Mammals and the Cretaceous-Tertiary Mass Extinction
John Hunter (Department of Evolution, Ecology, and Organismal Biology, Ohio State University at Newark, Newark, Ohio, USA)
22 May 2012
There are 3 groups of mammals today: monotremes, marsupials, and placentals.
The monotremes are the duck-billed platypus and the echidna. They were widespread across Gondwana in the past. They are now restricted to Australasia.
Marsupials are in Australasia and in North America today.
There are 20 or so orders of placental mammals, living and extinct.
Placentals include primates, which turn out to be related to bats and flying lemurs.
Placentals also include the ungulates - the hoofed mammals. The ungulates are an artificial group - they consist of 3 separate groups, actually.
Whales turn out to be in the Artiodactyla.
Mammals have been around since the Late Triassic. A Middle Triassic busted-up mammal braincase has been found in southwestern USA. Well-preserved mammal fossils start in the Late Triassic.
Mammals diversify after the Cretaceous.
Modern placental orders go to near the base of the Paleocene.
Marsupials and monotremes do go into the Cretaceous.
Early mammals include:
Morganucodonts (Late Triassic to Early Jurassic) - they are stem mammals. They have teeth with cusps that slide past each other. These mammals were likely insectivores (insect eaters).
Docodonts (Late Triassic-Late Jurassic) - they have broad, crushing teeth and were likely frugivores (fruit eaters). Their bodies ranged from small & shrew-like to beaver-sized.
Australosphenids (stem monotremes) - they are known from across Gondwana - Australia, Madagascar, South America.
Eutriconodonts (Jurassic-Cretaceous) - bigger & badder mammals, up to ~possum-sized.
Multituberculates (Jurassic-Eocene) - “rodents” of the Mesozoic, although they wouldn’t be mistaken for living rodents. These mammals had cheek teeth with cusps in multiple rows, like a modern rodent molar. Multituberculate mammals had splayed-out-to-the-side limbs. They could reverse their hind feet to hang from tress, like squirrels today.
Stem eutherians (Jurassic-Cretaceous)
Mammals in the Mesozoic were not ecologically dominant - they were small-bodied, nocturnal, and had a small biomass compared with the archosaurs/dinosaurs.
After the Cretaceous-Tertiary extinction, there was a survivorship period in the mammals.
Survivors of the extinction included marsupials (they were in & are still in North America) and multituberculates. The multituberculate mammals persisted until the Eocene, when rodents migrated into North America from Asia. The rodents probably outcompeted the multituberculates.
Mammals diversified in the Paleogene-Neogene.
The Cretaceous-Tertiary transition had the greatest number of originations per unit species and the greatest number of extinctions per unit species - lots of turnover.
Mammal body sizes increased through time. The largest single increase occurred at the Cretaceous-Tertiary. After that, there was continued increase in mammal body sizes through the Cenozoic.
See Schulte et al. (2010) - Science 327(1214).
There was a large impact in the Gulf of Mexico, at the Yucatan Peninsula, at 65.5 million years ago - now the Chicxulub Impact Crater.
The timing of the impact corresponds with extinction of Cretaceous-only species of plants and animals, and corresponds with a sudden spike in iridium (Ir) in Cretaceous-Tertiary boundary sediments, and also corresponds with a d13C excursion.
The iridium spike and extinction event also correspond with periods of volcanism in India - the Deccan Traps. Imagine an area the size of Texas suddenly being filled with lava, with lots of poison gases being released. There were multiple episodes of Deccan Traps volcanism - not all of them correspond with the extinction horizon.
Deccan Traps volcanism could have caused the Cretaceous-Tertiay mass extinction, some say.
Others say Deccan Traps volcanism did not cause the mass extinction.
Close to Chicxulub, boundary sections have dozens of meters thick tsunami deposits - very coarse grained sediments - formed quickly and dewatered quickly.
Far away from Chicxulub, for example at Agost, Spain, the boundary section is quite thin.
See Archibald et al. (2010) - Science 328 (21 May 2010). This paper says that the Cretaceous-Tertiary mass extinction had multiple causes - impact + volcanism + marine regression + climate change.
