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TRANSITIONS & RADIATIONS - Coggle Diagram
TRANSITIONS & RADIATIONS
origin of mammals from reptiles
many characters differ between extant mammals and living Sauropsida (living reptiles and birdS)
best represented transition in the fossil record
usually interpreset in Neo-darwinian terms
earliest fossils in the transition occur in the late carboniferous
mammal like reptiles (synapsids)
by latest triassic (100 my later) skeletons of small animal like megazostrodon show that all the skeletal characters of crown-group mammals
decrease in body size relative to the range of sizes of forms at the preceding node
dentition and jaw musculature more like that of a carnivore than a herbivore
more mammalian patterns of locomotion and feeding
characters relating to the ribcage, olfactory and auditory systems, and brain size appear in some sequence
overall pattern of correlated progression? no indication of a key innovation or macromutational jump
in between carboniferous and triassic a complex series of originations and extinctions of synapsids, but an erratic trend through time towards an increasing number of mammalian characters
sidor & hopson
compared number of character state changes between nodes with the estimated amount of time between nodes: good correlation
implies approximately constant rate of evolution in this lineage
other trends along the axis
carrier's constraint
most amphibians and lizards have a fish like gait
anterior and posterior limb pairs move out of phase
compresses the lungs on alternate sides of the animal - impossible to breath while moving
cevolution of diaphragm and erect stance allow animal to breathe while running
maintain continuous activity
three chambered heart in reptiles
no point in a powerful mechanism to pump blood through the lungs as these are not ventilating when the animal is most active
mammal characters are all intimately related
fundamentally all mammals are reptiles that have evolved a very high ability to regulate their body temperature, internal chemistry and position in space in very heterogeneous terrestrial envrionments
thermoregulation
facilitated by high metabolic rate
variable insulation
skin blood flow
high metabolic rate
high rate of food collection made possible by
advanced lovomotion
sensory systems
brain coordination
high rate of ingestion
advanced dentition
advanced jaw musculature
higher rate of gas exchange
larger lungs
diaphragm
modified locomotory gait (breath while running)
double circulation that allows rapid delivery of oxygen and metabolites to tissues
higher blood pressure
higher ultrafiltration rate at kidneys
enables the rapid and precise regulation of body fluids
evolution of modern mammals
larger herbivores/omnivores -> smaller carnivores
correlated progression towards more mammalian characters of the skeleton
and by implication of soft part anatomy
trend towards improved thermorefulation and homeostasis
trend towards high investment in the young
prolonged nutrition
protections of vulnerable juveniles
lactation and use of a burrow
in stem group mammals
modern monotremes
vivipary
in crown group taxa
the successful lineage is just one of several other lines of mammal like reptiles that did not persist to the present
herbivores and large carnivores with their own high levels of diversity and abundance
acquired some but not all mammal characters, flourished and became extinct
warning: it is only in retrospect that we pick out the lineage leading to crown group mammals. why these and why not others?
neodarwiniain (natural selection vs. species selection)
natural selection
the more homeostatic the organism, the better adapted to its environment
problem 1
overall rate to reach crown group very slow (100My)
problem 2
why was it only small carnivores that survived
why didnt the other mammal like groups go all the way to being mammals?
species selection
relatively small, relatively more homeostatic carnivores have a greater probability of speciating
today such organisms form populations with low home ranges and vagility
lower population densities than equivalent ectothermic species with lower food requirements
herbivores with more abundant food resources
implies that small carnivores were no better adapted to their environment than large carnivores or herbivores
explains slow rate of change (100my)
perhaps one speciation per million years
origin of birds
birds emerged from within the therapod dinosaurs
all dinosaurs are stem group birds because they all have at least some modern avian characters
eg. digitigrade hind legs
archaeopteryx
discovered 1861
Thomas Henry huxley noticed the similarity to the coelurosaur
compsognathus
new fossils from china reveal many groups of coelurosaurs with filamentous structures on the skin
these may be related to feathers of their precursors
other workers claim these are the remains of collagen fires in the skin
there were many groups of feathered coelurosaurs after archaeopteryx
could archaopteryx fly?
