The Main Karoo Basin of South Africa contains a near-continuous sequence of continental deposition spanning ~80 Myr from the mid-Permian to the Early Jurassic. The terrestrial vertebrates of this sequence provide a high-resolution stratigraphic record of regional origination and extinction, especially for the mid–late Permian. Until now, data have only been surveyed at coarse stratigraphic resolution using methods that are biased by nonuniform sampling rates, limiting our understanding of the dynamics of diversification through this important time period. Here, we apply robust methods (gap-filler and modified gap-filler rates) for the inference of patterns of species richness, origination rates, and extinction rates to a subset of 1321 reliably-identified fossil occurrences resolved to approximately 50 m stratigraphic intervals. This data set provides an approximate time resolution of 0.3–0.6 Myr and shows that extinction rates increased considerably in the upper 100 m of the mid-Permian Abrahamskraal Formation, corresponding to the latest part of the Tapinocephalus Assemblage Zone (AZ). Origination rates were only weakly elevated in the same interval and were not sufficient to compensate for these extinctions. Subsampled species richness estimates for the lower part of the overlying Teekloof Formation (corresponding to the Pristerognathus and Tropidostoma AZs) are low, showing that species richness remained low for at least 1.5–3 million years after the main extinction pulse. A high unevenness of the taxon abundance–frequency distribution, which is classically associated with trophically unstable postextinction faunas, in fact developed shortly before the acme of elevated extinction rates due to the appearance and proliferation of the dicynodont Diictodon. Our findings provide strong support for a Capitanian (“end-Guadalupian”) extinction event among terrestrial vertebrates and suggest that further high-resolution quantitative studies may help resolve the lack of consensus among paleobiologists regarding this event.

The two major approaches to studying macroevolution in deep time are the fossil record and reconstructed relationships among extant taxa from molecular data. Results based on one approach sometimes conflict with those based on the other, with inconsistencies often attributed to inherent flaws of one (or the other) data source. Any contradiction between the molecular and fossil records represents a failure of our ability to understand the imperfections of our data, as both are limited reflections of the same evolutionary history. We therefore need to develop conceptual and mathematical models that jointly explain our observations in both records. Fortunately, the different limitations of each record provide an opportunity to test or calibrate the other, and new methodological developments leverage both records simultaneously. However, we must reckon with the distinct relationships between sampling and time in the fossil record and molecular phylogenies. These differences impact our recognition of baselines and the analytical incorporation of age estimate uncertainty.

For mammals today, mountains are diverse ecosystems globally, yet the strong relationship between species richness and topographic complexity is not a persistent feature of the fossil record. Based on fossil-occurrence data, diversity and diversification rates in the intermontane western North America varied through time, increasing significantly during an interval of global warming and regional intensification of tectonic activity from 18 to 14 Ma. However, our ability to infer origination and extinction rates reliably from the fossil record is affected by variation in preservation history. To investigate the influence of preservation on estimates of diversification rates, I simulated fossil records under four alternative diversification hypotheses and six preservation scenarios. Diversification hypotheses included tectonically controlled speciation pulses, while preservation scenarios were based on common trends (e.g., increasing rock record toward the present) or derived from fossil occurrences and the continental rock record. For each scenario, I estimated origination, extinction, and diversification rates using three standard methods—per capita, three-timer, and capture–mark–recapture (CMR) metrics—and evaluated the ability of the simulated fossil records to accurately recover the underlying diversification dynamics. Despite variable and low preservation probabilities, simulated fossil records retained the signal of true rates in several of the scenarios. The three metrics did not exhibit similar behavior under each preservation scenario: while three-timer and CMR metrics produced more accurate rate estimates, per capita rates tended to better reproduce true shifts in origination rates. All metrics suffered from spurious peaks in origination and extinction rates when highly volatile preservation impacted the simulated record. Results from these simulations indicate that elevated diversification rates in relation to tectonic activity during the middle Miocene are likely to be evident in the fossil record, even if preservation in the North American fossil record was variable. Input from the past is necessary to evaluate the ultimate mechanisms underlying speciation and extinction dynamics.

Despite a global fossil record, Metatheria are now largely restricted to Australasia and South America. Most metatherian paleodiversity studies to date are limited to particular subclades, time intervals, and/or regions, and few consider uneven sampling. Here, we present a comprehensive new data set on metatherian fossil occurrences (Barremian to end Pliocene). These data are analyzed using standard rarefaction and shareholder quorum subsampling (including a new protocol for handling Lagerstätte-like localities).

