Marine reptiles and birds evolution & diversity

Definition of Amniotes

A clade of tetrapods (descendants of four-limbed and backboned animals) that have an egg equipped with an amnion, allowing them to lay eggs on land.

Two main lineages of Amniotes

Synapsids (mammal-line) and Reptiles (sauropsids).

Key characteristics of Reptiles

Skin covered in scales; amniotic eggs (soft or calcareous shells); direct development (no larval stage).

Anapsid Skull Condition

No temporal fenestrae (holes) behind the orbit. (Primitive condition, e.g., early reptiles, turtles*).

Synapsid Skull Condition

One lower temporal fenestra. (Mammals and mammal-like reptiles).

Diapsid Skull Condition

Two temporal fenestrae (Upper and Lower). (Lizards, snakes, crocs, birds).

Euryapsid Skull Condition

One upper temporal fenestra. (Modified diapsid condition found in Ichthyosaurs and Plesiosaurs).

The "Turtle Problem" in phylogeny

Turtles have Anapsid skulls but genetic data suggests they are modified Diapsids, sister to Archosaurs or Lepidosaurs.

Odontochelys (Fossil Importance)

Triassic stem-turtle. Had a plastron (belly shell) but NO carapace (back shell), and had teeth. Proves the shell evolved from the belly up.

Archelon

Extinct giant Cretaceous sea turtle (4m wide) with a leathery shell and reduced bone structure for weight saving.

Salt Gland Location: Sea Birds & Marine Iguanas

Nasal (sneeze out salt).

Salt Gland Location: Sea Turtles

Lachrymal (tear ducts).

Salt Gland Location: Sea Snakes

Sublingual (under the tongue).

Salt Gland Location: Saltwater Crocodiles

Lingual (on the tongue).

Laticaudinae (Sea Kraits)

Amphibious sea snakes; lay eggs on land (oviparous); have ventral scales for crawling.

Hydrophiinae (True Sea Snakes)

Fully aquatic sea snakes; give birth at sea (viviparous); lack functional ventral scales.

Marine Iguana Adaptations

Laterally flattened tail for swimming; blunt snout for grazing algae; strong claws for holding rocks in swell.

Ichthyosaur Body Shape

Thunniform (tuna-shaped); convergent evolution with dolphins.

Ichthyosaur Tail Structure

Reversed Heterocercal tail (vertebral column bends into the LOWER lobe).

Hyperdactyly

An increase in the number of digits (fingers/toes) in a limb; seen in Ichthyosaurs.

Hyperphalangy

An increase in the number of phalanges (bones) within a digit; seen in Ichthyosaurs and Sea Turtles.

Sclerotic Rings

Bony rings inside the eye of Ichthyosaurs; supported the eye shape against water pressure and allowed for massive eye size.

Viviparity in Ichthyosaurs

Confirmed by fossils (e.g., Stenopterygius) showing embryos being born tail-first (to prevent drowning).

Placodont Feeding Strategy

Durophagy: crushing hard-shelled prey (molluscs/crustaceans) using flat, broad teeth.

Plesiosaur Locomotion

Underwater flight using all four flippers (unique among marine vertebrates).

Plesiosauromorph vs. Pliosauromorph

Plesiosauromorph: Long neck, small head. Pliosauromorph: Short neck, massive head (apex predator).

Mosasaur Phylogeny

Giant aquatic Anguimorph lizards (Squamates); closely related to modern Monitor Lizards and Snakes.

Mosasaur Feeding Mechanism

Ratchet feeding: Flexible lower jaw and pterygoid teeth (on roof of mouth) walk prey down the throat.

Globidens

A Mosasaur species with rounded, crushing teeth specialized for Durophagy.

Carsosaurus

An extinct lizard fossil with 4 embryos in the body cavity, proving viviparity evolved in semi-aquatic lineages leading to Mosasaurs.

Secondary Invasion

The evolutionary process where terrestrial animals (like reptiles) re-adapt to life in the marine environment.

Number of reptile species that have re-entered the marine realm

Around 100 (out of >12,000 extant species).

Metabolic advantage of aquatic air-breathing reptiles

Extracting oxygen from air is much easier than extracting it from water.

Locomotion advantage in water vs. land

Moving in water is much more energy efficient than on land.

Decompression adaptation in marine reptiles

Problems of decompression (the bends) are offset by the physiological advantages of air breathing.

Phylogeny: Earliest terrestrial turtle fossils

Found in Germany (Upper Triassic); already had a fully developed shell.

Phylogeny: First marine turtles

Appeared in the Jurassic (The Pleurostrenidae).

The 4 Marine Turtle Families established by the Cretaceous

1. Toxochelyidae (Extinct), 2. Protostegidae (Extinct), 3. Cheloniidae (Extant), 4. Dermochelyidae (Extant).

Scientific name of the Leatherback Turtle

Dermochelys coriacea.

Relationship between Temperature and Incubation Period in turtles

Non-linear; Incubation period goes down as temperature goes up.

Thermal tolerance range for turtle embryos

25–27°C (lower limit) to 33–35°C (upper limit).

Effect of tidal inundation on turtle nests

Reduces Temperature and Oxygen (O₂) levels in the sand column.

Morreale et al. (1982) significance

Highlighted that current conservation practices regarding Temperature-Dependent Sex determination might threaten sea turtles.

ESD Mechanism: Bonellia verdis (Marine Worm)

Chance/Location: Larvae on females become male; larvae on sea floor become female.

ESD Mechanism: M. incognita (Plant Parasite)

Nutrient dependent: Sparse nutrients = Male; Plentiful nutrients = Female.

Temperature-Dependent Sex Determination (TSD) Pattern: Turtles

High Temp = Female; Low Temp = Male. (Pivotal range 28–30°C).

