Eutherian Mammals (Placentals) Evolved Faster Right After Dinosaurs Went Extinct

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

Authors:

Halliday et al

Abstract:

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.

pop sci link.

Therian Mammals Started an Ecomorphical Evolutionary Radiation PRIOR to the Extinction of the Dinosaurs

Therian mammals experience an ecomorphological radiation during the Late Cretaceous and selective extinction at the K–Pg boundary

Authors:

Grossnickle et al

Abstract:

It is often postulated that mammalian diversity was suppressed during the Mesozoic Era and increased rapidly after the Cretaceous–Palaeogene (K–Pg) extinction event. We test this hypothesis by examining macroevolutionary patterns in early therian mammals, the group that gave rise to modern placentals and marsupials. We assess morphological disparity and dietary trends using morphometric analyses of lower molars, and we evaluate generic level taxonomic diversity patterns using techniques that account for sampling biases. In contrast with the suppression hypothesis, our results suggest that an ecomorphological diversification of therians began 10–20 Myr prior to the K–Pg extinction event, led by disparate metatherians and Eurasian faunas. This diversification is concurrent with ecomorphological radiations of multituberculate mammals and flowering plants, suggesting that mammals as a whole benefitted from the ecological rise of angiosperms. In further contrast with the suppression hypothesis, therian disparity decreased immediately after the K–Pg boundary, probably due to selective extinction against ecological specialists and metatherians. However, taxonomic diversity trends appear to have been decoupled from disparity patterns, remaining low in the Cretaceous and substantially increasing immediately after the K–Pg extinction event. The conflicting diversity and disparity patterns suggest that earliest Palaeocene extinction survivors, especially eutherian dietary generalists, underwent rapid taxonomic diversification without considerable morphological diversification.

When Placental Mammals Evolved

Dating Placentalia: Morphological clocks fail to close the molecular-fossil gap

Authors:

Puttick et al

Abstract:

Dating the origin of Placentalia has been a contentious issue for biologists and paleontologists. While it is likely that crown-group placentals originated in the Late Cretaceous, nearly all molecular clock estimates point to a deeper Cretaceous origin. An approach with the potential to reconcile this discrepancy could be the application of a morphological clock. This would permit the direct incorporation of fossil data in node dating, and would break long internal branches of the tree, so leading to improved estimates of node ages. Here, we use a large morphological dataset and the tip-calibration approach of MrBayes. We find that the estimated date for the origin of crown mammals is much older (∼130–145 Ma) than fossil and molecular clock data (∼80–90 Ma). Our results suggest that tip-calibration may result in estimated dates that are more ancient than those obtained from other sources of data. This can be partially overcome by constraining the ages of internal nodes on the tree; however, when this was applied to our dataset, the estimated dates were still substantially more ancient than expected. We recommend that results obtained using tip-calibration, and possibly morphological dating more generally, should be treated with caution.

Placental Mammal Divergence Explained?

Placental mammals consist of three main groups that diverged rapidly, evolving in wildly different directions: Afrotheria (for example, elephants and tenrecs), Xenarthra (such as armadillos and sloths) and Boreoeutheria (all other placental mammals). The relationships between them have been a subject of fierce controversy with multiple studies coming to incompatible conclusions over the last decade leading some researchers to suggest that these relationships might be impossible to resolve.

There are thus many outstanding questions such as which is the oldest sibling of the three? Did the mammals go their separate ways due to South America and Africa breaking apart? And if not, when did placentals split up?

"This has been one of the areas of greatest debate in evolutionary biology, with many researchers considering it impossible to resolve," said lead author Dr Tarver of Bristol's School of Earth Sciences. "Now we've proven these problems can be solved - you just need to analyse genome-scale datasets using models that accurately reflect genomic evolution."

link.

