Parade of the Tetrapods

  • Presented by Rosita Roldan-Gan, M.D.

    • Faculty, De La Salle University - Manila

    • Fellow, Philippine Society of Physiologists

    • Associate Member, Philippine College of Emergency Medicine

Tetrapods

  • Definition:

    • Vertebrates characterized by a chiridium: a muscular limb with well-defined joints and digits (fingers and toes).

  • Etymology:

    • Literally means "four-footed."

    • Includes groups with secondary limb loss (e.g., snakes).

  • Geological Origin:

    • Emerged during the Late Paleozoic, after the formation of Pangaea.

    • Early tetrapods lived mostly in shallow freshwater, with limited excursions onto land.

Tetrapod Phylogenetics

  • Phylogenetic Definition:

    • Tetrapoda = clade of species more closely related to each other than to rhipidistians.

Adaptive Radiation and Diversity

  • Radiation:

    • Extensive radiation followed the initial transition from water, leading to diverse forms including:

    • Fully terrestrial vertebrates

    • Amphibious groups

    • Aquatic and flying descendants

  • Importance of maintaining the term Tetrapoda:

    • Alternative terminologies have failed as they excluded hand- and foot-bearing groups.

Tetrapod Taxonomy and Phylogenetic Challenges

  • Early Classification Attempts:

    • Initially based on vertebral anatomy alone (e.g., temnospondyls, lepospondyls).

    • Abandoned due to susceptibility to functional convergence (analogy) rather than homology.

  • Labyrinthodonts:

    • Named for complex tooth structure.

    • Now recognized as a paraphyletic stem group.

    • Term retained for early tetrapods documenting the transition from sarcopterygians to terrestrial forms.

Evidence for Early Tetrapod Evolution

  • Fossil Record:

    • Exclusively known from fossils; molecular data unavailable.

    • Two major gaps:

    • Approximately 100 million years between living amphibians and the oldest fossils.

    • Romer's gap: the first 30 million years of the Early Carboniferous.

  • Trace Fossils:

    • Hundreds of footprints and trackways throughout the Paleozoic.

    • Indicate land walking by early tetrapods, though cannot be assigned to specific species.

Primitive Tetrapods: Labyrinthodonts

  • General Characteristics:

    • Retained bony scales, mainly abdominal.

    • Often large-bodied with proportionately large skulls.

    • Example: Eogyrinus, a Carboniferous species up to 5 meters long.

  • Ontogeny:

    • Juveniles possessed lateral line systems; adults lacked them.

    • Suggests aquatic larvae and more terrestrial adults, similar to modern amphibians.

Early Devonian Labyrinthodonts

  • Acanthostega:

    • Late Devonian; described as a “four-footed fish.”

    • Retained Fish-like Traits:

    • Unconstricted notochordal vertebra.

    • Tail fin with radial fin rays.

    • Lateral line system.

    • Labyrinthodont teeth.

    • Intracranial joint.

    • Tetrapod Traits:

    • Dermal skull bone pattern.

    • Limbs with digits.

    • Weight-bearing girdles.

  • Respiration and Hearing:

    • Stapes derived from the hyomandibula; functioned as a skull brace rather than for airborne hearing.

    • Possibly involved in airflow via spiracle.

    • Ecology:

    • Possessed internal gills; likely fully aquatic.

    • Evolutionary Significance:

    • Digits evolved before terrestriality.

    • Exhibited polydactyly (eight fingers and toes).

  • Ichthyostega:

    • Large early tetrapod.

    • Features:

    • Unconstricted notochord extending into braincase.

    • Vertebral column adapted for dorsoventral flexion (function unknown).

    • Tail fin with radial rays; lateral line system; labyrinthodont teeth; internal gills.

    • Digit number: Seven toes.

Transition from Water to Land

  • Skeletal Adaptations:

    • More ossified limbs and girdles.

    • Vertebral column became more prominent.

  • Neck Development:

    • Loss of skull–shoulder girdle connection.

    • Development of a mobile neck allowing independent head movement.

    • Also seen in some rhipidistians (e.g., Tiktaalik).

  • Loss of Opercular Bones:

    • Corresponded with the loss of internal gills.

  • Respiration and Reproduction:

    • Inherited lungs and aquatic reproduction.

    • External fertilization; many small eggs laid in water.

    • Fossil larvae show external gills, similar to modern salamanders.

Key Points on Tetrapods

  • Early tetrapods illustrate a gradual, mosaic transition from fish to land vertebrates.

