The enigmatic evolutionary relationships of Palaeocene mammals and their relevance for the Tertiary radiation of placental mammals

Understanding the general pattern of how a clade evolves over time is a central aim of palaeontology and evolutionary biology. The observation that the tree of life is asymmetric in species distribution necessitates that rates of evolution, speciation, and extinction vary through time and across phylogeny. The way this variation is distributed can help to inform on historic events, selection pressures, and relationships. Often, at the origination of a clade, it is supposed that there is an ‘early burst’ of diversification, before rates of speciation and morphological evolution slow down as the clade ages. One example of a supposed ‘early burst’ is that of placental mammals, but the internal relationships of the earliest members of this group have prevented further study of macroevolutionary parameters. In this thesis, by building the largest cladistic data matrix to date, I test the relationships of mammals from the earliest Cenozoic, and from the resulting phylogenies, test the hypotheses that the end-Cretaceous mass extinction resulted in an adaptive radiation of placental mammals. I show that Phenacodontidae are most parsimoniously ancestral to Perissodactyla, that a division between Boreoeutheria and Atlantogenata is better supported than one between Xenarthra and Epitheria or Afrotheria and Exafroplacentalia at the root of Placentalia, and that all “condylarths” can be placed, with varying degrees of confidence, as stem members of laurasiatherian orders. I show that there was an increase in rate of morphological evolution immediately after the end-Cretaceous mass extinction, that Placentalia is extremely likely to have originated less than 70 million years ago, and that the rise of Placentalia was associated with an increase in morphospace occupation, and, with a lag, mean pairwise dissimilarity of taxa. These conclusions support the contention that the end-Cretaceous mass extinction was not just an important time in Earth’s ecological history, but crucial to the diversification of mammals to the level observed today

Eutherian morphological disparity across the end-Cretaceous mass extinction

© 2015 The Linnean Society of London. In the aftermaths of mass extinction events, during radiations of clades, and in several other evolutionary scenarios, there is often a decoupling of taxonomic diversity and morphological disparity. The placental mammal radiation after the end-Cretaceous mass extinction is one of the archetypal adaptive radiations, but the change in morphological disparity of the entire skeleton has never been quantified across this important boundary. We reconstruct ancestral morphologies of 680 discrete morphological characters onto dated phylogenies of 177 mostly Cretaceous and Palaeogene eutherians (placental mammals and their stem relatives). Using a new approach to incorporate morphologies representing ghost lineages, we assess three measures of morphological disparity (sum of ranges, sum of variances and mean pairwise dissimilarity) across stage-level time bins within the Cretaceous and Palaeogene. We find that the range-based metric suggests that eutherian disparity increased immediately after the end-Cretaceous mass extinction, while both variance-based metrics declined from the Campanian to the Maastrichtian, but showed no change in disparity from the Maastrichtian to the Puercan - the first North American Land Mammal Age of the Paleocene. Increases in variance-based metrics lag behind the range-based metric and per-lineage accumulation rate, suggesting that the response of mammals to the Cretaceous-Palaeogene event was characterized by an early radiation that increased overall morphospace occupation, followed later by specialization that resulted in increased dissimilarity

Late Artinskian–Early Kungurian (Early Permian) warming and maximum marine flooding in the East Gondwana interior rift, Timor and Western Australia, and comparisons across East Gondwana