Global metatherian diversity was lowest during the Cretaceous, and increased sharply in the Paleocene, when the South American record begins. Global and South American diversity rose in the early Eocene then fell in the late Eocene, in contrast to the North American pattern. In the Oligocene, diversity declined in the Americas, but this was more than offset by Oligocene radiations in Australia. Diversity continued to decrease in Laurasia, with final representatives in North America (excluding the later entry of Didelphis virginiana) and Europe in the early Miocene, and Asia in the middle Miocene. Global metatherian diversity appears to have peaked in the early Miocene, especially in Australia. Following a trough in the late Miocene, the Pliocene saw another increase in global diversity. By this time, metatherian biogeographic distribution had essentially contracted to that of today.

Comparison of the raw and sampling-corrected diversity estimates, coupled with evaluation of “coverage” and number of prolific sites, demonstrates that the metatherian fossil record is spatially and temporally extremely patchy. Therefore, assessments of macroevolutionary patterns based on the raw fossil record (as in most previous studies) are inadvisable.

Ecologists and paleontologists alike are increasingly using the fossil record as a spatial data set, in particular to study the dynamics and distribution of geographic range sizes among fossil taxa. However, no attempts have been made to establish how accurately range sizes and range-size dynamics can be preserved. Two fundamental questions are: Can common paleo range-size reconstruction methods accurately reproduce known species’ ranges from locality (i.e., point) data? And, are some reconstruction methods more reliable than others? Here, we develop a methodological framework for testing the accuracy of commonly used paleo range-size reconstruction methods (maximum latitudinal range, maximum great-circle distance, convex hull, and alpha convex hull) in different extinction-related biogeographic scenarios. We use the current distribution of surface water bodies as a proxy for “preservable area,” in which to test the performance of the four methods. We find that maximum great-circle distance and convex-hull methods most reliably capture changes in range size at low numbers of fossil sites, whereas convex hull performs best at predicting the distribution of “victims” and “survivors” in hypothetical extinction scenarios. Our results suggest that macroevolutionary and macroecological patterns in the relatively recent past can be studied reliably using only a few fossil occurrence sites. The accuracy of range-size reconstruction undoubtedly changes through time with the distribution and area of fossiliferous sediments; however, our approach provides the opportunity to systematically calibrate the quality of the spatial fossil record in specific environments and time intervals, and to delineate the conditions under which paleobiologists can reconstruct paleobiogeographical, macroecological, and macroevolutionary patterns over critical intervals in Earth history.

Certain taxa are noticeably common within collections, widely distributed, and frequently long-lived. We have examined these dominant genera as compared with rarer genera, with a focus on their temporal histories. Using occurrence data from the Paleobiology Database, we determined which genera belonging to six target groups ranked among the most common within each of 49 temporal bins based on occurrences. The turnover among these dominant taxa from bin to bin was then determined for each of these groups, and all six groups when pooled. Although dominant genera are only a small fraction of all genera, the patterns of turnover mimic those seen in much larger compilations of total biodiversity. We also found that differences in patterns of turnover at the top ranks among the higher taxa reflect previously documented comparison of overall turnover among these classes. Both dominant and nondominant genera exhibit, on average, symmetrical patterns of rise and fall between first and last appearances. Dominant genera rarely begin at high ranks, but nevertheless tend to be more common when they first appear than nondominant genera. Moreover, dominant genera rarely are in the top 20 when they last appear, but still typically occupy more localities than nondominant genera occupy in their last interval. The mechanism(s) that produce dominant genera remain unclear. Nearly half of dominant genera are the type genus of a family or subfamily. This is consistent with a simple model of morphological and phylogenetic diversification and sampling.

The Signor-Lipps effect states that even a sudden mass extinction will invariably appear gradual in the fossil record, due to incomplete fossil preservation. Most previous work on the Signor–Lipps effect has focused on testing whether taxa in a mass extinction went extinct simultaneously or gradually. However, many authors have proposed scenarios in which taxa went extinct in distinct pulses. Little methodology has been developed for quantifying characteristics of such pulsed extinction events. Here we introduce a method for estimating the number of pulses in a mass extinction, based on the positions of fossil occurrences in a stratigraphic section. Rather than using a hypothesis test and assuming simultaneous extinction as the default, we reframe the question by asking what number of pulses best explains the observed fossil record.