Alcidedorbingya inopinata: the Oldest Known Pantodont Eutherian From Danian Paleocene Paleogene Bolivia

Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology

Authors:

Muizon et al

Abstract:

The study of exceptional remains (several skulls and a complete skeleton) of a 65 MY old placental mammal from Bolivia, provides a good understanding of the cranial anatomy, biology and evolution of the first mammals which lived in South America just after the large dinosaurs disappeared, 66 MY ago. This mammal is a Pantodont named Alcidedorbingya, in honour of Alcide d’Orbigny, famous naturalist voyager of the Museum during the XIXth century. Pantodonts constitute a group of mammals from the beginning of the Tertiary, which disappeared 40 MY ago. They lived essentially in northern continents (China, North America, and Europe). They were of small (rabbit) to large size (cow) and were omnivorous. Alcidedorbignya is the only austral pantodont, the oldest, and the smallest known.

The CT scanner of the MNHN allows the authors, a reconstruction of the arterial and venous passageways of the basicranium, thus approaching an ancestral pattern of blood circulation of the first placental mammals. The complete skeleton of Alcidedorbignya, was also CT scanned, allowing virtual reconstructions of several life positions. Alcidedorbignya was terrestrial, but also capable of a bipedal position on its hind limbs and of climbing as is observed in the Recent squirels. This ability allowed Alcidedorbignya to escape the numerous crocodiles, which lived with it in a large Amazonian-like alluvial plain.

These results have been obtained thanks to the unprecedented completeness of the specimens, taking into account their geological age, in which the most common remains are only isolated teeth.

Did Placentals and Marsupials Diverge 170 Million Years ago Starting at the Aalenian/Bajocian Jurassic?

The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference

Authors:

Tarver et al

Abstract:

Placental mammals comprise three principal clades: Afrotheria (e.g. elephants and tenrecs), Xenarthra (e.g. armadillos and sloths) and Boreoeutheria (all other placental mammals), the relationships among which are the subject of controversy and a touchstone for debate on the limits of phylogenetic inference. Previous analyses have found support for all three hypotheses, leading some to conclude that this phylogenetic problem might be impossible to resolve, due to the compounded effects of Incomplete Lineage Sorting (ILS) and a rapid radiation. Here we show, using a genome scale nucleotide dataset, microRNAs, and the reanalysis of the three largest previously published amino-acid datasets, that the root of Placentalia lies between Atlantogenata and Boreoeutheria. Although we found evidence for ILS in early placental evolution, we are able to reject previous conclusions that the placental root is a hard polytomy that cannot be resolved. Reanalyses of previous datasets recover Atlantogenata + Boreoeutheria and show that contradictory results are a consequence of poorly fitting evolutionary models; instead, when the evolutionary process is better-modelled, all datasets converge on Atlantogenata. Our Bayesian molecular clock analysis estimates that marsupials diverged from placentals 157-170 Ma, crown Placentalia diverged 86-100 Ma, and crown Atlantogenata diverged 84-97 Ma. Our results are compatible with placental diversification being driven by dispersal rather than vicariance mechanisms, postdating early phases in the protracted opening of the Atlantic Ocean.

Placental Mammals Exploded in Diversity After the KT/K-Pg Mass Extinction

Resolving the relationships of Paleocene placental mammals

Authors:

Halliday et al

Abstract:

The ‘Age of Mammals’ began in the Paleocene epoch, the 10 million year interval immediately following the Cretaceous–Palaeogene mass extinction. The apparently rapid shift in mammalian ecomorphs from small, largely insectivorous forms to many small-to-large-bodied, diverse taxa has driven a hypothesis that the end-Cretaceous heralded an adaptive radiation in placental mammal evolution. However, the affinities of most Paleocene mammals have remained unresolved, despite significant advances in understanding the relationships of the extant orders, hindering efforts to reconstruct robustly the origin and early evolution of placental mammals. Here we present the largest cladistic analysis of Paleocene placentals to date, from a data matrix including 177 taxa (130 of which are Palaeogene) and 680 morphological characters. We improve the resolution of the relationships of several enigmatic Paleocene clades, including families of ‘condylarths’. Protungulatum is resolved as a stem eutherian, meaning that no crown-placental mammal unambiguously pre-dates the Cretaceous–Palaeogene boundary. Our results support an Atlantogenata–Boreoeutheria split at the root of crown Placentalia, the presence of phenacodontids as closest relatives of Perissodactyla, the validity of Euungulata, and the placement of Arctocyonidae close to Carnivora. Periptychidae and Pantodonta are resolved as sister taxa, Leptictida and Cimolestidae are found to be stem eutherians, and Hyopsodontidae is highly polyphyletic. The inclusion of Paleocene taxa in a placental phylogeny alters interpretations of relationships and key events in mammalian evolutionary history. Paleocene mammals are an essential source of data for understanding fully the biotic dynamics associated with the end-Cretaceous mass extinction. The relationships presented here mark a critical first step towards accurate reconstruction of this important interval in the evolution of the modern fauna.

Mammalian Insectivores (and Marsupial!) From Miocene Neogene Germany

Insectivore palaeoecology. A case study of a Miocene fissure filling in Germany

Authors:

Kleitmann et al

Abstract:

Many Miocene localities yielded considerable numbers of eulipotyphlan fossils. The group as a whole is generally considered to be indicative for humid environments, but little is known about the preferences of specific taxa. We discuss the insectivores found in the German fissure filling Petersbuch 28, including an insectivorous marsupial, in an attempt to refine the knowledge of the preferred environments of insectivores. For this, we compared the assemblage in quantitative analyses with other insectivore assemblages of similar age. Our results show that, in full accordance with previous hypotheses, dimylids, most moles and shrews were shown to be indicators for humid environments, like swamps or humid forests, whereas the hedgehogs, the moles Desmanodon and Theratiskos and the shrew Oligosorex were more common in dryer environments.

When *DID* Placental Mammals Really Evolve?

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

Authors:

Beck et al

Abstract:

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.

Macroevolution of Ungulates in North America

The fossil record and macroevolutionary history of North American ungulate ungulate mammals: standardizing variation in intensity and geography of sampling

Author:

Marcot

Abstract:

The record of the taxonomic evolution of North American ungulates is critical to our understanding of mammalian evolution and environmental change throughout the Cenozoic. The distribution of sampling in the ungulate fossil record over time and geographic space and the degree to which this biases the observed patterns of taxonomic evolution is poorly understood. To address these issues, I placed fossil collections and occurrences drawn from the Paleobiology Database into 2-Myr time intervals between 55 and 1 Ma. I determined the variation in numbers of fossil collections and occurrences, using three metrics to measure geographic variation: first, the area of the convex hull containing all collections in an interval, to determine the areal coverage of sampling; second, the mean pairwise geographic distance among collections as a measurement of the dispersion of collections within that area; and third, the interval-to-interval migration of the geographic centroid of all collections, to calculate changes in the geographic location of sampling. Each of these showed considerable variation over the Cenozoic, and both the area of the convex hull (ACH) encompassing all collections in an interval, and mean pairwise distance (MPWD) among them showed increasing trends over time.

To minimize the effect of variation in numbers of fossil samples over time, I used standard sample-standardization procedures. To minimize the effect of geographic variation in sampling over time, I standardized the area of sampling among intervals. I also employed both standardizations sequentially. Each standardization procedure had surprisingly little effect on observed patterns of taxonomic richness and rates. This indicates that, for North American ungulates, neither variation in number nor geographic distribution of fossil samples exerts an overwhelming influence on perceived macroevolutionary patterns. These results confirm the ungulate fossil record as a critical and faithful record for our understanding of Cenozoic environmental change and the mammalian evolutionary response.

New Study Claims Placental Mammals Originated in the Cretaceous

Neither phylogenomic nor palaeontological data support a Palaeogene origin of placental mammals.