  • Digits, limbs, and neck mobility evolved before full terrestrial life.

Modern Amphibians

  • Lissamphibia:

    • A clade that includes all living amphibians and some fossil forms.

  • Phylogenetic Origin:

    • Arose within the labyrinthodont radiation, specifically from temnospondyls.

  • Loss of Ancestral Features:

    • Many labyrinthodont traits (e.g., infolded labyrinthine teeth) were lost by the time lissamphibians appeared.

  • Terminology:

    • “Amphibian” is restricted to living lissamphibians: salamanders, frogs, and caecilians.

Diversity and Distribution of Living Amphibians

  • Major Living Groups:

    • Salamanders

    • Frogs

    • Caecilians

  • Age and Diversity:

    • Date back over 200 million years to the Jurassic.

    • Nearly 4,000 living species with diverse life histories.

  • Geographic Distribution:

    • Occur in tropical and temperate regions worldwide.

    • Absent from some oceanic islands.

Reproductive and Physiological Characteristics of Amphibians

  • Egg Characteristics:

    • Lack shells and amniotic membranes.

    • Laid in water or moist environments.

  • Fertilization:

    • Frogs: external fertilization.

    • Most salamanders and probably all caecilians: internal fertilization.

  • Respiration:

    • Typically possess paired lungs; some salamander families may have lungs reduced or absent.

    • Cutaneous respiration occurs through moist skin.

  • Skin Glands:

    • Mucous glands maintain moisture.

    • Granular (poison) glands produce toxic or unpleasant chemicals.

Evolutionary Significance of Modern Amphibians

  • Transitional Role:

    • Modern amphibians resemble intermediates between fishes and later tetrapods.

  • Morphological Changes:

    • Loss of many skull and pectoral girdle bones.

    • Scales absent except in caecilians.

    • Smaller body size relative to ancient tetrapods.

  • Fossil Record:

    • No definite fossil ancestor linking living amphibians directly to lepospondyls or labyrinthodonts.

    • Salamanders first appear in Upper Jurassic; frogs in Triassic, already showing modern skeletal design and saltatory locomotion.

Shared Characteristics of Living Amphibians

  • Small body size

  • Respiration through skin

  • Pedicellate Teeth:

    • Suture dividing base and crown.

  • Presence of an Auricular Opisthocom:

    • An extra ear bone.

  • Metamorphosis:

    • Larva-to-adult transformation; subtle in salamanders, dramatic in frogs.

  • Reduced Skull and Girdle Bones.

Taxonomy of Class Amphibia

  • Class Amphibia

    • Subclass Labyrinthodontia

    • Order Ichthyostegalia

    • Order Temnospondyli

    • Order Anthracosauria

    • Subclass Lissamphibia

    • Order Proanura

    • Order Anura (Salientia)

    • Order Urodela (Caudata)

    • Order Apoda (Gymnophiona)

Urodela (Caudata) — Salamanders

  • Includes all salamanders; aquatic forms in Salamandridae called newts.

  • Body Form:

    • Paired limbs and long tail; resembles Paleozoic tetrapods.

  • Ectotherms with a low metabolic rate.

  • Feeding Mechanisms:

    • Terrestrial: tongue protrusion; aquatic: suction feeding via rapid jaw opening.

    • Carnivorous as both larvae and adults; feed on worms, small arthropods, and mollusks.

  • Skull Characteristics:

    • Broader and more open than ancestral tetrapods.

    • Many bones lost or fused; no tympanum (eardrum) or temporal notch.

  • Reproductive Biology:

    • Primitive forms: external fertilization.

    • Advanced forms: internal fertilization via spermatophore.

Salientia (Anura) — Frogs and Toads

  • Frogs and toads; adults lack tails (Anura = “no tail”).

  • Locomotion:

    • Long hindlegs for jumping; hence “salientians.”

  • Fertilization:

    • Mostly external, except in genus Ascaphus.

  • Larval Stage:

    • Tadpole specialized for scraping algae; exploits temporary food resources in ponds.

  • Metamorphosis:

    • Adult features: ectothermic, stout body, tongue-based feeding.

  • Characteristics:

    • Tympanum usually present, especially developed in males.

    • Example Species:

    • Conraua goliath (Goliath Frog)

    • Ascaphus truei (Coastal Tailed Frog)

Salientia (Anura) — Frog vs. Toad

  • Toads:

    • Warty skin and parotoid glands.

  • Frogs:

    • Smoother skin, lack parotoid glands.

