Robin M. D. Beck

The delayed rise of present-day mammals

Did the end-Cretaceous mass extinction event, by eliminating non-avian dinosaurs and most of the existing fauna, trigger the evolutionary radiation of present-day mammals? Here we construct, date and analyse a species-level phylogeny of nearly all extant Mammalia to bring a new perspective to this question. Our analyses of how extant lineages accumulated through time show that net per-lineage diversification rates barely changed across the Cretaceous/Tertiary boundary. Instead, these rates spiked significantly with the origins of the currently recognized placental superorders and orders approximately 93 million years ago, before falling and remaining low until accelerating again throughout the Eocene and Oligocene epochs. Our results show that the phylogenetic ‘fuses’ leading to the explosion of extant placental orders are not only very much longer than suspected previously, but also challenge the hypothesis that the end-Cretaceous mass extinction event had a major, direct influence on the diversification of today’s mammals.

A Dated Phylogeny of Marsupials Using a Molecular Supermatrix and Multiple Fossil Constraints

Abstract Phylogenetic relationships within marsupials were investigated based on a 20.1-kilobase molecular supermatrix comprising 7 nuclear and 15 mitochondrial genes analyzed using both maximum likelihood and Bayesian approaches and 3 different partitioning strategies. The study revealed that base composition bias in the 3rd codon positions of mitochondrial genes misled even the partitioned maximum-likelihood analyses, whereas Bayesian analyses were less affected. After correcting for base composition bias, monophyly of the currently recognized marsupial orders, of Australidelphia, and of a clade comprising Dasyuromorphia, Notoryctes, and Peramelemorphia, were supported strongly by both Bayesian posterior probabilities and maximum-likelihood bootstrap values. Monophyly of the Australasian marsupials, of Notoryctes + Dasyuromorphia, and of Caenolestes + Australidelphia were less well supported. Within Diprotodontia, Burramyidae + Phalangeridae received relatively strong support. Divergence dates calculated using a Bayesian relaxed molecular clock and multiple age constraints suggested at least 3 independent dispersals of marsupials from North to South America during the Late Cretaceous or early Paleocene. Within the Australasian clade, the macropodine radiation, the divergence of phascogaline and dasyurine dasyurids, and the divergence of perameline and peroryctine peramelemorphians all coincided with periods of significant environmental change during the Miocene. An analysis of “unrepresented basal branch lengths” suggests that the fossil record is particularly poor for didelphids and most groups within the Australasian radiation.

Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific

New Zealand (NZ) has long been upheld as the archetypical example of a land where the biota evolved without nonvolant terrestrial mammals. Their absence before human arrival is mysterious, because NZ was still attached to East Antarctica in the Early Cretaceous when a variety of terrestrial mammals occupied the adjacent Australian portion of Gondwana. Here we report discovery of a nonvolant mammal from Miocene (19–16 Ma) sediments of the Manuherikia Group near St Bathans (SB) in Central Otago, South Island, NZ. A partial relatively plesiomorphic femur and two autapomorphically specialized partial mandibles represent at least one mouse-sized mammal of unknown relationships. The material implies the existence of one or more ghost lineages, at least one of which (based on the relatively plesiomorphic partial femur) spanned the Middle Miocene to at least the Early Cretaceous, probably before the time of divergence of marsupials and placentals >125 Ma. Its presence in NZ in the Middle Miocene and apparent absence from Australia and other adjacent landmasses at this time appear to reflect a Gondwanan vicariant event and imply persistence of emergent land during the Oligocene marine transgression of NZ. Nonvolant terrestrial mammals disappeared from NZ some time since the Middle Miocene, possibly because of late Neogene climatic cooling.

Australia's oldest marsupial fossils and their biogeographical implications.

Background We describe new cranial and post-cranial marsupial fossils from the early Eocene Tingamarra Local Fauna in Australia and refer them to Djarthia murgonensis, which was previously known only from fragmentary dental remains. Methodology/Principal Findings The new material indicates that Djarthia is a member of Australidelphia, a pan-Gondwanan clade comprising all extant Australian marsupials together with the South American microbiotheres. Djarthia is therefore the oldest known crown-group marsupial anywhere in the world that is represented by dental, cranial and post-cranial remains, and the oldest known Australian marsupial by 30 million years. It is also the most plesiomorphic known australidelphian, and phylogenetic analyses place it outside all other Australian marsupials. Conclusions/Significance As the most plesiomorphic and oldest unequivocal australidelphian, Djarthia may approximate the ancestral morphotype of the Australian marsupial radiation and suggests that the South American microbiotheres may be the result of back-dispersal from eastern Gondwana, which is the reverse of prevailing hypotheses.

