Animal evolution: Life in the mesozoic Flashcards
~252 MYA Permian mass extinction occurred
Difference between background and mass extinction
Background extinction
*Contrasted with “mass extinction” and represents a baseline rate of species loss seen in the fossil record and/or estimated from molecular phylogenies (see next weeks lecture)
*The vast majority of extinction that has occurred on Earth is due to background extinction
Mass extinction
*Periods identified in the fossil record as experiencing a much elevated rate of extinction (loss of species/unit time) relative to the background
*There have been 5 major mass extinctions in the history of earth: ordivician, devonian, permian, triassic, cretaceous
Mass extinction
*End-Ordovician
*Devonian
*Permian
*Triassic
*Cretaceous/Paleogene
see info in notes
Permian mass extinction
Permian mass extinction
* Most dramatic mass extinction event in Earth’s history in all
environments — perhaps 96% of species were lost - affected land and marine
*Northern & southern continents collided forming Pangaea = changing ocean currents
*Conditions for life deteriorated towards the end of the Permian.
- Massive, widespread volcanic activity. Ash blocked sunlight and cooled the climate, resulting in the largest glaciers recorded. - we know this as in Siberia there are lava flows the size of the USA dating to this time period
- Atmospheric O2 declined from 30% to 12% so that all but the lowest land would be uninhabitable for many terrestrial animals – not enough O2 to survive at higher altitudes
End of Permian followed by the Mesozoic era
Mezozoic = Cretaceous, Jurassic and Triassic combined
Post-Permian ‘winners’
Some animals survived the mass extinction better – amniote reptiles specifically
Reptilia
Reptilia:
*amniotic
*air breathing vertebrates with four limbs, adapted primarily for terrestrial life.
phylogenetically include birds, but often separated because modern reptiles are ectothermic, birds are endothermic.
However when we look closely not only are birds dinosaurs but they are descendents of therapod dinosaurs – they are dinosaurs
What is a dinosaur?
*A clade nested within archosaurs
*Characterised by an upright stance made possible by hip modifications, long tails, and big leg muscles
*Conventionally, ‘crown’ dinosaurs arbitrarily defined as any descendants of the common ancestor of Iguanodon & Megalosaurus
Stem vs crown groups
Stem groups = more closely related to one crown group than another but are not nested within that group
Major dinosaur clades
^ ornithischians = sturdy herbivores e.g. triceratops
^sauropods = gigantic species long necked e.g. diplodocus
^theropods = walk on two legs e.g. t-rex
Brief dinosaur timeline
*Triassic: first dinosaurs appear (245 Ma), begin to radiate; end Triassic mass extinction (associated with breaking-up of Pangea) these are the earliest records at current time – new discoveries could prove that they evolved even earlier
*Jurassic: dinosaurs explosively radiate, body size increases, first birds evolve (non-dino reptiles that radiate: pterosaurs, marine reptiles [e.g., ichthyosaurs])
*Cretaceous: Dinosaurs dominate terrestrial ecosystems and reach maximum diversity—appearance of Tyrannosaurus rex; up until the end Cretaceous extinction (meteor impact).
slow initial diversification, increasing in Jurassic, peaking in cretaceous followed by extinction of all dinosaurs except birds
hypotheses about dinosaurs
Evolutionary biologists find patterns in hip bones, tooth formation etc. And need to test these methods – but how can you research species with no extant relatives and DNA too ancient for analysis?
Modern approaches to testing hypotheses about dinosaurs:
1.What other Pan-Aves were around when dinosaurs began to diversify?
Muller et al 2013 (Nature) discovered Venetoraptor gassenae and reconstructed the phylogeny below combining their fossil with other existing records
the group Muller’s dinosaur belonged to was once thought insignificant but now it has been found in multiple countries and now is gaining importance
Principle coordinate analysis (PCo) used to compare dinosaurs
closer points are more morphologically similar
A lot of diversity suggesting dinosaurs exploited many ecological niches
Modern approaches to testing hypotheses about dinosaurs:
2.Were ceratopsian (e.g. Triceratops) skull frills used for species recognition?
Knapp et al. 2008:
Question:
Have the elaborate structures on ceratopsian skulls (e.g., Triceratops) evolved to prevent wasteful interactions with other species (“species recognition”)?
Hypothesis development
*(Reproductive) character displacement
*Prediction: display characters should be more divergent in coexisting (sympatric) species.
Characteristics should be more expressed where species coexist in sympatry. (Knapp et al 2008)
Methods
*Measured dimensions of display characters, other visible characters (e.g. leg bones), and internal characters (Skeletal traits not visible to other species.)
*Devised several indices of coexistence
*Compared evolutionary divergence in traits between sympatric taxa to divergence in traits between non-coexisting taxa (Knapp et al 2008)
Results
* Positive allometry observed in the comparison of ~60 skulls – as body size increases frill size increases in a non-linear fashion and we see this in extant sexually selective traits as in the size of deer antlers or peacock tails
*No evidence for character displacement in display traits
*Display traits did, however, diverge more rapidly overall
Conclusion
—suggesting an intraspecific role in social or sexual selection
Modern approaches to testing hypotheses about dinosaurs:
3.Were some theropod dinosaurs nocturnal predators?