  • Families:

    • 44 families of frogs and toads

    • Family Ranidae (common larger frogs in North America)

    • Family Hylidae (tree frogs)

    • Family Bufonidae (true toads; thicker skins and prominent warts)

      • Examples:

      • Bufo bufo (Common Toad)

      • Agalychnis callidryas (Red-eyed Tree Frog)

      • Pelophylax esculentus (Common Green Frog)

Gymnophiona (Apoda) — Caecilians

  • Description:

    • Limbless, wormlike amphibians; "apodans" = "no feet."

  • Habitat:

    • Damp tropical regions; burrowing lifestyle.

  • Skull Characteristics:

    • Solid and compact, unlike frogs and salamanders.

  • Reproductive Biology:

    • Internal fertilization; males possess a copulatory organ.

    • Primitive forms lay eggs with aquatic larvae; advanced forms produce live terrestrial young.

  • Phylogenetic Debate:

    • Treated here as temnospondyl-derived, though some argue for lepospondyl origin.

    • Example Species:

    • Caecilia pulchraserrana (Common Caecilian)

Amniotes

  • Vertebrates whose embryos are enclosed in extraembryonic membranes.

  • Amniotic Egg:

    • Includes the embryo plus membranes, typically packaged in a calcareous or leathery shelled egg.

  • Fossil Evidence:

    • Direct reproductive evidence is rare, especially in basal fossil groups.

    • Taxonomic placement relies on phylogenetic analyses using multiple characters, not reproduction alone.

Major Amniote Lineages

  • Two Primary Radiations:

    • Sauropsida:

    • Birds, dinosaurs, modern reptiles, and many Mesozoic forms.

    • Synapsida:

    • A monophyletic lineage including therapsids and modern mammals.

  • Timing of Divergence:

    • Diverged by the Carboniferous, possibly earlier.

Sauropsida

  • Definition:

    • Amniotes including all birds and reptiles, plus their immediate fossil ancestors.

  • Major Subdivisions:

    • Parareptilia:

    • Extinct.

    • Eureptilia:

    • All living sauropsids belong to this group, specifically the Diapsida.

  • Extinction and Survival:

    • Parareptilia died out ~200 million years ago.

Skull Fenestration and Amniote Classification

  • Importance of the Temporal Region:

    • Area behind the eye in the skull, historically central to determining amniote relationships.

  • Key Variables:

    • Number of temporal fenestrae (openings).

    • Position of temporal arches (bars) formed by skull bones.

Major Skull Types

  • Anapsid Skull:

    • Fully roofed by bone; no openings.

    • Found in:

    • Primitive amniotes

    • Turtles and their allies.

  • Synapsid Skull:

    • Single pair of temporal openings, bordered above by a temporal bar formed by squamosal and postorbital bones.

    • Characteristic of mammalian ancestors.

  • Diapsid Skull:

    • Two pairs of temporal openings.

    • Upper temporal bar: squamosal + postorbital bones.

    • Lower temporal bar: jugal + quadratojugal bones.

    • Includes:

    • Pterosaurs, dinosaurs, birds, and all living reptiles except turtles.

  • “Euryapsid” Skull (Reinterpreted):

    • Once considered separate; now recognized as a modified diapsid skull with a loss of the lower temporal bar.

    • Seen in: Plesiosaurs and Ichthyosaurs.

    • Independent loss of the lower temporal opening in both groups.

Evolutionary Interpretation and Changing Terminology

  • Modern Phylogenies:

    • Incorporate newly described fossils.

    • Based on large numbers of characters.

  • Taxonomic Revisions:

    • Traditional terms may be restricted or abandoned, e.g., Reptilia recognized as a taxonomic grade, not a true clade.

    • Phylogenetic Insight:

    • Crocodiles share more traits with birds than with lizards, snakes, or turtles.

    • Necessitates reevaluation of traditional groupings.

Anthracosaurs and Cotylosauria

  • Transitional between non-amniote and amniote tetrapods.

  • Anthracosaurs:

    • Historically included Seymouromorpha and other late non-amniote groups.

    • Often envisioned as closely related to, or ancestral to, amniotes.

  • Cotylosauria:

    • Means "stem reptiles."

    • Originally proposed as basal amniotes but used variably in past literature.

    • Abandoned clade of reptiles, e.g., Seymouria baylorensis skeleton.

Sauropsids Key Points

  • The amniotic egg represents a critical evolutionary innovation.