Using a two-step algorithm, we are able to estimate not just the number of extinction pulses but also a confidence level or posterior probability for each possible number of pulses. In the first step, we find the maximum likelihood estimate for each possible number of pulses. In the second step, we calculate the Akaike information criterion and Bayesian information criterion weights for each possible number of pulses, and then apply a k-nearest neighbor classifier to these weights. This method gives us a vector of confidence levels for the number of extinction pulses—for instance, we might be 80% confident that there was a single extinction pulse, 15% confident that there were two pulses, and 5% confident that there were three pulses. Equivalently, we can state that we are 95% confident that the number of extinction pulses is one or two. Using simulation studies, we show that the method performs well in a variety of situations, although it has difficulty in the case of decreasing fossil recovery potential, and it is most effective for small numbers of pulses unless the sample size is large. We demonstrate the method using a data set of Late Cretaceous ammonites.

Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

Mass extinctions affect the history of life by decimating existing diversity and ecological structure and creating new evolutionary and ecological pathways. Both the loss of diversity during these events and the rebound in diversity following extinction had a profound effect on Phanerozoic evolutionary trends. Phylogenetic trees can be used to robustly assess the evolutionary implications of extinction and origination.

We examine both extinction and origination during the Late Ordovician mass extinction. This mass extinction was the second largest in terms of taxonomic loss but did not appear to radically alter Paleozoic marine assemblages. We focus on the brachiopod order Strophomenida, whose evolutionary relationships have been recently revised, to explore the disconnect between the processes that drive taxonomic loss and those that restructure ecological communities.

A possible explanation for this disconnect is if extinction and origination were random with respect to morphology. We define morphospace using principal coordinates analysis (PCO) of character data from 61 Ordovician–Devonian taxa and their 45 ancestral nodes, defined by a most parsimonious reconstruction in Mesquite. A bootstrap of the centroid of PCO values indicates that genera were randomly removed from morphospace by the Late Ordovician mass extinction, and new Silurian genera were clustered within a smaller previously unoccupied region of morphospace. Diversification remained morphologically constrained throughout the Silurian and into the Devonian. This suggests that the recovery from the Late Ordovician mass extinction resulted in a long-term shift in strophomenide evolution. More broadly, recovery intervals may hold clues to understanding the evolutionary impact of mass extinctions.

Numerical simulations of neutral metacommunities are used here to predict the effects of growth and shrinkage of metacommunities, as well as their separation and merging caused by continental collision and rifting and their secondary eustatic effects. Although growth and shrinkage of metacommunities predictably change diversity, separating and merging metacommunities have counterintuitive effects. Separating and merging metacommunities change diversity within the individual areas, especially so for smaller areas, but they cause no change in total diversity of the system, contrary to previous predictions. The response times of metacommunities are likely to be geologically undetectable except for enormously large systems. These models can be used to predict the plate-tectonic effects on the diversity of terrestrial, coastal-marine, deep-marine, and oceanic-island systems. Of these, global and regional coastal-marine systems are the most acutely sensitive to the changes in area and fragmentation caused by plate tectonics. Oceanic-island systems also experience global and regional changes in diversity during supercontinent breakup and assembly, with the global effects driven by the changing length of volcanic arcs, and the regional effects also driven by secondary eustatic changes in shallow-marine area. Although individual terrestrial provinces or continents may experience substantial changes in diversity from rifting and collision, global terrestrial diversity should be unchanged except for the relatively modest contributions caused by the secondary eustatic effects on land area. These changes in diversity may be reinforced or counteracted by the changing latitudinal position of metacommunities.

The quantity of biomass in an ecosystem is constrained by energy availability. It is less clear, however, how energy availability constrains taxonomic and functional diversity. Competing models suggest biodiversity is either resource-limited or far from any bound. We test the hypothesis that functional diversity in marine bivalve communities is constrained by energy availability, measured as particulate organic carbon (POC) flux, in the modern oceans. We find that POC flux predicts the relative prevalence of ecological modes in both the Atlantic and Pacific Oceans. Moreover, the associations of ecological modes with POC fluxes are similar between the Atlantic and Pacific despite being based on independent sets of species, indicating a direct causal relationship. We then use the relationship between POC flux and the prevalence of functional groups in the modern to test the hypothesis that the trend of increasing functional diversity in bivalves across the past 500 Myr has occurred in response to increased POC flux. We find no evidence that the earliest-appearing modes of life are preferentially associated with low-POC environments or that the mean POC flux experienced by marine bivalves has increased across geological time. To reconcile the close association between ecological mode and POC flux in the modern oceans with the lack of evidence for increasing POC fluxes across time, we propose that POC flux has not increased substantially over time but, rather, the increase in bivalve functional diversity enabled bivalves to become more abundant, to occupy a broader range of environments, and to capture a greater fraction of the total POC flux. The results here suggest at the geographic scale of oceans and through geologic time bivalve diversity was not bounded by food availability.