Current status of species-level representation in faunas from selected fossil localities in the Riversleigh World Heritage Area, northwestern Queensland

Current lists of species-level representation in faunas from 80 Cenozoic fossil localities at the Riversleigh World Heritage Area have been compiled by review of recorded occurrences of taxa obtained from both published and unpublished sources. More than 290 species-level taxa are represented, comprising mammals, amphibians, reptiles, birds, fishes, molluscs and crustaceans. The data are presented for the purpose of ongoing palaeoecological and biochronological studies.

Ancient dates or accelerated rates? Morphological clocks and the antiquity of placental mammals

Analyses of a comprehensive morphological character matrix of mammals using ‘relaxed’ clock models (which simultaneously estimate topology, divergence dates and evolutionary rates), either alone or in combination with an 8.5 kb nuclear sequence dataset, retrieve implausibly ancient, Late Jurassic–Early Cretaceous estimates for the initial diversification of Placentalia (crown-group Eutheria). These dates are much older than all recent molecular and palaeontological estimates. They are recovered using two very different clock models, and regardless of whether the tree topology is freely estimated or constrained using scaffolds to match the current consensus placental phylogeny. This raises the possibility that divergence dates have been overestimated in previous analyses that have applied such clock models to morphological and total evidence datasets. Enforcing additional age constraints on selected internal divergences results in only a slight reduction of the age of Placentalia. Constraining Placentalia to less than 93.8 Ma, congruent with recent molecular estimates, does not require major changes in morphological or molecular evolutionary rates. Even constraining Placentalia to less than 66 Ma to match the ‘explosive’ palaeontological model results in only a 10- to 20-fold increase in maximum evolutionary rate for morphology, and fivefold for molecules. The large discrepancies between clock- and fossil-based estimates for divergence dates might therefore be attributable to relatively small changes in evolutionary rates through time, although other explanations (such as overly simplistic models of morphological evolution) need to be investigated. Conversely, dates inferred using relaxed clock models (especially with discrete morphological data and MrBayes) should be treated cautiously, as relatively minor deviations in rate patterns can generate large effects on estimated divergence dates.

The Miocene mammal Necrolestes demonstrates the survival of a Mesozoic nontherian lineage into the late Cenozoic of South America

The early Miocene mammal Necrolestes patagonensis from Patagonia, Argentina, was described in 1891 as the only known extinct placental “insectivore” from South America (SA). Since then, and despite the discovery of additional well-preserved material, the systematic status of Necrolestes has remained in flux, with earlier studies leaning toward placental affinities and more recent ones endorsing either therian or specifically metatherian relationships. We have further prepared the best-preserved specimens of Necrolestes and compared them with newly discovered nontribosphenic Mesozoic mammals from Argentina; based on this, we conclude that Necrolestes is related neither to marsupials nor placentals but is a late-surviving member of the recently recognized nontherian clade Meridiolestida, which is currently known only from SA. This conclusion is supported by a morphological phylogenetic analysis that includes a broad sampling of therian and nontherian taxa and that places Necrolestes within Meridiolestida. Thus, Necrolestes is a remnant of the highly endemic Mesozoic fauna of nontribosphenic mammals in SA and extends the known record of meridiolestidans by almost 45 million years. Together with other likely relictual mammals from earlier in the Cenozoic of SA and Antarctica, Necrolestes demonstrates the ecological diversity of mammals and the mosaic pattern of fauna replacement in SA during the Cenozoic. In contrast to northern continents, the Cenozoic faunal history of SA was characterized by a long period of interaction between endemic mammalian lineages of Mesozoic origin and metatherian and eutherian lineages that probably dispersed to SA during the latest Cretaceous or earliest Paleocene.