Substantial new information is presented on upper Artinskian–Kungurian deposits in Timor-Leste and in the Canning, Southern Carnarvon and northern Perth basins of Western Australia. These basins, situated between about 35°S and 55°S palaeolatitude, formed part of the East Gondwana interior rift, a precursor to the rift that 100 my later formed the Indian Ocean in this region. Timor lay near the main axis of the East Gondwana interior rift, whereas the Western Australian basins were marginal splays from the rift axis. The main depocentres developed as a result of faulting that was initiated during the Late Pennsylvanian. Detailed lithostratigraphic and biostratigraphic analyses have been made on the newly recognized Bua-bai limestone and the type Cribas Group in Timor, the Noonkanbah Formation in the Canning Basin, the Byro Group in the Merlinleigh Sub-basin of the Southern Carnarvon Basin, and the Carynginia Formation in the northern Perth Basin. In Timor the succession, which is highly disrupted by faulting, was deposited under open-marine conditions probably in a shelf–basin setting. Restricted, very shallow-water seas flooded the Canning Basin and the Merlinleigh–Byro–Irwin sub-basins of the Southern Carnarvon and northern Perth basins and had highly variable oxygen levels and salinities typical of estuarine environments. A similar pattern of warming and bathymetric change is recognized in all studied basins. During the early part of the late Artinskian cool conditions prevailed, with water temperatures 0–4 °C forming sea ice in the Merlinleigh–Byro–Irwin rift. Rapid warming during the latter part of the late Artinskian was accompanied by maximum marine flooding close to the Artinskian–Kungurian boundary. Climatic and bathymetric conditions then allowed carbonate mounds, with larger fusulines and a variety of algae, to develop in the northern part of the rift system, and Tubiphytes, conodonts, and brachiopods with Tethyan affinities to migrate into the marginal-rift basins despite the generally adverse water quality at these depositional sites. Comparison between the stratigraphic record from the East Gondwana interior rift and coeval records from Lhasa and Sibumasu indicate a similar pattern of climate change during the Carboniferous to end Cisuralian. Similar trends probably are present in Eastern Australia although there is confusion over the correlation of some units

Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous-Palaeogene mass extinction.

The effect of the Cretaceous-Palaeogene (K-Pg) mass extinction on the evolution of many groups, including placental mammals, has been hotly debated. The fossil record suggests a sudden adaptive radiation of placentals immediately after the event, but several recent quantitative analyses have reconstructed no significant increase in either clade origination rates or rates of character evolution in the Palaeocene. Here we use stochastic methods to date a recent phylogenetic analysis of Cretaceous and Palaeocene mammals and show that Placentalia likely originated in the Late Cretaceous, but that most intraordinal diversification occurred during the earliest Palaeocene. This analysis reconstructs fewer than 10 placental mammal lineages crossing the K-Pg boundary. Moreover, we show that rates of morphological evolution in the 5 Myr interval immediately after the K-Pg mass extinction are three times higher than background rates during the Cretaceous. These results suggest that the K-Pg mass extinction had a marked impact on placental mammal diversification, supporting the view that an evolutionary radiation occurred as placental lineages invaded new ecological niches during the Early Palaeocene

New record of Egertonia (Elopiformes, Phyllodontidae) from the Late Cretaceous of South India

We report a new occurrence of the phyllodontid teleost fish Egertonia from the Late Cretaceous Kallamedu Formation of the Cauvery Basin, South India. This is the oldest occurrence of Phyllodontidae in India, and only the second Cretaceous Gondwanan occurrence of this genus, following a toothplate previously described from the Late Cretaceous Maevarano Formation, Madagascar. The presence of phyllodontid fish supports a fluvial-deltaic or brackish environment for the lower part of the Kallamedu Formation, a rich deposit including typically Gondwanan taxa, such as simosuchid crocodiles, bothremydid turtles and abelisaurid dinosaurs, as well as an anomalous troodontid dinosaur. Egertonia adds another taxon of primarily Laurasian distribution to the Kallamedu fauna and further expands the list of taxa known from the Late Cretaceous of both India and Madagascar, strengthening the degree of faunal similarity between the two landmasses in the latest Cretaceous

Rapid morphological evolution in placental mammals post-dates the origin of the crown group.

Resolving the timing and pattern of early placental mammal evolution has been confounded by conflict among divergence date estimates from interpretation of the fossil record and from molecular-clock dating studies. Despite both fossil occurrences and molecular sequences favouring a Cretaceous origin for Placentalia, no unambiguous Cretaceous placental mammal has been discovered. Investigating the differing patterns of evolution in morphological and molecular data reveals a possible explanation for this conflict. Here, we quantified the relationship between morphological and molecular rates of evolution. We show that, independent of divergence dates, morphological rates of evolution were slow relative to molecular evolution during the initial divergence of Placentalia, but substantially increased during the origination of the extant orders. The rapid radiation of placentals into a highly morphologically disparate Cenozoic fauna is thus not associated with the origin of Placentalia, but post-dates superordinal origins. These findings predict that early members of major placental groups may not be easily distinguishable from one another or from stem eutherians on the basis of skeleto-dental morphology. This result supports a Late Cretaceous origin of crown placentals with an ordinal-level adaptive radiation in the early Paleocene, with the high relative rate permitting rapid anatomical change without requiring unreasonably fast molecular evolutionary rates. The lack of definitive Cretaceous placental mammals may be a result of morphological similarity among stem and early crown eutherians, providing an avenue for reconciling the fossil record with molecular divergence estimates for Placentalia