no supracoracoideus skin (this both raises and twists the wing during the flight of modern birds)
wishbone U-shaped with little or no springing action
in modern birds repositions the shoulder joints after the upstroke
also helps to pump air through the lungs
weak breastbone
unable to raise forelimbs very high
modern birds can bring them together at the top prior to each downstroke
no long primary feathers at the ends of the wings
plainly designed to fly even if rather poorly but had almost no bird-like features
no keeled sternum
little or no fusion of wrist or ankle bones
not much in the way of special skull adaptations
alvarezsaurids
have all these and more but whatever they were adapted for it was plainly not flight
scapula-coracoid angle appears to have been only slightly obtuse in
mononykus
(the most derived member of the clade
clavicles are not known and the furcula may have disappeared. alvarezsaurids remained notably scrawny over their considerable size range
arms not used for flight but for something else
long term success in both Mongolia and Argentina on large continents
broad geographic dispersion
no advanced rodents at that time (the bane of many more recent flightless bird species)
retained a keeled sternum and a robust pectoral girdle
troodotids had feathers but were unable to fly
how did powered flight evolve in birds?
early versions
fast running predatory reptile might evolve scales on the arms
generate lift as the animal ran
but lift reduces traction with ground and acceleration is lost
recent theories
porto-bird ran fast then jumped after small prey in the air
once in the air feathers on the arms or tail may have been used to steer the animal
arm movements may have been added later
but
energy intensive
requires high air speed (25mph) before rudders offer any effective control
display and fighting hypothesis
long colourful display feathers on the arms and tail
flapping display may have encouraged powerful pectorals and the supracoracoideus system
display improved by lengthening arms and tail
display cannot be just a bluff
strong forwards and downwards strokes of the arms are used by modern birds during aggressive displays
same movements as in in takeoff
may have lifted proto-birds up above opponents, where back legs could be brought into play
quicker wing beats -> more effective fighting
claws in
archaeopteryx
were well developed and quite formidable - strong sharp teeth
first flights may have been at low speeds, brief and close to the ground
first flights may have been at low speeds, brief and close to the ground
arboreal hypothesis
favoured until relatively recently. new fossil finds cast doubt
aerodyamically difficult to evolve from gliding flight to flapping
nothing in the ancestry of birds that suggests arboreal adaptations
cursorial hypothesis - ground up
early versions
fast running predatory reptile might evolve scales on the arms
generate lift as the animal ran
lift reduces traction with ground and acceleration is lost
recent theories
porto-bird ran fast then jumped after small prey in the air
feathers on the arms or tail may have been used to steer the animal
BUT energy intensive and requires high air speed (25mph) before rudders offer any effective control
wing assisted incline running hypothesis
prehistoric birds run up vertical surfaces by flapping their wings to maintain thrust to keep feet moving on vertical and 90º+ surfaces
to fly or not to fly
huge benefits/advantages
huge costs in terms of energetic and anatomical resources
giving up flight confers huge energy bonus. many lineages of birds have become flightless
most later (cenozoic) flightless birds share several features
loose keeled sternum
obtuse angle between scapula and coracoid, with loss of the furcula
larger and fatter
most modern flightless birds evolve in isolated locations (often islands). tend to fill ecological vacuum very quickly but do not fare well when predators or competitors are introduced
heterochrony in bird evolution
archaeopteryx shows greater similarity with juvenile of the coelurosaur
ceolophysis
than with the adult
skill shape and tooth morphology
smaller size (paedomorphosis)
longer forearms (peramorphosis)
like mammals - stem group birds were all small and carnivorous
correlated progression in bird evolution
no evidence for a directed trend from dinosaurs to birds
some characters evolved in some groups, other characters in others
one line went all the way to modern birds, others became extinct
like mammals the transition had an erratic aspect
correlated changes between different parts of the locomotory system (fore and hind limbs)
other systems
feeding
ventilation
tempperature
physiology
uncertain if correlated but looks likely
flight probably evolved several times in the groups of dinosaurs immediately surrounding the ornuthurae