Debate over which mammals roamed with the dinosaurs

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Did the first mammal with a placenta live alongside the dinosaurs — or did it emerge after a gigantic asteroid wiped them out? This is the subject of a heated debate that pits scientists who contend that fossils are the ultimate timekeeper of life’s history against researchers who say that genetics offers more reliable dates.

Such disputes have been waging for decades, since researchers first began gleaning evolutionary detail from proteins and DNA. But the skirmish over placental mammals — animals that give birth to live offspring that are in late stages of development, including whales, mice and humans — began with a paper published early last year, arguing that the group diversified only after those dinosaurs that did not evolve into birds went extinct, 65 million years ago.

For that study, Maureen O’Leary, an evolutionary biologist at Stony Brook University in New York, and her team spent several years characterizing and analyzing thousands of traits in dozens of living and fossil mammals. The team combined those characteristics with genetic data to build a giant tree of life, showing how different placental mammals related to one another.

But to establish when the different creatures evolved, the researchers looked only at the fossil record. They concluded that the earliest placental mammals appeared only after the asteroid impact that killed the dinosaurs and marked the end of the Cretaceous period and the beginning of the Palaeogene. After this, the team said, the placentals quickly diversified, and a menagerie of mammals filled the habitat niches left by the dinosaurs.

Written By: Ewen Callaway
continue to source article at nature.com

10 COMMENTS

  1. Ex articulo:

    [Phil] Donoghue and evolutionary geneticists Mario dos Reis and Ziheng Yang of University College London publish a study in Biology Letters2, saying that O’Leary’s team made a fatal error in assuming that lineages of species date back no further than their oldest fossils. The fossils should instead mark the minimum age for a lineage, says Donoghue, because it is likely that animals existed before that, but were not preserved as fossils or their remains have yet to be discovered.

    Reading this was for me one of those “of course!” moments, when something suddenly made more sense. Of course each fossilized animal had ancestors, so of course the oldest fossil of a lineage indicates its minimum age (unless there be evidence that the animal were made whole in one moment – perhaps “Made by God” in ancient Hebrew divinely embossed upon the pelvis).

  2. Here’s a fun little site: TimeTree.org. You put in two separate extant species, and it indicates quickly numerical information on when the species’ ancestors, based on genetic studies, most likely diverged. I’ve already put in the red kangaroo and the common chimpanzee, since the marsupials were the last extant non-placental lineage to split away from the lineage that gave rise to the placental mammals.

    Also, I think I’d go with the DNA over the fossils. Fossils can be biased samples that are missing key species, but DNA contains a very thorough record.

    • All measurement has inherent error. But, i am with you, the nature of the two sets of data suggests (to me) that the molecular clock yields a number with a bit of error (tolerable error) meanwhile, the fossil is a much fuzzier entity as far as age of a species is concerned. The article and the commenters are quite right to point out that the fossil you hold in your hand (although reliable dated) is one of a species whose representatives could have roamed the planet for millions of years before the specimen that fossilized lived.

      In reply to #2 by Zeuglodon:

      Here’s a fun little site: TimeTree.org. You put in two separate extant species, and it indicates quickly numerical information on when the species’ ancestors, based on genetic studies, most likely diverged. I’ve already put in the red kangaroo and the common chimpanzee, since the marsupials were the…

  3. Why can’t both techniques be employed in a complimentary way to approach the subject from different angles?

    That way any facets that can’t be investigated by one method can be explored by the other and vice verse; and who knows, a third new discipline might evolve.

    That’s not a rhetorical question, it’s one from a lay person, so if it’s nonsense, as usual, I stand to be corrected please.

    • In reply to #3 by Stafford Gordon:

      Why can’t both techniques be employed in a complimentary way to approach the subject from different angles?

      They can, but it depends very much on the question you want answered. Genetic studies are unsurpassed when it comes to establishing family trees, so you can build quite a good picture of when the major speciations occurred throughout prehistory. For instance, the latest evidence from the field suggests that the monotremes split away from the rest of the mammals during the Upper Triassic, roughly the same period when mammals emerged from mammal-like reptiles, and the marsupials split away during the Middle Jurassic. Just take a look at the link and pick a chapter for an example of more genetic studies establishing relationships between extant species.

      Palaeontology is more helpful if you want to recreate the conditions of the past, for instance to get an inkling of what those ancestral species might have looked like and what ecosystem and climate they had to work with. For instance, without fossils, we might never have suspected that gliding mammals appeared as early as the Jurassic period, nor that there were some mammals that were bigger than some dinosaurs. Without fossils, other groups of mammals like multituberculates would have remained unknown, and but for 35 million years, we might have recognized four major groups of mammals rather than three (monotremes, placentals, and marsupials).

      However, since the question is when lineages like the placentals emerged, genetic studies have the advantage here.

      • In reply to #6 by Christiana Magdalene Moodley:

        Oops,sorry, I meant homo sapiens.In reply to #4 by Christiana Magdalene Moodley:

        Don’t ask people at the Creation Museum! They”ll say it was the mammal, homosexual sapiens;)

        I think you were right the first time! ^_^

        Steve

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