Bedding-plane assemblages of Ediacaran fossils from Mistaken Point, Newfoundland, are among the oldest known records of complex multicellular life on Earth (dated to ~565 Ma). The in situ preservation of these sessile but otherwise deeply enigmatic organisms means that statistical analyses of specimen positions can be used to illuminate their underlying ecological dynamics, including the interactions between taxa.

Fossil assemblages on Mistaken Point D and E surfaces were mapped to millimeter accuracy using differentiated GPS. Spatial correlations between 10 well-defined taxa (Bradgatia, Charniid, Charniodiscus, Fractofusus, Ivesheadiomorphs, Lobate Discs, Pectinifrons, Plumeropriscum, Hiemalora, and Thectardis) were identified using Bayesian network inference (BNI), and then described and analyzed using spatial point-process analysis. BNI found that the E-surface community had a complex web of interactions and associations between taxa, with all but one taxon (Thectardis) interacting with at least one other. The unique spatial distribution of Thectardis supports previous, morphology-based arguments for its fundamentally distinct nature. BNI revealed that the D-surface community showed no interspecific interactions or associations, a pattern consistent with a homogeneous environment.

On the E surface, all six of the abundant taxonomic groups (Fractofusus, Bradgatia, Charniid, Charniodiscus, Thectardis, and Plumeropriscum) were found to have a unique set of interactions with other taxa, reflecting a broad range of underlying ecological responses. Four instances of habitat associations were detected between taxa, of which two (Charniodiscus–Plumeropriscum and Plumeropriscum–Fractofusus) led to weak competition for resources. One case of preemptive competition between Charniid and Lobate Discs was detected. There were no instances of interspecific facilitation. Ivesheadiomorph interactions mirror those of Fractofusus and Charniodiscus, identifying them as a form-taxonomic grouping of degradationally homogenized taphomorphs. The absence of increased fossil abundance in proximity to these taphomorphs argues against scavenging or saprophytic behaviors dominating the E-surface community.

Comprising Holostei and Teleostei, the ~32,000 species of neopterygian fishes are anatomically disparate and represent the dominant group of aquatic vertebrates today. However, the pattern by which teleosts rose to represent almost all of this diversity, while their holostean sister-group dwindled to eight extant species and two broad morphologies, is poorly constrained. A geometric morphometric approach was taken to generate a morphospace from more than 400 fossil taxa, representing almost all articulated neopterygian taxa known from the first 150 million years—roughly 60%—of their history (Triassic‒Early Cretaceous). Patterns of morphospace occupancy and disparity are examined to: (1) assess evidence for a phenotypically “dominant” holostean phase; (2) evaluate whether expansions in teleost phenotypic variety are predominantly abrupt or gradual, including assessment of whether early apomorphy-defined teleosts are as morphologically conservative as typically assumed; and (3) compare diversification in crown and stem teleosts. The systematic affinities of dapediiforms and pycnodontiforms, two extinct neopterygian clades of uncertain phylogenetic placement, significantly impact patterns of morphological diversification. For instance, alternative placements dictate whether or not holosteans possessed statistically higher disparity than teleosts in the Late Triassic and Jurassic. Despite this ambiguity, all scenarios agree that holosteans do not exhibit a decline in disparity during the Early Triassic‒Early Cretaceous interval, but instead maintain their Toarcian‒Callovian variety until the end of the Early Cretaceous without substantial further expansions. After a conservative Induan‒Carnian phase, teleosts colonize (and persistently occupy) novel regions of morphospace in a predominantly gradual manner until the Hauterivian, after which expansions are rare. Furthermore, apomorphy-defined teleosts possess greater phenotypic variety than typically assumed. Comparison of crown and stem teleost partial disparity indicates that, despite a statistically significant increase in crown teleost disparity between the Late Jurassic and earliest Cretaceous, stem teleosts remained important long-term contributors to overall teleost disparity during this time.