Authors:

dos Reis et al

Abstract:

O'Leary et al. (O'Leary et al. 2013 Science 339, 662-667. (doi:10.1126/science.1229237)) performed a fossil-only dating analysis of mammals, concluding that the ancestor of placentals post-dated the Cretaceous-Palaeogene boundary, contradicting previous palaeontological and molecular studies that placed the ancestor in the Cretaceous. They incorrectly used fossil ages as species divergence times for crown groups, while in fact the former should merely form minimum-age bounds for the latter. Statistical analyses of the fossil record have shown that crown groups are significantly older than the oldest ingroup fossil, so that fossils do not directly reflect the true ages of clades. Here, we analyse a 20 million nucleotide genome-scale alignment in conjunction with a probabilistic interpretation of the fossil ages from O'Leary et al. Our combined analysis of fossils and molecules demonstrates that Placentalia originated in the Cretaceous.

Chris Taylor on the Litopterns

link.

Go Read.

Dormaalocyon latouri: A Close Relative to the Ancestor of All Modern Carnivores From the Paleocene Paleogene

New fossils from Belgium have shed light on the origin of some of the most well-known, and well-loved, modern mammals. Cats and dogs, as well as other carnivorous mammals (like bears, seals, and weasels), taxonomically called 'carnivoraformes', trace their ancestry to primitive carnivorous mammals dating back to 55 million years ago (the beginning of the time period called the Eocene). A study, published in the most recent issue of the Journal of Vertebrate Paleontology, discusses the origins of this group and describes new specimens of one of the earliest of these primitive taxa.

The species, dubbed Dormaalocyon latouri, had previously been found at the Belgian locality of Dormaal (thus the name of the genus). New specimens found by lead author Floréal Solé and his colleagues, allow for a better characterization of the animal, and its placement in the evolutionary history of carnivores. "Its description allows better understanding of the origination, variability and ecology of the earliest carnivoraforms", says Solé.

The new specimens include over 250 teeth and ankle bones. More teeth allow for a description of the entire tooth row of Dormaalocyon, while previous finds only included two upper molars. The new finds even include the deciduous teeth (or 'baby teeth'). The fact that these teeth are very primitive looking, and from a very early time, implies that Dormaalocyon is close to the origin of carnivoraforms, and that this origin may have been in Europe.

The ankle bones suggest that Dormaalocyon was arboreal, living and moving through the trees. Previous reconstructions of the environment at Dormaal 55 million years ago inferred a warm, humid, and wooded area. This was a time soon after an event called the Paleocene-Eocene Thermal Maximum (or PETM). This extremely warm period affected the evolution of many mammal groups, including carnivoraforms. Dr. Solé believes that the fact that Dormaalocyon was arboreal, and that carnivoraforms made their way to North America around this time, "supports the existence of a continuous evergreen forest belt at high latitudes during the PETM".

link.

Academic Bun Fight: Evolution of Placental Mammals Since the KT/K-Pg Event

This is one of those topics I wish Darren Naish would give an opinion on.   I normally like to start the day with a precambrian paper of some kind, but I don't see one I'm, uh, digging.  However, this bun fight is sticky and interesting.

O'Leary et al published a paper on the evolution of placental mammals across the KT/K-Pg boundary (The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals). They used both genomic and phenomic data to create a supermatrix to produce a phylogeny of placental mammals using 86 different critters and over 4500 characters.  They used far, far more data than any study before, from what I gather.  One of the radical arguments is there was one (*1*) ancestor to modern placentals.

There has now been a critique of the paper (Technical Comment on “The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals".  The basic theme is O'Leary et al probably overreached with their analysis.  One of the cautions stated is similar lifestyles will cause convergent evolution.   

The reply from the original authors is here (Response to Comment on “The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals”) where they offer their counter arguments. I am not so sure I buy into the comment of "Accumulated negative evidence indicates that [placental] fossils are absent."  That comment strikes me as very wrong. 

But then, I am not an expert and this is why I wish Darren would comment. 

