Resetting Evolution’s Clock? End-Cretaceous Extinction and the Origin of Modern Mammals
John Hunter (Department of Evolution, Ecology, and Organismal Biology, Ohio State University at Newark, Newark, Ohio, USA)
10 November 2011
The Cretaceous-Tertiary (K-T) event occurred at 65.5 m.y. - an impact event. There is still some debate among mammalian paleontologists whether this even was significant, in terms of mammal evolution.
The first mammals appeared at >200 Ma.
Basic terminology - the concept of geologic time (deep time). Earth is 4.55 billion years old. The Moon formed at 4.527 billion years. During most of Earth’s history, single-celled organisms were present. In the last 500 million years or so, more complex life has been present on Earth.
There are three eras of Earth history during which complex life was around - Paleozoic, Mesozoic, Cenozoic. The Mesozoic was the time of dinosaurs and mammal-like reptiles. The Cenozoic was the time of modern mammals.
The first vertebrates appeared at about 450 Ma.
Concept of evolution - in the mid-1800s, Darwin presented a mechanism that explained biologic change through time. Darwin’s book - On the Origin of Species. Darwin also appreciated the concept of extinction.
John Hunter works on mammals. Mammals are vertebrates with hair, two sets of teeth, and nurse their young.
Linnaeus established a biologic classification scheme. It’s old-fashioned now. Linnaeus was not an evolutionist. The Linnaean hierarchy (kingdom, phylum, class, order, family, genus, species) is a pre-evolutionary classification.
A more explicitly evolutionary classification is based on names of groups of organisms of common descent.
Today, there are three groups of living mammals:
2) Marsupials - diversified in Australia and South America
Together, the marsupials and placentals share a common ancestor and are known as the Theria.
There are four groups of living marsupials. These include the Ameridelphia (oppossum), the Marsupicarnivora (Tasmanian devil), bandicoots, and the diprotodonts (koala). The marsupicarnovirans + bandicoots + diprotodonts share a common ancestor - the Australidelphia.
Placental mammals (Placentalia) are much more complex. The majority of modern mammal diversity is in the placental group. Placentals are divided into 20 or so different orders. These include carnivorans, primates, chiropterans (bats), dermopterans (flying lemurs), lagomorphs (rabbits), rodents, Cetartiodactyla (= cetaceans + artiodactyls - whales are now known to be related to artiodactyls; a quick, terrestrial to freshwater to fully marine transition is represented by fossils; the transition occurred in Asia).
How are the different placental orders related? Not sure. Most modern orders first appeared close to the K-T (65 m.y.).
For two-thirds of their history, mammals were small and rat-like. This changed at 65 m.y. Early Cenozoic faunas are post-disaster faunas.
In the earliest Cenozoic (first 10 m.y.), little changed among mammals. There were some surviving lineages from the Mesozoic - multituberculates. Earliest Cenozoic mammals also included placentals (their ancestors were present in the Cretaceous), archaic ungulates, small-bodied insectivorous eutherians (proteutherians). These creatures were present in open woodland North America.
During the Paleocene to mid-Eocene, Earth was a hothouse world, especially at the Paleocene-Eocene thermal maximum (PETM). The PETM event was followed by the highest mammalian diversity. From the mid-Eocene to Holocene, Earth was an icehouse world.
Traditional view of placental orders in the Cenozoic - placental mammals radiated out from ~K-T boundary. The first actual fossils are between 65 and 55 m.y.
New view - estimates of divergence times (based on molecular clock studies) indicate that placental orders go back into the Mesozoic (see Cooper & Fortey, 1998). These divergence time estimates are usually two or three times older than what the first fossil occurrences show.
Crown groups vs. stem groups
Sinodelphys - an archaic marsupial; marsupial relative; stem-group marsupial mammal.
Eomaia - an archaic placental; stem-group placental mammal.
Both of these forms are stem-group mammals. Can also have stem-group monotremes & stem-group mammals.
Mammalia - vertebrates with mammary glands.
Theria - vertebrates with live birth
The earliest mammal fossils are >200 m.y. old.
In the shrew-like mammal fossil Morganucodon (= Morganucodonta), the ear bones are still attached to the lower jaw. The hind limbs are sprawled, like a lizard.
