11-8 Diversification

Paleontologists refer to the Paleocene Epoch that followed the End Cretaceous extinction as the dark epoch since it is so difficult to find fossils; however, some larger mammals began to evolve soon after the extinction. Most of the land animals that survived were small insect eating animals such as mammal insectivores, birds, and lizards, but not even many of these animals survived. The Periptychidae were herbivores that appeared in the San Juan basin of New Mexico, 500,000 years after the K-Pg boundary.[1] Primitive stem carnivorans Ravenictis and Pristinictis also appear in the early Paleocene. Eventually carnivores diversified into canines and felines.

The Boreoeutheria (placental mammals) evolved on the northern continents (Figure 11‑38), which were disconnected from Africa and South America. The Boreoeutheria include the Euarchontaglires (primates and rodents), the Ferae (bears, cats, dogs), and the Ungulates, which includes the Perrisodactyla (horses), and the cetartiodactyls (cows, whales, pigs). The nonplacental marsupials evolved in South America and Australia (possums, kangaroos). The placental Xenartha (sloths, armadillos, and anteaters) also evolved in South America, and the placental Afrotheria (elephants) evolved in Africa. Most of the world is now ruled by the Boreoeutheria (Figure 11‑39), which occupied Africa and South America when they reconnected to North America and Asia.

Although most diversification took place after the End Cretaceous extinction, the divergence of major groups such as carnivores, ungulates and rodents took place in the Cretaceous. Liu estimated the diversification of the Boreoeutheria based on DNA molecular clock analysis. [2] He placed the origin of placental mammals at 180 Ma, which is the middle of the Jurassic. Liu calculated that Rodentia, Lagomorpha (rabbits), Euliptyphlans (insectivores) and Primates diverged in the Late Cretaceous. Liu calculated that Perrisodactyla (horses) and Cetartiodactyla (cows and whales) diverged at the time of the End Cretaceous extinction, and that Carnivora and several other orders diverged just after the End Cretaceous extinction. He found that the animals native to Africa (Afrotheria) and South America (Xenarthra) had an early divergence from the other placental mammal groups and confirmed the grouping of Afrotheria and Xenarthra within Atlantogenata. In summary, Liu’s and other molecular clock studies agree with Halliday that placental mammal diversification began in the Cretaceous.

A significant number of paleontologists think that because no placental mammal fossils are found in the Cretaceous, as indicated by the presence of an epipubic bone in Cretaceous fossils, that all placental mammals evolved from a single placental mammal ancestor after the End Cretaceous Extinction.[3]

“We find that only the stem lineage to Placentalia crossed the K-Pg boundary and then speciated in the early Paleocene. We estimate that the minimum age of the diversification of crown Placentalia is just younger than the K-Pg boundary, or ~36 My younger than molecular clock–based mean estimates derived from supertree and supermatrix analyses... We recognize Protungulatum donnae as the oldest undisputed species within crown Placentalia, and this species dates to the earliest Paleocene within an interval extending from the K-Pg boundary to ~200,000 to ~400,000 years later. Integration of fossils as primary data in the phylogeny indicates that ~10 interordinal speciation events might have occurred in as little as 200,000 years.

Because of the discrepancy between epipubic bones, other eutherian characteristics, and molecular clock studies, there are three proposed models of placental mammal evolution: the explosive, long fuse, and short fuse models. O’Leary favors the explosive model since there are no placental mammal fossils in the Cretaceous. The short fuse model has the diversification of placental mammals taking place deep in the Cretaceous. The long-fuse model, favored by Halliday, has the diversification of major orders taking place in the Cretaceous but the diversification within orders taking place after the End-Cretaceous extinction.

· Explosive model. Origination and diversification of placental mammal orders within 10 million years of the K-Pg boundary

· Long-fuse model. Mammal orders originated in the Cretaceous but intraorder diversification took place after the K-Pg boundary.

· Short-fuse model. Orders originated and diversified deep in the Cretaceous.

Professor Stearns from Yale described the four different types of placenta in placental mammals and the times at which placental mammal characteristics appeared in the fossil record (https://youtu.be/w2dDwLKk1YA)

1. Cotyledonary placenta

a. Cattle and sheep

2. Diffuse placenta, similar shape with cotyledonary

a. Horses and pigs

3. Zonary placenta

a. Dogs and cats

b. Elephants, rock hyraxes, aardvarks (Afrotheria)

4. Discoid placenta

a. Rodents and primates

Invasiveness refers to the number of layers in the mother’s uterus that are dissolved by the placenta. More invasive placentas have longer gestation periods. Humans and rodents have the maximum invasiveness (3 layers are dissolved). Thus, the number of layers (invasiveness) is ancient and has not changed in the Cenozoic Eon.

The villous condition is a derived condition in some mammals in which veins form that improve the rate of mass transfer between the mother and the fetus. There are a few groups within each type of placenta that have a villous condition. Humans, apes, and chimpanzees have deep extravillous invasion with rearrangement of blood vessels, but not gibbons, which means that this condition only evolved in the great apes.

As strange as it may sound, viruses enabled the placental condition in eutherians. An endogenous retrovirus is a virus that has been incorporated into the human genome by implanting itself into the egg or sperm. A virus triggers the formation of a wall between the baby and the mother in the uterus and protects the baby from the mother’s immune system (https://youtu.be/CDA1ISk0rlI). Perhaps something like this triggered the full placental mammal condition at the time of the End Cretaceous extinction and allowed the fetus to live outside of an egg in the womb.

The fact that there are four different types of placenta in placental mammals indicates that they the condition have begun to originate in the major groups of mammals prior to the End Cretaceous extinction. On the other hand, this could be a case of convergent evolution for animals of similar shape and conditions.

References

[1] Halliday, Thomas, Paul Upchurch, and Anjali Goswami. Resolving the relationships of Paleocene placental mammals. Biol. Rev. (2015), 92, pp. 521–550. 521 doi: 10.1111/brv.12242 Halliday, Resolving.

[2] Liu, Liang, Jin Zhang, Frank E. Rheindt, Fumin Lei, Yanhua Qu, Yu Wang, Yu Zhang et al. "Genomic evidence reveals a radiation of placental mammals uninterrupted by the KPg boundary." Proceedings of the National Academy of Sciences114, no. 35 (2017): E7282-E7290.

[3] O'Leary, Maureen A., Jonathan I. Bloch, John J. Flynn, Timothy J. Gaudin, Andres Giallombardo, Norberto P. Giannini, Suzann L. Goldberg et al. "The placental mammal ancestor and the post–K-Pg radiation of placentals." Science 339, no. 6120 (2013): 662-667.

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