Carbon Isotope Analysis of Ouled Abdoun Basin, Morocco Has Implications for Africa's Earliest Placentals

First carbon isotope chemostratigraphy of the Ouled Abdoun phosphate Basin, Morocco; implications for dating and evolution of earliest African placental mammals

Authors:

1. Johan Yans (a)
2. M’Barek Amaghzaz (b)
3. Baadi Bouya (b)
4. Henri Cappetta (c)
5. Paola Iacumin (d)
6. László Kocsis (e)
7. Mustapha Mouflih (f)
8. Omar Selloum (b)
9. Sevket Sen (g)
10. Jean-Yves Storme (a)
11. Emmanuel Gheerbrant (g)

Affiliations:

a. University of Namur, Department of Geology, NaGRIDD, 61 rue de Bruxelles, 5000 Namur, Belgium

b. Groupe Office Chérifien des Phosphates (OCP), Centre Minier de Khouribga, Service Géologique, Khouribga, Morocco

c. Université Montpellier II, UMR 5554, France

d. Parma University, Earth Sciences, Parco Area delle Scienze, 157, 43100 Parma, Italy

e. University of Lausanne, Faculty of Geoscience and Environment, Institute of Earth Sciences, UNIL - Geopolis, CH-1015 Lausanne, Switzerland

f. University Hassan II-Mohammedia, Faculty of Sciences Ben M'sik, Casablanca, Morocco

g. UMR-CNRS 7207, CR2P Centre de Recherches sur la Paléobiodiversité et les Paléoenvironnements, Muséum National d'Histoire Naturelle, Département Histoire de la Terre, CP38, 8 rue Buffon, 75005 Paris, France

Abstracts:

The well-known Maastrichtian-Ypresian vertebrate-bearing phosphate series, in the Ouled Abdoun Basin, Morocco, is classically dated using regional selachian biostratigraphic zonation. These marine sediments yielded Paleocene and Eocene mammals comprising the earliest known placentals from Africa. This study provides the first insight into the organic carbon isotope chemostratigraphy (δ¹³C_org) of the Moroccan phosphate series and a refined dating of its vertebrate-bearing levels. Four Paleocene-Eocene sections in the NE Ouled Abdoun quarries show consistent δ¹³C_org long term evolutions, from the base to the top: 1) positive trend in phosphorite Bed IIa, beginning with the lower Bone Bed yielding mammals such as Eritherium, Ocepeia, Abdounodus, Lahimia, of early Thanetian and Selandian age; 2) transitional negative trend in the Intercalary phosphorite Beds II/I that includes the Otodus obliquus and Phosphatherium escuilliei Bone Bed of earliest Ypresian age; 3) negative trend to the lowermost δ¹³C_org values that are correlative to the early-middle Ypresian interval including ETM 2 and ETM 3 hyperthermal events in the global record; 4) positive trend in chert-enriched facies containing the middle Ypresian EECO global climatic event. Our chemostratigraphic study of the Ouled Abdoun phosphate series provides a new chronostratigraphic framework for calibrating the beginning of the evolution of placental mammals in Africa. The lower Bone Bed level from the Paleocene phosphorite Bed IIa yielding Eritherium is not younger than early Thanetian, and is most likely Selandian. The Phosphatherium Bone Bed in the Intercalary Beds II/I is earliest Ypresian. The phosphorite Bed 0, from which Daouitherium probably came, is early-middle Ypresian, just below the EECO. This suggests that the first large proboscideans evolved after the PETM, during mid-Ypresian warming events. The δ¹³C_org study does not support the presence of Lutetian in the NE Ouled Abdoun phosphate series and suggests that a noticeable part of the upper Thanetian is absent.