TSD Pattern: Alligators and Lizards

High Temp = Male; Low Temp = Female (Reversed compared to turtles).

TSD Pattern: Crocodiles

Intermediate Temp = Male; High or Low Extremes = Female.

Why turtles are ideal for electronic tagging

They are large/powerful, surface regularly (for satellite transmission), and have a bulky carapace for attachment.

Information provided by Telemetry/Satellite Tracking

Migratory pathways, depth utilisation, energetics, foraging strategies, and interaction with fisheries.

Case Study: Galapagos Green Turtles and Ship Strikes

Damage incidence is greater inshore than offshore; acoustic/radio receivers used to monitor behavior in high-risk areas.

Key Conservation Group: ARCHELON

Mediterranean Association to Save the Sea Turtle (http://www.archelon.gr).

Key Conservation Group: Archie Carr Centre

Archie Carr Centre for Sea Turtle Research (University of Florida).

Scientific name of the Marine Iguana

Amblyrhynchus cristatus

Phylogenetic lineage of Amblyrhynchus

A monospecific lineage endemic to the Galapagos, forming a clade with Conolophus (land iguanas).

Time of divergence from land-based cousins

Around 4 million years ago.

Geological formation of Galapagos

Tectonic plates moving over a mantle hotspot, creating a chain of islands with decreasing age from east to west.

Ecological intersection in Galapagos (Examples)

Corals/reef fish with Penguins; Fur seals with Hammerhead sharks.

MacLeod et al. (2015) findings

Evidence for incipient within-island speciation and between-island hybridization; island-specific genetic diversity appeared ~50,000 years ago.

Miralles et al. (2017) taxonomy revision

Identified 11 population clusters defined as sub-species of marine iguana.

Comparison: Anolis agassizi (Malpelo Island)

Lizard that feeds intertidally, partly on crustacea.

Comparison: Diploglossus hancocki (Malpelo Island)

Skink that feeds wholly on crustacea.

Comparison: Crypoblepharus cognatus (Nosi Bé)

Skink that feeds on insects, crustacea, and fish.

Adaptation: Long claws

Hanging onto rocks.

Adaptation: Laterally compressed tail

Swimming and diving.

Adaptation: Salt gland location

Nostril.

Adaptation: Diet composition

Red and green algae.

Adaptation: Teeth shape

Tricuspid teeth.

Adaptation: Dark skin function

UV protection and warming.

Foraging Strategy: IF

Intertidal feeding.

Foraging Strategy: SF

Subtidal feeding.

Thermoregulation challenges

Facing both cold water and hot arid land with cool nights.

Impact of El Niño (ENSO) on algae

Loss of green & red algae; replacement by indigestible Brown species.

Physiological consequences of El Niño on iguanas

Starvation, skeleton shrinkage, death, and breeding failure.

Carpenter (1966) behavior study

Studied head nod patterns in marine iguanas.

Phylogenetic position of Birds

Birds (Aves) are a lineage of Theropod dinosaurs within the clade Saurischia.

Saurischia vs. Ornithischia

Saurischia = "Lizard-hipped" (pubis points forward); Ornithischia = "Bird-hipped" (pubis points backward). Note: Birds evolved from Saurischians.

Eoraptor key features

Early dinosaur with hollow bones and five digits in the hand (though IV and V were reduced).

Coelurosaurs key evolutionary step

The appearance of proto-feathers (hollow cylindrical feathers) and enlarged brains.

Paraves (Deinonychosaurs) key evolutionary steps

Evolution of the semilunate carpal (wrist), feathers closed with barbules/hooks, and nest-brooding.

Archaeopteryx lithographica

The Late Jurassic fossil considered the "first bird" (or transitional form), exhibiting a mosaic of reptilian and avian characters.

Reptilian features of Archaeopteryx

Teeth, long bony tail, gastralia (belly ribs), and unfused fingers with claws.

Avian features of Archaeopteryx

Asymmetrical flight feathers, furcula (wishbone), and wings.

Semilunate Carpal

A wrist bone modification in Maniraptorans (like Velociraptor and Birds) that allows the hand to fold sideways (precursor to the wing stroke).

Evolution of Feathers sequence

1. Hollow filaments (Sinosauropteryx) → 2. Tufted feathers → 3. Interlocking with barbules → 4. Asymmetrical flight feathers.

Primary original function of feathers

Insulation (thermoregulation) and display; flight was a secondary exaptation.

Avian Respiration System

Unidirectional airflow using anterior and posterior air sacs; gas exchange occurs in parabronchi.

Skeletal evidence for avian respiration in dinosaurs

Post-cranial pneumaticity (hollow vertebrae invaded by air sacs) found in Theropods and Sauropods.

Pygostyle

The fused caudal (tail) vertebrae found in modern birds (and some derived Cretaceous birds), replacing the long bony tail.

Carpometacarpus

The fusion of wrist and hand bones found in modern birds.

Paleognaths

The "Old Jaw" birds; includes Ratites (Ostrich, Emu, Kiwi) and Tinamous.

Neognaths

The "New Jaw" birds; includes Galloanserae (chickens/ducks) and Neoaves (all other modern birds).

Galloanserae

The clade containing Galliformes (landfowl) and Anseriformes (waterfowl).

Convergent Evolution in Dinosaurs and Birds

The backward rotation of the pubis occurred in Ornithischian dinosaurs and independently in the Theropod ancestors of birds.

K-Pg Extinction Event

The mass extinction ~66 Ma that wiped out non-avian dinosaurs; only the Neornithes (modern birds) survived.

Huxley’s 5 things about birds

• Fluffy body covering

• Birds can fly (bipinate feathers)

• Birds brood their nests

• Birds take care of their young

• Birds have hollow bones (air-sac system)