Birds are feathered dinosaurs
The class Aves (birds) represents a prodigous radiation
*After K-T event, birds diversified dramatically and today (ca. 10,000 spp.), occupy a wide range of ecological niches; most are capable of flight and this has been important in facilitating that radiation and their widespread distribution.
We know now that birds are descendants of Therapods so, were any (extinct) Therapods nocturnal?
Measured scleral rings in skulls—larger rings reflect larger pupil sizes – observed in nocturnal birds and other extinct Theropods
They also measured endosseus cochleaar ducts in skulls—longer ducts reflect important role of auditory cues (e.g., while hunting) contemporary birds that are auditoral hunters have this feature and hunt in the dark (e.g. owls)
combining these measurements they compared Theropods to living birds
Some fossil species fall in place with extant nocturnal bird species with large sclera and long cochlear duct then on the right they created a plot to show it is likely that some theropods were nocturnal
Birds are dinosaurs - a short history
*1859 – publication of Darwin’s Origin of Species
*1861 – Discovery of Archaeopteryx lithographicus, a fossil bird with reptilian features, at the Solnhofen formation (a Lagerstätte) in Germany (Jurassic, 150 MYA). Exquisite detail – feathered wings and body evident. Undoubtedly, a flying bird. However also sharing reptilian traits with Theropods such as a long bony tail and sharp-toothed jaw.
*1868: Thomas Huxley proposes that birds are descended from dinosaurs.
*1926: Gerhard Heilmann casts doubt on this claim—argues birds descended from a different reptile group
*1960s-1980s: Revival of birds-as-dinosaurs hypothesis, steady trickle of new fossils and cladistics arguments
(Birds are Theropods: Many synapomorphies e.g. Feathers, wishbones and three-fingered hands)
Wishbones are thought to have initially evolved to handle physical impact when hunting – a shock absorber, in birds it has become an essential bone connecting flight muscles (exaptation evolution)
*1990s: Multiple regions with a wealth transitional fossils (re)discovered in Liaoning China
“These dinosaurs had the great misfortune to live in a dense forest surrounding a wonderland of ancient lakes, a landscape that was periodically obliterated by volcanoes. Some of these eruptions spewed out tsunamis of ash, which combined with water to flood the landscape in a viscous ooze that buried everything in sight. The dinosaurs were captured going about their everyday business, preserved Pompeii-style. That’s why the details of the feathers are so pristine.”-Brusatte 2018
*1996: Bird stem fossil discovered in Liaoning; clear evidence of simple, filament-like feathers on a Theropod dinosaur.
*2003: Microraptor fossil discovered ‘Four-winged biplane’ arboreal dromeosaur fossils (a stem bird lineage), Microraptor gui, with feathered legs and asymmetric pennaceous (quilled) feathers, discovered in China. Probably glided between trees.
*2010: Recently, microscopic investigation of the shape and density of melanosomes has made it possible to reconstruct plumage colour from fossils. (Shape and density of melanosomes dictate the colours in extant species which these fossil forms can be compared with)
Modern approaches to testing hypotheses about dinosaurs:
4.Did pennaceous feathers evolve for flight?
Feathered dinosaurs: Hypotheses for feather evolution: research method proposals
1.Proto-flight (gliding, escape from predators) - look for association between large flight muscles and feather anatomy, anatomical comparison with gliding birds or flightless birds
2.Insulation - correlate with the local temperature at that time period, see how extant bird feathers correlate with their local temperature conditions
3.Social signalling (within or between species) - similar to triceratops analysis, pigment analysis – bright colours are often associated with signalling, compare extant feather patterns with fossils
*2014: Foth et al. Identify the 11th Archaeopteryx specimen based reconstruction of a phylogeny - Based on these results, the evolution of pennaceous feathers is generally decoupled from the origin of flight and might be related to other biological roles.” These feathers evolved long before any flying birds.
Pennaceous feathers: an exaptation:
*Pigment reconstruction shows plumage diversity suggesting a role in social signalling
*By simply constructing a phylogeny and mapping trait data, the authors were able to build a case for rejecting a hypothesis for feather evolution. Hence pernaceous feathers were thought to be exaptation
Summary
1.The End-Permian mass extinction was the largest in Earth’s history—nearly 96% of all species were lost.
2.Amniotes are tetrapods with a variety of adaptations that made life on land possible.
3.Dinosaurs arose and radiated during the Mesozoic, dominating terrestrial ecosystems during the Jurassic and Cretaceous.
4.Recent analyses on new fossils show that other lineages within Pan-Aves were taxonomically and ecologically diverse around the time that dinosaurs first evolved.
5.Recent analyses suggest that Ceratopsian skull ornaments evolved as social signals within species, not for species recognition.
6.Birds are dinosaurs. They descended from theropod ancestors.
7.Fossil beds from NE China (Liaoning) have yielded extraordinarily well-preserved specimens showcasing the diversity of the avian stem lineage.
8.Feathers, even well-developed colourful pennaceous feathers, were present even in non-volant paravian dinosaurs—suggests a role for social interactions.
