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:
1) Monotremes
2) Marsupials - diversified in Australia
and South America
3) Placentals
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.
Extinction/speciation rates
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
\ \ \ /
\ \ \ /
\ \______________________/
\_______________/
/
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.