Docodonta - early mammals with bulbous teeth; included a beaver-sized swimmer; included a digging form.
Monotremes - the living platypus has no teeth. Fossil monotremes with teeth are now known from South America, Australia, and Madagascar. Embryonic living monotremes also have teeth.
Jurassic-Cretaceous stem therians - Eutriconodonta; includes a form found with baby dinosaurs in its stomach.
Jurassic-Eocene stem therians - multituberculates; they survived the K-T event; they fluorished after K-T.
Crown therians - the first ones are traditionally considered to be at 125 Ma. They are now known at 160 Ma - Juramaia - a new find.
Late Cretaceous fossil mammals are now known with epipubic bones - support a pouch in marsupials.
Eutherian mammal history is often considered to be a story set against a backdrop of continental movements, sea level changes, climate changes, etc.
K-T boundary - one of a series of big extinction events. It was the biggest one that mammals have ever experienced. Each extinction was a “resetting” for life, especially in the oceans.
Mesozoic (Late Cretaceous) - average mammal species duration = 4 m.y.; extinction rate 0.25/lineage*m.yr.
Cenozoic - average mammal species duration = 2.5 m.y.; extinction rate 0.4/lineage*m.yr.
The K-T was a resetting of the rate at which species originate and go extinct.
The highest increase in mammal species originations was in the post-K-T.
Chicxulub Crater article (2010) - Science 327: 1214.
Archibald et al. (21 May 2010) - Science 328.
Naysayers of the K-T impact causing extinction story focus on Deccan Traps volcanism. Deccan Traps volcanism peaked at certain times. Non-K-T Deccan Traps volcanism peaks do not correlate with extinctions. The K-T extinction occurs at a peak in Deccan Traps volcanism at 65 m.y.
Very proximal to Chicxulub - sections have meters-thick breccias (tsunami deposits).
Farther away, sections have thinner tsunami breccias.
Was the K-T a single event? Or a Murder on the Orient Express situation? (everything did the extinction) Causative factors - impact, volcanism, marine regression, climate change.
Seas were relatively low at the end-Cretaceous. Marine regressions are problematic as an extinction mechanism. Regressions apparently encourage terrestrial diversification.
Sheltering Hypothesis - some terrestrial animals survived an intense thermal pulse from the K-T impact by being in water or going underground. Animals that could shelter - did. This included fish, turtles, crocodilians, amphibians, snakes, etc. These did survive the immediate aftermath of the impact.
Did surviving mammals do this? Did surviving mammals dig & shelter underground?
Size of the elbow - can tell if a mammal was a digger or a swimmer. Diggers have long lever arms. There are lots of elbows in the fossil record but they are usually broken. But can determine lever arm length by looking at ulnar length - use ulna length as a proxy (this is known based on studying modern forms).
Don’t see digging/swimming mammals above the K-T. Surviving mammals aren’t explained by the Sheltering Hypothesis.
Looked at the St. Mary River Formation (Upper Cretaceous) - mammal fossils are associated with dinosaur nests.
Regional stratigraphy shows a series of transgressive & regressive sedimentary successions. Mammal diversity increased during sea level lows.
Looked at K-T mammal faunal changes in the Little Missouri Badlands of North Dakota.
Hunter (1999) - NDAS
Worked with amateur fossil hunters that started a museum in Bowman, North Dakota.
In North Dakota, the yellowish-colored Ft. Union Formation has the K-T boundary in it - it’s the basal Cenozoic. The Hell Creek Formation is below. The Ft. Union Fm. has a cm-scale set of layers at the K-T boundary. Iridium, shocked quartz, spherules - all are found at many K-T sites in the area.
A fossil pollen change is the most ubiquitous marker at the K-T boundary- it occurs everywhere - a nice pollen change.
Worked at Mud Buttes, North Dakota.
A yellowish-colored tonstein (ash horizon; clay horizon) occurs sometimes at the K-T in this area.
The Bug Creek anthill fossil record was looked at a while ago - shows a gradual decline in dinosaurs and a gradual increase in mammal diversity. This pattern was found at 5 to 6 localities (Bob Sloan research). The fossils in this study are from channels in the Hell Creek Formation - the Hell Creek was eroded into. The fossils turn out to have been secondarily deposited - Cretaceous dinosaur fossils with Cenozoic mammal fossils. This created the illusion of gradual change at the K-T. This can occur in places.