A simple rule governs the evolution and development of hominin tooth size

The variation in molar tooth size in humans and our closest relatives (hominins) has strongly influenced our view of human evolution. The reduction in overall size and disproportionate decrease in third molar size have been noted for over a century, and have been attributed to reduced selection for large dentitions owing to changes in diet or the acquisition of cooking1, 2. The systematic pattern of size variation along the tooth row has been described as a ‘morphogenetic gradient’ in mammal, and more specifically hominin, teeth since Butler3 and Dahlberg4. However, the underlying controls of tooth size have not been well understood, with hypotheses ranging from morphogenetic fields3 to the clone theory5. In this study we address the following question: are there rules that govern how hominin tooth size evolves? Here we propose that the inhibitory cascade, an activator–inhibitor mechanism that affects relative tooth size in mammals6, produces the default pattern of tooth sizes for all lower primary postcanine teeth (deciduous premolars and permanent molars) in hominins. This configuration is also equivalent to a morphogenetic gradient, finally pointing to a mechanism that can generate this gradient. The pattern of tooth size remains constant with absolute size in australopiths (including Ardipithecus, Australopithecus and Paranthropus). However, in species of Homo, including modern humans, there is a tight link between tooth proportions and absolute size such that a single developmental parameter can explain both the relative and absolute sizes of primary postcanine teeth. On the basis of the relationship of inhibitory cascade patterning with size, we can use the size at one tooth position to predict the sizes of the remaining four primary postcanine teeth in the row for hominins. Our study provides a development-based expectation to examine the evolution of the unique proportions of human teeth

The osteology of ‘Periptychus carinidens’: a robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America

Periptychus is the archetypal genus of Periptychidae, a clade of prolific Paleocene 'condylarth' mammals from North America that were among the first placental mammals to radiate after the end-Cretaceous extinction, remarkable for their distinctive dental anatomy. A comprehensive understanding of the anatomy of Periptychus has been hindered by a lack of cranial and postcranial material and only cursory description of existing material. We comprehensively describe the cranial, dental and postcranial anatomy of Periptychus carinidens based on new fossil material from the early Paleocene (Torrejonian) of New Mexico, USA. The cranial anatomy of Periptychus is broadly concurrent with the inferred plesiomorphic eutherian condition, albeit more robust in overall construction. The rostrum is moderately elongate with no constriction, the facial region is broad, and the braincase is small with a well-exposed mastoid on the posterolateral corner and tall sagittal and nuchal crests. The dentition of Periptychus is characterized by strongly crenulated enamel, enlarged upper and lower premolars with a tall centralised paracone/protoconid. The postcranial skeleton of Periptychus is that of a robust, medium-sized (~20 Kg) stout-limbed animal that was incipiently mediportal and adopted a plantigrade stance. The structure of the fore- and hindlimb of Periptychus corresponds to that of a typically terrestrial mammal, while morphological features of the forelimb such as the low tubercles of the humerus, long and prominent deltopectoral crest, pronounced medial epicondyle, and hemispherical capitulum indicate some scansorial and/or fossorial ability. Most striking is the strongly dorsoplantarly compressed astragalus of Periptychus, which in combination with the distal crus and calcaneal morphology indicates a moderately mobile cruropedal joint. The anatomy of Periptychus is unique and lacks any extant analogue; it combines a basic early placental body plan with numerous unique specializations in its dental, cranial and postcranial anatomy that exemplify the ability of mammals to adapt and evolve following catastrophic environmental upheaval