Miniaturization has been defined as the evolution of extremely small adult size in a lineage. It does not simply imply the decrease of the body size but also involves structural modifications to maintain functional efficiency at a strongly reduced size. Miniaturization has been proposed as a key factor in the origin of several major tetrapod clades. Current hypotheses propose that the living amphibians (lissamphibians) originated within a clade of Paleozoic dwarfed dissorophoid temnospondyls. Morphological traits shared by these small dissorophoids have been interpreted as resulting from constraints imposed by the extreme size reduction, but these statements were based only on qualitative observations. Herein, we assess quantitatively morphological changes in the skull previously associated with miniaturization in the lissamphibian stem lineage by comparing evolutionary and ontogenetic allometries in dissorophoids. Our results show that these features are not comparable to the morphological consequences of extreme size reduction as documented in extant miniature amphibians, but instead they resemble immature conditions of larger temnospondyls. We conclude that the truncation of the ancestral ontogeny, and not constraints related to miniaturization, might have been the factor that played a major role in the morphological evolution of small dissorophoids. Based on our results, we discuss the putative role of miniaturization in the origin of lissamphibians within Dissorophoidea.

Understanding temporal patterns in biodiversity is an enduring question in paleontology. Compared with studies of taxonomic diversity, long-term perspectives on ecological diversity are rare, particularly in terrestrial systems. Yet ecological diversity is critical for the maintenance of biodiversity, especially during times of major perturbations. Here, we explore the ecological diversity of Cretaceous herbivorous dinosaurs leading up to the K-Pg extinction, using dental and jaw morphological disparity as a proxy. We test the hypothesis that a decline in ecological diversity could have facilitated their rapid extinction 66 Ma. We apply three disparity metrics that together capture different aspects of morphospace occupation and show how this approach is key to understanding patterns of morphological evolution. We find no evidence of declining disparity in herbivorous dinosaurs as a whole—suggesting that dinosaur ecological diversity remained high during the last 10 Myr of their existence. Clades show different disparity trends through the Cretaceous, but none except sauropods exhibits a long-term decline. Herbivorous dinosaurs show two disparity peaks characterized by different processes; in the Early Cretaceous by expansion in morphospace and in the Campanian by morphospace packing. These trends were only revealed by using a combination of disparity metrics, demonstrating how this approach can offer novel insights into macroevolutionary processes underlying patterns of disparity and ecological diversity.

Vertebrates with terrestrial or freshwater ancestors colonized the sea from the Early Triassic onward and became competitively dominant members of many marine ecosystems throughout the Mesozoic and Cenozoic eras. The circumstances that led to initial marine colonization have, however, received little attention. One hypothesis is that mass extinction associated with ecosystem collapse provided opportunities for clades of amphibians, reptiles, birds, and mammals to enter marine environments. Another is that competitive pressures in donor ecosystems on land and in freshwater, coupled with abundant food in nearshore marine habitats, favored marine colonization. Here we test these hypotheses by compiling all known secondarily marine amniote clades and their times of colonization. Marine amniotes are defined as animals whose diet consists primarily of marine organisms and whose locomotion includes swimming, diving, or wading in salt water. We compared the number of clades entering during recovery phases from mass extinctions with the rate of entry of clades during nonrecovery intervals of the Mesozoic and Cenozoic. We conservatively identify 69 marine colonizations by amniotes. The only recovery interval for which prior mass extinction could have been a trigger for marine entry is the Early Triassic, when four clades colonized the sea over 7 Myr, significantly above the rates at which clades entered during other intervals. High nearshore productivity was a greater enticement to colonization than was a low diversity of potential marine competitors or predators in nearshore environments of a highly competitive terrestrial or freshwater donor biota. Rates of marine entry increased during the Cenozoic, in part because of rising productivity and in part thanks to the participation of warm-blooded birds and mammals, which broadened the range of thermal environments in which initial colonization of the sea became possible.

Previous attempts to quantify the test-flattening trend in Heterostegina depressa with water depth have been rather unsuccessful. Due to its broad depth distribution, H. depressa is a perfect model species to calibrate test flattening as a bathymetric signal for fossil assemblages. This might enable us to better reconstruct paleoenvironments of fossil communities of larger foraminifera or even provide clues to the degree of transport in allochthonous deposits. In this study, we used growth-independent functions to describe the change of test thickness throughout ontogeny. Four growth-invariant characters, deriving from these functions, clearly quantify a transition of individuals with thicker to thinner central parts along the water-depth gradient. This transition is probably controlled by light intensity, because the photosymbionts of H. depressa (diatoms) are most effective at low irradiation levels. Thus, specimens at shallower depths grow thicker to reduce light penetration, whereas specimens living deeper than the light optimum increase their surface by flattening to obtain better exposure to light.

