text: Abstract 
   
   To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria. ||||| Humankind’s common ancestor with other mammals may have been a roughly rat-size animal that weighed no more than a half a pound, had a long furry tail and lived on insects. 
   
   In a comprehensive six-year study of the mammalian family tree, scientists have identified and reconstructed what they say is the most likely common ancestor of the many species on the most abundant and diverse branch of that tree — the branch of creatures that nourish their young in utero through a placenta. The work appears to support the view that in the global extinctions some 66 million years ago, all non-avian dinosaurs had to die for mammals to flourish. 
   
   Scientists had been searching for just such a common genealogical link and have found it in a lowly occupant of the fossil record, Protungulatum donnae, that until now has been so obscure that it lacks a colloquial nickname. But as researchers reported Thursday in the journal Science, the animal had several anatomical characteristics for live births that anticipated all placental mammals and led to some 5,400 living species, from shrews to elephants, bats to whales, cats to dogs and, not least, humans. 
   
   A team of researchers described the discovery as an important insight into the pattern and timing of early mammal life and a demonstration of the capabilities of a new system for handling copious amounts of fossil and genetic data in the service of evolutionary biology. The formidable new technology is expected to be widely applied in years ahead to similar investigations of plants, insects, fish and fowl. 
   
   Given some belated stature by an artist’s brush, the animal hardly looks the part of a progenitor of so many mammals (which do not include marsupials, like kangaroos and opossums, or monotremes, egg-laying mammals like the duck-billed platypus). 
   
   Maureen A. O’Leary of Stony Brook University on Long Island, a leader of the project and the principal author of the journal report, wrote that a combination of genetic and anatomical data established that the ancestor emerged within 200,000 to 400,000 years after the great dying at the end of the Cretaceous period. At the time, the meek were rapidly inheriting the earth from hulking predators like T. rex. 
   
   Within another two million to three million years, Dr. O’Leary said, the first members of modern placental orders appeared in such profusion that researchers have started to refer to the explosive model of mammalian evolution. The common ancestor itself appeared more than 36 million years later than had been estimated based on genetic data alone. 
   
   Although some small primitive mammals had lived in the shadow of the great Cretaceous reptiles, the scientists could not find evidence supporting an earlier hypothesis that up to 39 placental mammalian lineages survived to enter the post-extinction world. Only the stem lineage to Placentalia, they said, appeared to hang on through the catastrophe, generally associated with climate change after an asteroid crashed into Earth. 
   
   The research team drew on combined fossil evidence and genetic data encoded in DNA in evaluating the ancestor’s standing as an early placental mammal. Among characteristics associated with full-term live births, the Protungulatum species was found to have a two-horned uterus and a placenta in which the maternal blood came in close contact with the membranes surrounding the fetus, as in humans. 
   
   The ancestor’s younger age, the scientists said, ruled out the breakup of the supercontinent of Gondwana around 120 million years ago as a direct factor in the diversification of mammals, as has sometimes been speculated. Evidence of the common ancestor was found in North America, but the animal may have existed on other continents as well. 
   
   The publicly accessible database responsible for the findings is called MorphoBank, with advanced software for handling the largest compilation yet of data and images on mammals living and extinct. “This has stretched our own expertise,” Dr. O’Leary, an anatomist, said in an interview. 
   
   “The findings were not a total surprise,” she said. “But it’s an important discovery because it relies on lots of information from fossils and also molecular data. Other scientists, at least a thousand, some from other countries, are already signing up to use MorphoBank.” ||||| What's cute and furry and has a name only a paleontologist could love? 
   
   A tiny theoretical creature dubbed the "hypothetical placental mammal ancestor" is stealing the hearts of some evolutionary biologists — and annoying others — as it raises new debate over just when our early mammal ancestors began diversifying across the globe. 
   
   In a paper published this week in the journal Science, an international team of researchers described how they used a vast database of fossil and anatomical data, as well as DNA evidence, to reverse-engineer an ancestor to the largest group of living mammals. 
   
   The scientists theorized that this hypothetical critter would have begun to diversify into new species of mammals after the demise of the dinosaurs roughly 66 million years ago — a concept at odds with some current thinking. 
   