Early Paleocene first appearances of mammals include Purgatorius (Plesiadapiformes) - a primate that possibly goes into the Cretaceous (based on one tooth in a probably-contaminated fossil washing screen) - and Protungulatum (Condylarthra) - now known in the Cretaceous.
John Hunter does mammal fossil field work in southwestern North Dakota, in the Williston Basin.
The area has Paleocene- and Cretaceous-aged sedimentary rocks. The contact in the area potentially has Cretaceous-Tertiary fossil records. The area is mostly rangelands/grasslands. It’s usually only the river valleys that have adequate rock exposures, such as along the Yellowstone River and the Little Missouri River, etc. Going north in this field area, one encounters younger sites. Going south in this field area, one encounters older sites.
The area has the Hell Creek Formation (upper Upper Cretaceous) - it has dinosaurs + lots of mammal sites. Above that is the Fort Union Group (Paleocene) - its has lots of mammal sites.
Marine rocks occur below the Hell Creek Fm. Marine rocks also come in from the east during the Paleocene - there’s a regression to Hudson Bay in the Middle Paleocene.
In the field, the Hell Creek Fm. outcrops are drab grays. Above that, the Fort Union Group outcrops have tans + grays.
The Pioneer Trails Region Museum in Bowman, North Dakota has a paleontology curator, Dean Pearson. John Hunter works with him, plus Joe Hartman (works on invertebrate fossils), Dan Peppe (works on fossil plants & paleomagnetic analysis), Antoine Bercovici (works on fossil pollen), and others.
The asteroid layer is a tonstein at the Cretaceous-Tertiary (K-T) boundary in some sections - iridium has been identified in these sections. A good K-T section occurs at Mud Buttes, North Dakota - it has shocked quartz + a pollen change + an iridium spike + spherules.
Sandstone channels can cut out parts of these sections, including the K-T transition. So, the K-T asteroid layer is not found everywhere.
Mammal fossils: mostly isolated teeth + jaw fragments.
See Hunter (1999) - North Dakota Academy of Science.
See Pearson et al. (2002) - Geological Society of America Special Paper 361.
There is very little ecological change in Hell Creek Formation fossils through time. There is no evidence of dinosaurs (or other groups) declining.
At one locality, <1 meter above the K-T boundary, lenses of sediments have the earliest mammals and plants recovered from the post-Cretaceous - these are “day after” biotas. They are the closest-to-the-Cretaceous biotas of Paleocene age identified so far.
The Hell Creek Fm.-Fort Union Group contact is arbitrary - it’s placed at a mappable coal bed. This is somewhat correlative with the Z Coal. “Z Coal” is a term used in eastern Montana - the coal bed splits and joins. The formation contact (= coal bed) is within 1 to 0.5 meters of the iridium spike. So, the “Z Coal” and the iridium spike are pretty close.
Bercovici et al. (2009) - Cretaceous Research.
Floras have been recovered in these close-to-the-Cretaceous sands.
The fossil record looked at just above the K-T is environmentally sensitive.
Wilkening locality - 5341 fossil vertebrate specimens were recovered (mostly fish, but some mammals, too). The fossils were retrieved by screening/sieving. A freshwater ponding event occurred after K-T at this locality. Found a new species of Mesodma, a multituberculate mammal. Got archaic placental mammals and archaic ungulates. The site is dominated by one species of multituberculate - Mesodma. This animal was 80% of the mammal fauna at the Wilkening locality.
Seeing a surge in multituberculate mammals in the transitional period between the Late Cretaceous and the Early Paleocene. The multituberculates were an opportunistic group following the K-T extinction event. Also seeing a fern spore spike just above K-T. This is the first time a spike in mammals has been found after the K-T.
At other sites, reports of increased numbers of mammals after K-T were dubious - there was often evidence of reworking - hundreds or thousands of years of time may have been compressed, including through the boundary. This will disproportionately represent forms that go extinct.
The Wilkening locality is the first confident occurrence of a multituberculate mammal site after K-T.
Also looked at Merle’s Mecca locality - dates to ~100,000 years after K-T (= Eary Paleocene). It is more diverse that the Wilkening locality, in terms of mammal fossils. The mammals at Merle’s Mecca are also more derived.