New Placental Phylogeny Based on 4500 Characters, Genetic and Morphological Traits

A groundbreaking six-year research collaboration has produced the most complete picture yet of the evolution of placental mammals, the group that includes humans. Placental mammals are the largest branch of the mammalian family tree, with more than 5,100 living species. Researchers from Carnegie Museum of Natural History are among the team of 23 that took part in this extensive interdisciplinary effort that utilizes molecular (DNA) and morphological (anatomy) data on an extraordinary scale. By combining these two types of data scientists reconstructed, to an unprecedented level of detail, the family tree of placental mammals. This study explored thousands of characteristics of the anatomy of both living and extinct placental mammals.

This new project produced a more complete picture of mammalian history and provides a huge dataset that will become the starting point of research for a number of scientific questions, including those of vital importance today: how mammals may have survived climate change in the past and what may that mean for our future. The paper appears today in the journal Science.

The collaboration is part of the Assembling the Tree of Life (ATOL) project funded by the National Science Foundation.

Joining forces, joining research

Today's article reveals the final results of the six-year ATOL project. The study began with two teams organizing data from two distinct approaches to evolutionary research: molecular data (DNA), and morphological data (anatomical features).

"In the field of mammal research, there had been a big divide between people working with DNA and others working on morphology," explains John Wible, PhD, Curator of Mammals at Carnegie Museum of Natural History and co-author on the paper. "They just weren't working with each other until now."

The molecular team collected DNA sequences of living animals and the morphology team examined the anatomy of both living and extinct mammals. The molecular team only sampled living mammals, because genetic material can't be extracted from fossils older than 30,000 years,. Thus, to include fossils, morphological information was essential. Researchers in morphology deal with individual physical features, from bone length to types of teeth to the presence of stripes in the fur; each one of these features is termed a 'character.' By collecting as many characters as possible and comparing their variation among dozens of specimens, relationships between species can be tested and broader patterns emerge.

The ATOL project became a morphological powerhouse. Generally, a group of 500 characters is considered to be a large dataset. The morphology researchers on the ATOL project generated an unprecedented 4,500 characters. Once both DNA and morphological datasets were produced, the resulting combined matrix provided an unprecedented amount of information for each of the 83 mammals included in the study.

"It's not that we hadn't combined morphology with DNA before." clarifies co-author Michelle Spaulding, PhD, the Rea Post-doctoral Fellow at Carnegie Museum of Natural History. "This time, we ratcheted up the amount of morphological detail phenomenally, providing a larger anatomical base for the study as compared with DNA than is typical.."

With the new Tree of Life matrix, researchers now have greater context for the fragmentary fossils they have in hand—often scant evidence such as a few teeth or a skull fragment—potentially shedding light on little-known species that have yet to find a solid home in the evolutionary tree.

Basal Placental Mammal Reconstructed from New Uber Phylogeny

An international team of researchers has reconstructed the common ancestor of placental mammals—an extremely diverse group including animals ranging from rodents to whales to humans—using the world's largest dataset of both genetic and physical traits. In research to be published in the journal Science, the scientists reveal that, contradictory to a commonly held theory, placental mammals did not diversify into their present-day lineages until after the extinction event that eliminated non-avian dinosaurs, and about 70 percent of all species on Earth, some 65 million years ago. This finding, and the visualization of the placental ancestor—a small, insect-eating animal— was made with the help of a powerful cloud-based and publicly accessible database called MorphoBank. The Science article is the result of a multi-year collaborative project funded by the National Science Foundation's Assembling the Tree of Life program.

"Analysis of this massive dataset shows that placental mammals did not originate during the Mesozoic," said lead author Maureen O'Leary, an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University and a research associate at the American Museum of Natural History. "Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor—a small, insect-eating, scampering animal—shortly after the dinosaurs' demise."

There are two major types of data for building evolutionary trees of life: phenomic data, observational traits such as anatomy and behavior; and genomic data encoded by DNA. Some scholars have argued that integration of both is necessary for robust tree-building because examining only one type of data (genomic or phenomic) leaves out significant information. The evolutionary history of placental mammals, for example, has been interpreted in very different ways depending on the data analyzed. One leading analysis based on genomic data alone predicted that a number of placental mammal lineages existed in the Late Cretaceous and survived the Cretaceous-Paleogene (KPg) extinction. Other analyses place the start of placental mammals near this boundary, and still others set their origin after this event.