The North Dakota sites looked at don’t have that illusion.
John Hunter collected Cretaceous fossils in North Dakota. These fossil occurrences shown that mammals persisted up to the K-T. Rarer teaxa did disappear before the K-T bounary, but this can be blamed on sampling error. Rarefaction shows the persistence of mammal taxa to the K-T. Vertebrate diversity did not decline leading up to the K-T, contra what other paleontologists say.
This was the 2002 situation.
The pollen-defined K-T boundary is now better refined.
Looked at the oldest known post-K-T mammal site in North America - 83 cm above the K-T.
Leaves and mammals (including aquatics) were found in studied sections. Microstratigraphic sampling was done. Screen-washed samples and got small mammal teeth.
Bercovici et al. (2009) - Cretaceous Research.
Above the pollen-defined K-T boundary, there’s lots of mudstone - freshwater ponding event. This was conducive to preservation of leaves and terrestrial organisms. Found aquatic plants in mudstones, plus crocodilians and fish.
Found allochthonous marine dinoflagellates in the mudstone - are much, much older than the host rocks - reworked.
Found a new species of multituberculate - Mesodma n. sp. - like a mouse.
Found larger multituberculates as well.
These mammals and early floras started ~9 to 10 k.yr. after K-T. They continued for ~11 to 13 k.yr.
Sedimentation rate estimates for the area - ~79-89 m/m.y.
Previous resolution - the closest-to-K-T mammals known were hundreds of thousands of years after K-T. Now, can see that, near the K-T boundary, multituberculates had a spike in abundance. They did well in the post-K-T.
Metatherians (marsupials) crashed at K-T. Eutherians started increasing in abundance after K-T. They diversified.
The faunal transition in North Dakota’s K-T sections was abrupt. There was a catastrophic extinction. At the centimeter-scale, the record is environment-sensitive.
Recovery & radiation . . .
Challenges to this view include molecular studies. These studies show that lineages were present long before the K-T and the post-K-T mammal radiations were ecologic.
Molecular-based mammal phylogeny - very different from morphology-based classifications.
Molecular-based phylogeny: Afrotheria (an endemic African clade), Xenarthra, Laurasiatheria, Euarchontoglires. Molecular studies show these clades separated deep in the Cretaceous.
Afrotheria Xenarthra Euarchontoglires Laurasiatheria
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Afrotheria lineage split - at 103 Ma. Xenarthra lineage split - at 94 Ma.
The Euarchontoglires-Laurasiatheria split - at 79 Ma.
There have been several fossil responses to the molecular estimates of divergence times.
Molecular studies need to depend on significant ghost lineages.
The Mesozoic mammal record is too complete to have significant, unrecognized ghost lineages. The fossil record doesn’t fit the molecular-based divergence timing estimates.
Placental mammals originated in Laurasia.
How to explain the discrepancy between molecular studies and fossil studies?
1) Incomplete fossil record - not likely
2) Molecular clock is wrong - why? Likely because there was punctuated molecular evolution at speciation events.
3) Garden of Eden Hypothesis - . . .
4) Long Fuse Hypothesis - mammals in the Mesozoic are crown-group placentals, but their morphologies are so primitive that they’re not recognized as such.
Resetting evolution’s clock:
- there was a rapid appearance of modern mammals at K-T
- there was a change in diversity dynamics - disturbed habitats, increased rates of extinction and speciation, ecological opportunities
- . . .
Crocodilians have very acidic stomachs - can see corroded bones/teeth if they have gone through a crocodilian’s gut.
Tracking diversity through time - such studies have to be corrected to take into account how much rock record is being compared.
The best mammal K-T survival record is in North America. Only minor records occur elsewhere.
Smaller body-sized mammal lineages were more likely to have survived the K-T event.
Herbivorous lineages didn’t do well at K-T. Omnivorous forms were more likely to survive K-T and the aftermath.
Audience comment: is the K-T disaster related to increased radiation and increased mutations? If so, this would have consequences for molecular clock studies.
Answer: not sure.