   "This is fairly novel, to reconstruct an ancestor," said study coauthor and paleontologist Mike Novacek, the provost of science at the American Museum of Natural History in New York. "I don't know of any other." 
   
   Placental mammals, including humans, make up the largest branch of the mammalian family tree. Only marsupials and a small number of egg-laying mammals are excluded. 
   
   For the last several decades, scientists have debated just when it was that placental mammals began their explosive proliferation. Estimates based on DNA, or so-called molecular evidence, suggest that placental mammals appeared in the mid-Cretaceous period when dinosaurs like Tyrannosaurus rex and Triceratops still trod the Earth. 
   
   However, scientists have yet to uncover evidence of placental fossils during this period, according to the authors of the new study. They argue that this is because placental mammals began to diversify only several hundred thousand years after the demise of non-avian dinosaurs, in the Paleocene epoch. 
   
   "An environment that radically changed may have offered more opportunities for a new group to take form and radiate," Novacek said. 
   
   Using a vast online database called Morphobank, the team of 23 scientists built a composite of the hypothetical ancestor based on the morphology, or appearance, of existing and extinct animals, along with the DNA data. Study coauthor Maureen O'Leary, a paleontologist at Stony Brook University in New York, likened the process to reassembling a crime scene from thousands of bits of evidence. 
   
   Over a period of six years, the researchers developed a highly specific picture of an unassuming little animal whose descendants would one day include whales, bats, lions and people. 
   
   "The placental ancestor was a scampering species that had a diet of insects, a fleshy nose, a light underbelly in its fur, and a long tail," O'Leary said. "It was larger than a mouse, but smaller than a rat." 
   
   Based on their findings, the scientists said placental mammals began their spread about 36 million years later than estimates based on the DNA-derived molecular clock model. 
   
   The research team also contradicted the molecular clock-based theory that placental mammals developed into different species because of the fragmentation of the supercontinent Gondwana, millions of years before the dinosaurs vanished. In addition, the study concluded that a subset of placental mammals known as Afrotheria, which includes elephants and aardvarks, did not originate in Africa as commonly believed; instead, authors say they arose in the Americas. 
   
   Not all evolutionary biologists have fallen in love with the furry, hypothetical bug eater, however. Argument over the rise of placental mammals is likely to continue for some time. 
   
   "The bottom line is that this study is not convincing and will not settle the debate," said Mark Springer, an evolutionary geneticist at UC Riverside who was not part of the research team. 
   
   Using fossils to establish branches of a family tree can result in mistaken groupings of species, Springer said, because some traits are passed along by related species while other traits develop in parallel with more distantly related species. This is known as convergent evolution, and Springer said he was not convinced that study authors had addressed that issue. 
   
   Study coauthor Novacek said he did not expect the paper to end discussion on the matter, but he hoped it would add weight to an ongoing argument. 
   
   "I think this will reinforce the notion that the fossil record is not in agreement with what's been reconstructed by molecular clocks," Novacek said. "As data accumulates, the case for this late origin becomes better and better." 
   
   Another debate is brewing over whether the hypothetical creature deserves a less cumbersome moniker. 
   
   "People who think this organism is cute have asked me for a name," Novacek said. "I tell them we can't give it a Latin name, because it never really existed." 
   
   monte.morin@latimes.com
target: – Say hello to your great-great-great-great-great-grandparents (times a few million or so)—Protungulatum donnae, a rat-sized insect eater believed to have lived 66 million years ago. A new six-year study of the mammalian family tree, looking at DNA and anatomical evidence in greater detail than ever before, has identified Protungulatum donnae as the most likely common ancestor for all 5,400 placental mammal species, reports the New York Times. The critter, however, is only hypothetical, notes the LA Times, explaining that scientists used data to "reverse-engineer" it. The study examined 83 mammals and fossils for more than 4,500 traits, creating a database 10 times larger than any previous database. Because Protungulatum donnae would have emerged 200,000 to 400,000 years after the great extinction that ended the dinosaurs—about 36 million years later than previous estimates—scientists say this is a clear sign the rise of the mammals was tied to that mass extinction. The abstract to the original article is at Science magazine.