The environmental transformations that occurred during the Neogene had profound effects on spatiotemporal biodiversity patterns, yet the modulating role of traits (i.e., physiological, ecological, and life-history traits) remains little understood. We tested this idea using the Neogene fossil record of chondrichthyans along the temperate Pacific coast of South America (TPSA). Information for georeferenced occurrences and ecological and life-history information of 38 chondrichthyan fossil genera in 42 Neogene sites was collected. Global georeferenced records were used to estimate present-day biogeographic distributions of the genera and to characterize the range of oceanographic conditions in which each genus lives as a proxy of their realized niche. Biogeographic range shifts (Neogene–present) were evaluated at regional and local scales. The role of traits as drivers of different range dynamics was evaluated using random forest models. The magnitude and direction of biogeographic range shifts were different at both spatial scales. At a regional scale, 34% of genera contracted their ranges, disappearing from the TPSA. At a local scale, a similar proportion of genera expanded and contracted their southern endpoints of distribution. The models showed a high precision at both spatial scales of analyses, but the relative importance of predictor variables differed. At a regional scale, disappearing genera tended to have a higher tolerance to salinity, lower sea surface temperature (SST) range, and smaller body sizes. At a local scale, genera contracting their ranges tended to live at greater depths, tolerate lower levels of primary productivity, and show a reduced tolerance to higher and lower SST ranges. The magnitude and direction of the changes in the range distribution were scale dependent and variable across the genera. Hence, multiple environmental exogenous factors interacted with taxon traits during the Neogene, creating a mosaic of biogeographic dynamics.

The fossil record of mammal dentition provides crucial insight into key ecological and functional transitions throughout mammalian evolutionary history. For cetaceans, both extant clades differ markedly from their stem ancestors; neither retains the differentiated dentition or the tribosphenic molars characteristic of Mammalia. We used quantitative measures of dental complexity across fossil and living cetaceans to identify a trend toward dental simplicity through the Neogene. Both extant cetacean clades depart from the ancestral mammalian condition and concurrently converge upon a reduced and simplified dentition; modern mysticetes all have become entirely edentulous (at birth), and living odontocetes possess teeth as single-rooted, conical pegs. These two parallel trends accompany major shifts in feeding strategy (i.e., filter feeding in mysticetes and echolocation in odontocetes), suggesting that these evolutionary innovations for prey acquisition are enabling factors for the loss of prey processing and subsequent convergence on dental simplification.

A large sample of postembryonic specimens of Dalmanitina proaeva elfrida and D. socialis from the Upper Ordovician (Sandbian to Katian) Prague Basin allows for the first reasonably complete ontogenetic sequence of Dalmanitoidea (Phacopina). The material provides an abundance of morphological information, including well-preserved marginal spines in protaspides and meraspides, and hypostome external surfaces throughout. The development of D. proaeva elfrida is unusual due to variability in timing of the first trunk articulation. This broadens our developmental understanding of Phacopina, a diverse group of phacopid trilobites, and also allows us to study the evolution of their specializations in exoskeletal molting behavior. Adult phacopines, unlike most other trilobites, had fused facial sutures. This means that rather than molting through the sutural gape mode, characterized by opening of the facial sutures and separation of the librigenae, they disarticulated the entire cephalon in Salter’s mode of molting. For other phacopine clades (Phacopoidea) the transition to Salter’s mode occurs during the meraspid period or at the onset of holaspis, and its developmental timing is intraspecifically fixed. However, owing to the large sample size, we can see that facial suture fusion likely occurred later in Dalmanitina, usually during the holaspid period, and was intraspecifically variable with holaspides of varying sizes showing unfused sutures. Further, D. proaeva elfrida specimens showed an initial librigenal–rostral plate fusion event, where the librigenae began as separate entities but appear fused with the rostral plate as one structure (the “lower cephalic unit”) from M1, and are discarded as such during molting. Dalmanitoidea is considered to represent the first phacopine divergence, occurring earliest in the fossil record. This material therefore provides insight into how linked morphologies and behaviors evolved, potentially suggesting the timing of facial suture fusion in Phacopina moved earlier during development and became more intraspecifically fixed over geological time.