Early division of amniotes produced 2 major evolutionary lineages = Synapsida and
Sauropsida
Lineages distinguished in fossil record by mid-Carboniferous period
Adaptations necessary for life on land
Terrestrial environment provided new opportunities for new lifestyles
Amniotic egg may be a critical element of success of both groups –amniotic eggs are
larger than non-amniotic eggs and produce larger hatchlings that grow into larger
adults
Both lineages underwent great radiations in late Paleozoic and Mesozoic eras that
include animals now extinct with no modern equivalents
- Dinosaurs + Pterosaurs = Sauropsids
- Pelycosaurs + Therapsids = Synapsids
Sauropsid and synapsid lineages separated – amniotes evolved few derived
characters associated with terrestrial life
- Both lineages independently developed most of the derived characters for
terrestrial life (eg. respiratory and excretory systems to conserve water,
locomotor systems compatible with high rates of lung ventilation)
- Both have fast moving predators and fleet-footed prey, both have species
capable of powered flight
- Endothermal, evolving high metabolic rates and insulation to retain metabolic
heat
- Extensive parental care and complex social behaviour
Parallel evolutionary trends – differences in the way they carry out basic functions
show that the derived characters evolved independently
Classification and Phylogeny
Class: Sauropsida
Subclass – Anapsid Subclass – Diapsida
Testudinia (turtles and Lepidosauropmorpha
tortoises) ~300 species
Sphenodontia (tuataras) Squamata (lizards, snakes,
~2 species amphisbaenids, uropeltids)
~7900 species
Archosauromorpha
Crocodilia (crocodiles, Aves
gharials, caimans and
alligators) ~23 speies
, Problem with turtles is their unique
morphology, few characters to link
them with any other group of amniotes
– different traits also provide
conflicting signals
Traditional hypotheses place turtles as
sistergroup to all living amniotes
(Gaffney 1980)
More recent morphological and fossil
data agree that synapsids are
sistergroup to remaining amniotes, and
anapsids placed as sister group to
diapsids (Reisz, 1997)
- Recent paleontological studies
suggest the anapsid group is
weakly supported
Molecular phylogenies favour a more
derived position for turtles within
reptiles (Zardoya and Meyer, 2001)
- Phylogenetic analyses based on 11
nuclear proteins, nuclear 18S and
28S rRNA genes suggest that
crocodiles are closest living relatives of turtles (Hedges and Polling, 1999) (Cao
et al., 2000), nuclear protein coding genes (Shen et al., 2011)
- Recent paleotonlogical and molecular data agree on the derived position of
turtles as diapsids – new placement of turtles has profound implications for
reconsruction of amniote evolution
Reproduction
Amniotic egg was a key innovation in the
success of sauropsids and synapsids
Lepidosaurs have transverse cloacal slit
rather than longitudinal slit (all other
tetrapods)
Range of reproductive modes observed
Oviparity: development occurs inside female’s body and supported entirely by yolk – ie.
lecithotrophy; assumed to be ancestral condition
Viviparity: eggs retained in oviducts and development is supported by transfer of
nutrients from mother to fetuses – matrotrophy
, - Evolved more than 100 times among squamates
- Squamates have specialised chorioallantoic placentae (eg. Mabuya heathi –
Brazilian skink)
- Can allow mother to use own thermoregulatory behaviour to control
temperature of the embryos during development (can reduce time for
development in cold climates)
- Can also lower reproductive output and may reduce agility of female (higher risk
of predation)
Intermediate conditons: retention of eggs for a time after they have been fertilised
and production of precocial young nourished primarily by material in yolk
Parthenogenesis
All-female species of squamates identified in six families of lizards and one snake
Many species appear to have their origin as interspecific hybrids
- Hybrids are diploid (2n), one set of chromosomes from each parental species
- Eg. whiptail lizard Aspidoscelis tesselatus product of hybridisation of A.tigris
and A.septemvittatus (Cole, 1988)
- Some are triploid (3n)
Temperature-Dependent Sex Determination
TSD is more limited in lepidosaurs than in turtles or crocodilians
Vivparous lizards can determine sex of offspring based on their own
thermoregulatory behaviour
Thermoregulation – Ectotherms
Behavioural mechanisms
involved in ectothermal
temperature regulation –
maintain a steady temperature
Alter amount of solar radiation
by changing
- Orientation to the sun:
animal oriented
perpendicular to sun’s rays
intercepts the maximum amount of solar radiation and orientation parallel
intercepts mimimum
- Body contour: many lizards can spread or fold their ribs to change the shape of
the trunk to increase surface area exposed to the sun
Horned lizards (Phrynosoma) – by changing o rientation and contour, radiant
heat gain increases by sixfold (McNab, 2002)
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