The Phylogenetic Affinities of the Enigmatic Mammalian Clade Gondwanatheria

Gondwanatheria is a group of extinct mammals known from the Cretaceous and Paleogene of Gondwana. Resolution of the phylogenetic affinities of gondwanatherians has proven problematical, with the group currently considered Mammalia incertae sedis. We briefly review the morphology of known gondwanatherians, and argue that isolated upper premolars and a partial dentary preserving a blade-like p4 originally referred to the ferugliotheriid gondwanatherian Ferugliotherium windhauseni but subsequently identified as Multituberculata incertae sedis do indeed belong to F. windhauseni. We also suggest that the recently described ?cimolodontan multituberculate Argentodites coloniensis, based on an isolated lower premolar, may in fact be an unworn p4 of Ferugliotherium or a closely related taxon. We present the first phylogenetic analyses to include gondwanatherians, using maximum parsimony and Bayesian methods. Both methods place Ferugliotherium and sudamericid gondwanatherians in a clade with cimolodontan and “plagiaulacidan” multituberculates, although relationships within this clade are largely unresolved. The Gondwanatheria + Multituberculata clade supported here may reflect the convergent evolution of similar dental features, but it is the best supported hypothesis based on currently available data. However, denser sampling of multituberculate taxa and the discovery of more complete gondwanatherian fossils will be required to clarify the precise relationship between gondwanatherians and multituberculates, specifically to determine whether or not gondwanatherians are members of Multituberculata. We hypothesize that the anterior molariforms of sudamericid gondwanatherians evolved from blade-like precursors similar to the p4 of Ferugliotherium, possibly in response to the appearance of grasses in Gondwana during the Cretaceous.

An ‘ameridelphian’ marsupial from the early Eocene of Australia supports a complex model of Southern Hemisphere marsupial biogeography

Recent molecular data strongly support the monophyly of all extant Australian and New Guinean marsupials (Eomarsupialia) to the exclusion of extant South American marsupials. This, together with available geological and fossil evidence, has been used to argue that the presence of marsupials in Australia is simply the result of a single dispersal event from South America during the latest Cretaceous or Palaeocene, without subsequent dispersals between the two continents. Here, I describe an isolated ankle bone (calcaneus) of a metatherian from the early Eocene Tingamarra Local Fauna in northeastern Australia. Strikingly, this specimen, QM F30060, lacks the ‘continuous lower ankle joint pattern’ (CLAJP), presence of which is a highly distinctive apomorphy of the marsupial clade Australidelphia, which includes Eomarsupialia, the living South American microbiotherian Dromiciops and the Tingamarran fossil marsupial Djarthia. Comparisons with a range of marsupials and stem-metatherians strongly suggest that the absence of the CLAJP in QM F30060 is plesiomorphic, and that this specimen represents the first unequivocal non-australidelphian (‘ameridelphian’) metatherian known from Australia. This interpretation is confirmed by phylogenetic analyses that place QM F30060 within (crown-group) Marsupialia, but outside Australidelphia. Based on these results, the distribution of marsupials within Gondwana cannot be explained by simply a single dispersal event from South America and Australia. Either there were multiple dispersals by marsupials (and possibly also stem-metatherians) between South America and Australia, in one or both directions, or, alternatively, there was a broadly similar metatherian fauna stretching across southern South America, Antarctica and Australia during the Late Cretaceous–early Palaeogene.

An exceptionally well-preserved short-snouted bandicoot (Marsupialia; Peramelemorphia) from Riversleigh's Oligo-Miocene deposits, northwestern Queensland, Australia

ABSTRACT We describe Galadi speciosus, gen. et sp nov., the second peramelemorphian (Yarala burchfieldi being the first) to be described from Oligo-Miocene deposits of Riversleigh World Heritage Property, northwestern Queensland. G. speciosus is represented by relatively complete craniodental material, including an exceptionally well-preserved skull. This taxon exhibits several apomorphies that clearly place it in the order Peramelemorphia, but it appears to be more plesiomorphic than any modern bandicoot. We present the first morphological phylogenetic analyses of Peramelemorphia, using 51 craniodental characters. Our analyses recover Yarala and Galadi speciosus outside crown group Peramelemorphia, with G. speciosus weakly supported as the sister taxon of the crown group. The craniodental morphology of G. speciosus, particularly its robust skull and proportionately short and broad snout, suggests that it filled a different ecological niche to extant bandicoots. We hypothesize that G. speciosus occupied a predominantly faunivorous, dasyurid-like niche in the Oligo-Miocene rainforests of Riversleigh, at a time when dasyurids appear to have been relatively rare.