provinces are not separated by a land barrier, exchange of species between them has been limited and intermittent. A synthesis of taxonomic, phylogenetic, stratigraphic, and biogeographic data on six gastropod and two bivalve groups reveals a dramatic shift in the pattern of interchange between these provinces. About 31% of early Miocene Caloosahatchian subgenus- and species-group-level taxa invaded the Gatunian Province by the late Miocene, but no taxa extended their ranges in the opposite direction. Beginning in the early Pliocene and continuing into the early Pleistocene, 40 taxa (roughly one-third of Gatunian diversity) invaded the Caloosahatchian Province from the Caribbean, whereas only four taxa extended their range from Florida into the Caribbean.
Comparisons between the ranked percentage of Gatunian invaders in Florida and the magnitude of regional extinction there for each of four middle Pliocene to early Pleistocene intervals reveal no consistent relation between invasion and prior or concurrent extinction. During the Pliocene, invaders not only compensated for extinctions, but also accounted for almost all the observed increase in standing diversity in Florida. Only after the large extinction event at the end of the Pliocene did invaders from the Gatunian Province not fully compensate for the loss of species.
Although the Miocene interaction between the Gatunian and Caloosahatchian biotas involved two fully tropical entities, the Plio–Pleistocene interaction exemplifies a general pattern in which tropical species often spread to higher latitudes during warm intervals, but warm-temperate or subtropical species rarely become established in the Tropics. Some evidence indicates that tropical Caribbean molluscs are exposed and adapted to more intense competition and predation than their subtropical counterparts in Florida, implying a role for individual-level biotic interactions in determining the predominant direction of interchange. Intensification of north-flowing currents in the western Atlantic may also contribute to the nearly one-way movement of taxa from the Caribbean to Florida during the Pliocene and early Pleistocene. The changing pattern of interchange from the Miocene to the Pliocene further reflects a change in the geography of species richness, with the richer province serving as the chief donor and the province with lower diversity acting as the main recipient of invaders. Diversity, ocean circulation, and the competitive environment thus account for the observed switch in the predominant direction of invasion in the western Atlantic during the Neogene.
The fact that almost 90% of Gatunian immigrants to Florida differentiated taxonomically there indicates that invasion is intermittent. Long-term consequences of this and many other cases of interchange between provinces include enrichment of the regional and global species pool and the spread of adaptations reflecting intense competition and predation.
A theoretical morphologic model defining patterns of shell sculptures in Bivalvia is introduced. It is based on the displacement of sculptural elements along the growing shell margin and introduction of new sculptural elements. The kinematics of the sculptural elements are defined in terms of the following parameters: maximum speed of displacement of sculptural elements, position along the growing shell margin where a migrating element attains maximum speed, and position of the divergence axis of the riblets. Computer models successfully mimicked most of the diverse patterns of bivalve shell sculptures. Morphometric analysis revealed that the displacement speed of a sculptural element is not constant but depends on the relative position of the element on the shell margin. It was revealed that the primary component of variation in bivalve shell sculptures could be explained by variation in the displacement speed of sculptural elements around the divergence axis of riblets.
This paper investigates trends in the evolution of body size and shape in the Plesiosauria, a diverse clade of Mesozoic marine reptiles. Using measures from well-preserved plesiosaur specimens, we document and interpret evolutionary patterns in relative head size, body size, and locomotor variables. Size increase is a significant trend in the clade as a whole, and in constituent clades. The trend in relative head size is of variance increase; observed head sizes are both smaller and larger than ancestral values. In the locomotor system, changes in propodial and girdle proportions appear concomitant with body size increase and are interpreted as allometric responses to the physical constraints of large body size. Other trends in the locomotor system are significantly correlated with both body size and relative head size. These locomotor trends evolved convergently in several clades of plesiosaurs, and may have had an ecomorphological basis, although data are lacking to constrain speculation on this point. The evolution of the locomotor system in plesiosaurs sheds new light on the response of aquatic tetrapods to the physical constraints of foraging at large body size.
Muscle moment arms are important determinants of muscle function; however, it is challenging to determine moment arms by inspecting bone specimens alone, as muscles have curvilinear paths that change as joints rotate. The goals of this study were to (1) develop a three-dimensional graphics-based model of the musculoskeletal system of the Cretaceous theropod dinosaur Tyrannosaurus rex that predicts muscle-tendon unit paths, lengths, and moment arms for a range of limb positions; (2) use the model to determine how the T. rex hindlimb muscle moment arms varied between crouched and upright poses; (3) compare the predicted moment arms with previous assessments of muscle function in dinosaurs; (4) evaluate how the magnitudes of these moment arms compare with those in other animals; and (5) integrate these findings with previous biomechanical studies to produce a revised appraisal of stance, gait, and speed in T. rex. The musculoskeletal model includes ten degrees of joint freedom (flexion/extension, ab/adduction, or medial/ lateral rotation) and 33 main muscle groups crossing the hip, knee, ankle, and toe joints of each hindlimb. The model was developed by acquiring and processing bone geometric data, defining joint rotation axes, justifying muscle attachment sites, and specifying muscle-tendon geometry and paths. Flexor and extensor muscle moment arms about all of the main limb joints were estimated, and limb orientation was statically varied to characterize how the muscle moment arms changed. We used sensitivity analysis of uncertain parameters, such as muscle origin and insertion centroids, to deterimine how much our conclusions depend on the muscle reconstruction we adopted. This shows that a specific amount of error in the reconstruction (e.g., position of muscle origins) can have a greater, lesser, similar, or no effect on the moment arms, depending on complex interactions between components of the musculoskeletal geometry. We found that more upright poses would have improved mechanical advantage of the muscles considerably. Our analysis shows that previously assumed moment arm values were generally conservatively high. Our results for muscle moment arms are generally lower than the values predicted by scaling data from extant taxa, suggesting that T. rex did not have the allometrically large muscle moment arms that might be expected in a proficient runner. The information provided by the model is important for determining how T. rex stood and walked, and how the muscles of a 4000–7000 kg biped might have worked in comparison with extant bipeds such as birds and humans. Our model thus strengthens the conclusion that T. rex was not an exceptionally fast runner, and supports the inference that more upright (although not completely columnar) poses are more plausible for T. rex. These results confirm general principles about the relationship between size, limb orientation, and locomotor mechanics: exceptionally big animals have a more limited range of locomotor abilities and tend to adopt more upright poses that improve extensor muscle effective mechanical advantage. This model builds on previous phylogenetically based muscle reconstructions and so moves closer to a fully dynamic, three-dimensional model of stance, gait, and speed in T. rex.