"There are over 5,100 living placental species and they exhibit enormous diversity, varying greatly in size, locomotor ability, and brain size," said Nancy Simmons, paper author and a curator in the Department of Mammalogy at the American Museum of Natural History. "Given this diversity, it's of great interest to know when and how this clade first began evolving and diversifying."

The new study combines genomic and phenomic data in a simultaneous analysis for a more complete picture of the tree of life.

"Despite the considerable contributions of DNA sequence data to the study of species relationships, phenomic data have a major role in the direct reconstruction of trees," said author Michael Novacek, senior vice president, provost for science, and a curator of paleontology at the American Museum of Natural History. "Such data include features preserved in fossils where DNA recovery may be impossible. The mammalian record is notably enriched with well-preserved fossils, and we don't want to build trees without using the direct evidence that these fossils contribute."

"Discovering the tree of life is like piecing together a crime scene—it is a story that happened in the past that you can't repeat," O'Leary said. "Just like with a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils, and anatomy. Combining all the evidence produces the most informed reconstruction of a past event."

The tree of life produced in this study shows that placental mammals arose rapidly after the KPg extinction, with the original ancestor speciating 200,000-400,000 years after the event.

"This is about 36 million years later than the prediction based on purely genetic data," said Marcelo Weksler, an author and research associate in the Museum's Department of Mammalogy who is now at the Museu Nacional-UFRJ in Brazil.

The finding also contradicts a genomics-based model called the "Cretaceous-Terrestrial Revolution" that argues that the impetus for placental mammal speciation was the fragmentation of supercontinent Gondwana during the Jurassic and Cretaceous, millions of years earlier than the KPg event.

"The new tree indicates that the fragmentation of Gondwana came well before the origin of placental mammals and is an unrelated event," said John Wible, paper author and curator of mammals at the Carnegie Museum of Natural History.

HERESY!  One does not DOUBT the Cretaceous Ecological Revolution!  Send in the Spanish Inquisition!  Oh.  Wait.  The 49ers shot them.
 
ahem.

Genomic Evidence for Large, Long-Lived Ancestors to Placental mammals.

Genomic Evidence for Large, Long-Lived Ancestors to Placental mammals.

Authors:

1. Romiguier J. (a)
2. Ranwez V. (a,b)
3. Douzery E.J.P. (a)
4. Galtier N. (a,*)

Affiliations:

a. CNRS – Université Montpellier 2 – UMR 5554 – ISEM – Montpellier, France

b. Montpellier SupAgro – UMR 1334 – AGAP – Montpellier, France

*corresponding author: e-mail: nicolas.galtier@univ-montp2.fr

Abstract:

It is widely assumed that our mammalian ancestors, which lived in the Cretaceous era, were tiny animals that survived massive asteroid impacts in shelters, and evolved into modern forms after dinosaurs went extinct, 65 Mya. The small size of most Mesozoic mammalian fossils essentially supports this view. Paleontology, however, is not conclusive regarding the ancestry of extant mammals, because Cretaceous and Paleocene fossils are not easily linked to modern lineages. Here we use full-genome data to estimate the longevity and body mass of early placental mammals. Analysing 36 fully-sequenced mammalian genomes, we reconstruct two aspects of the ancestral genome dynamics, namely GC-content evolution and nonsynonymous over synonymous rate ratio. Linking these molecular evolutionary processes to life history traits in modern species, we estimate that early placental mammals had a life-span above 25 years, and a body mass above one kilogram. This is similar to current primates, cetartiodactyls or carnivores, but markedly different from mice or shrews, challenging the dominant view about mammalian origin and evolution. Our results imply that long-lived mammals existed in the Cretaceous era, and were the most successful in evolution, opening new perspectives about the conditions for survival to the Cretaceous-Tertiary crisis.

FYI.  We are back to using title links here.