Mammals Evolved Three Times Faster after Dinosaur Extinction

     Our ancestors evolved three times faster in the 10 million years after the extinction of the dinosaurs than in the previous 80 million years, according to University College of London researchers.

     The research team found the speed of evolution of placental mammals (a group that today includes about 5000 species including humans) was constant before the extinction event but exploded after, resulting in the varied groups of mammals we see today.

     Lead (not lead ;-)) researcher, Dr. Thomas Halliday said: "Our ancestors, the early placental mammals, benefitted from the extinction of non-avian dinosaurs and dwindling numbers of competing groups of mammals. Once the pressure was off, placental mammals suddenly evolved rapidly into new forms."

     "In particular, we found a group called Laurasiatheria quickly increased their body size and ecological diversity, setting them on a path that would result in a modern group containing mammals as diverse as bats, cats, rhinos, whales, cows, pangolins, shrews and hedgehogs."

        Laurasiatheria is a superorder of placental mammals believed to have originated on the northern supercontinent of Laurasia. Thus it was the northern landmasses that produced much of the mammal explosion.

     The team found that the last common ancestor for all placental mammals lived in the late Cretaceous period, about three million years before the non-avian dinosaurs became extinct 66 million years ago. This date is about 20 million years younger than suggestions from previous studies which used molecular data from living mammals and assumed a near-constant rate of evolution.

     In this study the researchers analysed fossils from the Cretaceous to the present day, and used the dates of their occurrence in the fossil record to estimate the timing of divergences based on an updated tree of life. The new tree was released in 2015 and has the largest representation of Paleocene mammals to date.

     The scientists measured all the small changes in the bones and teeth of 904 placental fossils and mapped the anatomical differences between species on the tree of life. From measuring the number of character changes over time for each branch, they found the average rate of evolution for early placental mammals both before and after the dinosaur extinction event. They compared the average rate of evolution over the geological stages before the extinction and the geological stages after to see what impact it had.

     Senior author, Professor Anjali Goswami said, "Our findings refute those of other studies which overlooked the fossils of placental mammals present around the last mass extinction. Using rigorous methods, we've successfully tracked the evolution of early placental mammals and reconstructed how it changed over time. While the rate differed between species, we see a clear and massive spike in the rates of evolution right after the dinosaurs become extinct, suggesting our ancestors greatly benefitted from the demise of the dinosaurs. The huge impact of the dinosaur extinction on the evolution of our ancestors really shows how important this event was in shaping the modern world."

      Professor Paul Upchurch, co-author of the study, added: "Our large and refined data set allows us to build a clearer picture of evolutionary history. We plan on using it to study other large-scale evolutionary patterns such as how early placental mammals dispersed across the continents via land bridges that no longer exist today."

       I'd like to take that land bridge from South America to Africa in the southern land masses of Gondwanaland. 

      How about you? Does "Reunite Gondwanaland!" ring true to you?

Steph

Rudist Colonies: Fun, Wild Index Fossils

     Rudists are some of my favorite fossils, not just because it's a fun word to say, but because they are quite useful index fossils from the Jurassic and Cretaceous:

    Index fossils are the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved. John McPhee's analogy from Basin and Range  (1981) describes the concept well:

     "Imagine an E.L. Doctorow novel in which Alfred Tennyson, William Tweed, Abner Doubleday, Jim Bridger, and Martha Jane Canary sit down to a dinner prepared by Rutherford B. Hayes. ... a geologist could quickly decide -- as could anyone else -- that the dinner must have occurred in the middle 1870s, because Canary was 18 when the decade began, Tweed became extinct in 1878, and the biographies of the others do not argue with these limits."

    These marine bivalves were one of the main components of the widespread Tethys Sea between Laurasia and Gondwana about 200 million years ago as Pangaea was breaking up:

     Rudists were one of the main components  of the reefs that formed then:

      Rudists were widespread and had very different shapes making them excellent index fossils for fairly narrow time periods:

     The earlier forms were elongate, with both valves being similarly shaped, often pipe-shaped, while the later, reef-building Cretaceous forms had one valve that become a flat lid, with the other valve becoming an inverted spike-like cone. The size of these conical forms ranged widely from just a few centimeters to over a meter in length.

     Rudists' morphology consisted of a lower, roughly conical valve that was attached to the seafloor or to neighboring rudists, and a smaller upper valve that served as a kind of lid for the animal. The small upper valve could take a variety of different forms, including: a simple flat lid, a low cone, a spiral, and a star-shape.

     The earlier forms tended to be more solitary but the Cretaceous forms were generally more colonial. Rudist colony: they started it millions of years ago. The first naturists died off at the major Cretaceous-Paleogene extinction about 65 million years ago.

     Looking forward to your tales of nudist, er, rudist colonies. ;-)

Indexedly,

Steph

(Word Woman)

Holiday Hummer in the Colorado Mountains 7/3/14 (photo by C. Fiss)

     First clue (these are my photos) to location in the CO mts. See if you can win the geography quiz, at least a bit of a challenge this Sunday morning ;-):

Second clue:

More to come (if needed). Clue number three. Hint: It's very, very clear.