Fossil Discovery Confirms Mammals’ Original Ancestors

By Carl Engelking | September 10, 2014 12:17 pm
xianshou songae

Reconstruction of a new mammal species, Xianshou songae. This mouse-sized animal was a tree dweller in the Jurassic forests. (Credit: Zhao Chuang)

What looks like a rat, climbs trees and has a tail like a lemur? It’s your cousin, of course!

Before you take offense to that statement, please note that we’re referring to Xianshou songae, one of three newly discovered, extinct species that lived roughly 160 million years ago. This trio of species, unearthed in China, helps settle a long debate regarding the origin and earliest evolution of mammals. So by your cousin, we mean the earliest relatives of all mammals.

Unsolved Mystery

One of the major unsolved problems with the mammalian family tree concerns two groups of early, rodent-like mammals: multituberculata and haramiyida. It was believed these two groups of animals were related, but in 2013 scientists unearthed two complete haramiyid skeletons that had very different features. This discovery generated conflicting interpretations about the relationship between these two groups.

So what’s the big deal? Roughly 38 million years. If the differences between the two groups were significant enough, they’d be separated and mammals would have originated in the Middle Jurassic period, about 170 million years ago. If the two groups had a common ancestor, early mammals started walking the Earth much earlier, during the late Triassic period, about 208 million years ago.

Mystery Solved

Chinese scientists discovered the skeletons of six haramiyid fossils representing three different species that date from around 160 million years ago. The well-preserved skulls and skeletons revealed that haramiyida and multituberculata were indeed sister groups of each other.

However, haramiyida evolved very distinct features. Therefore, scientists concluded that they evolved from a common ancestor, confirming mammals originated some roughly 208 million years ago. They published their findings Wednesday in the journal Nature.

Check out the whole motley crew of our early relatives below.

Reconstruction of arboreal mammals in the Jurassic forests. The three individuals on the left side represent three new species of euharamiyidan mammals. The other two represent a gliding species and another euharamiyidan, respectively. Credit: Zhao Chuang

Reconstruction of arboreal mammals in the Jurassic forests. The three individuals on the left side represent three new species of euharamiyidan mammals. The other two represent a gliding species and another euharamiyidan, respectively. Credit: Zhao Chuang

CATEGORIZED UNDER: Environment, Living World
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  • Jim LeSire

    Well, yes, but before the mammals arose we were mammal-like reptiles, no? And before that…

    • Philip Olson

      Fish with mammal bone structure preceded the reptiles. Including the ball and socket shoulder as well as five fingers and toes and two bones in the forearm. Some traits are locked in tighter than others. I am guessing the shoulders and fingers were for picking food up off the seafloor as theses fish lived in roughly one to two feet of water before moving onto land.
      Eventually we will have the complete story, if only I live long enough for the blanks to be filled in.

  • Fraizer Josefski

    The title is misleading- they mean the First True Mammals . It’s A bit sloppy grammatically. Much like the tabloids – these online magz , too , need catchy Headlines .

  • Metalhead Nick

    I haven’t looked into this is a while, but I thought there was still some debate as to whether multituberculates were actually true mammals. Does this mean that’s settled? Also, if anyone whose more informed than I, I thought the distinction between mammals and mammal like reptiles is largely based on the jaw and eat bones. I read a while ago there is some thought that monotremes, while having a relatively mammalian structure, that the war bones still sit differently (lower if I recall) than in other mammals and may have been evolved independly of other mammals. Wouldn’t that relagate them to mammal like reptile status (along with the sweet of reptilian traits not just egg laying but having like ten sex chromosomes which I think still carry a z chromosome. I could be remembering this wrongly, but where exactly is the divide between synopsids and mammals?

    • Metalhead Nick

      Ear not eat or war… Stupid autofill

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