  • Early divergence produced two major amniote lineages:

    • Sauropsida and Synapsida.

  • Skull fenestration played a role in amniote classification.

  • Traditional taxonomic terms reflect grades rather than true evolutionary clades.

  • Illuminate the transition from non-amniote to amniote tetrapods.

Reptilia Fossil

  • Captorhinus specimens:

    • Applies to Parareptilia and Eureptilia; united by similarities of the braincase.

    • Distinguished from mesosaurs.

  • Historical Terminology:

    • Based on skull type.

    • Modern interpretation shows multicharacter analyses reveal skull type ≠ clade.

    • Example: Captorhinids have anapsid skulls but fall within Eureptilia.

    • Today:

    • Anapsida = skull type, not a valid taxon.

    • Diapsida = monophyletic lineage within Eureptilia.

    • Birds included within Diapsida; recognized as specialized derivatives of reptiles.

Reptilia

  • Characteristics:

    • Body covered with thick cornified epidermal cells in plaques, shields, or scales.

    • Pelvic girdle articulates with two sacral vertebrae.

    • Digits supplied with claws.

    • Metanephros is the new kidney.

    • Heart partially or completely divided into right and left chambers.

    • Has pulmonary and systemic circuits.

    • Ectothermic like fishes and amphibians.

    • Parapineal glands for thermoregulation in other organisms.

Diadectomorpha

  • Illustrates phylogenetic relationships of amniotes.

  • Phylogeny:

    • Major trends summarized through illustrations and figures (Figure 3.27).

  • Example groups: Thecodonts, Crocodylomorpha, Pterosaurs, Ornithischia, Saurischia, and others.

Taxonomy of Class Reptilia

  • Subclass Archosauria

    • Order Thecodontia

    • Order Pterosauria

    • Order Saurischia

    • Order Ornithischia

    • Order Crocodilia

  • Subclass Synapsida

    • Order Pelycosauria

    • Order Therapsida

  • Subclass Anapsida

    • Order Captorhinida

    • Order Testudinata (Chelonia)

  • Subclass Lepidosauria

    • Order Rhynchocephalia

    • Order Squamata

    • Suborder Sauria

    • Suborder Serpentes

  • Subclass Euryapsida

    • Order Sauropterygia

    • Order Ichthyosauria

Mesosaurs

  • Early sauropsids with a specialized aquatic lifestyle.

  • Key Features:

    • Primitive skull lacking temporal fenestrae.

    • Elongate snout with long teeth (potential for filter-feeding or fish capture).

    • Paddle-shaped feet, compressed tail, long neck.

    • Reproduction evidence supports vivipary (live birth).

    • Biogeography: Fossils found in South America and southern Africa, providing evidence for continental drift.

Parareptilia

  • Fossil-only reptilian lineage.

  • Distinctive Traits:

    • Eardrum supported by squamosal and retroarticular process.

    • Unique digit–ankle articulations.

  • Status:

    • No living representatives.

Eureptilia

  • Sauropsids excluding parareptiles and mesosaurs.

  • Diapsida:

    • Two temporal fenestrae, palatine fenestra in roof of mouth.

    • Oldest Diapsid: Petrolacosaurus (Late Carboniferous, Kansas).

    • Body approximately 20 cm, tail 20 cm; fully diapsid skull with complete temporal bars.

Eureptilian Radiation

  • Three Major Diapsid Lineages:

    • Archosauromorpha

    • Lepidosauromorpha

    • "Euryapsida" (informal; includes Mesozoic marine reptiles).

Archosauromorpha

  • Includes archosaurs and related fossil diapsids.

  • Key Trends:

    • Increasing bipedalism.

    • Hindlimbs become primary locomotor appendages.

  • Skull Features:

    • Diapsid skull with an antorbital fenestra and mandibular fenestra.

Archosaurs

  • "Ruling Reptiles"

  • Included Groups:

    • Thecodonts (paraphyletic).

    • Crocodiles: survivors of archosaur radiation (skull and ankle similar to primitive thecodonts).

    • Dinosaurs, birds, pterosaurs: all derived from thecodont ancestors.

Subclass Anapsida

  • Possess a skull with no temporal opening behind the orbits.

  • Primitive condition exhibited by basal reptiles and Chelonia (modern turtles).

  • Turtles, having diverged from another anapsid lineage, are the sole living members of this group.

Testudinata (Turtles)

  • Unique Shell:

    • Composed of carapace + plastron.

  • Groups:

    • Pleurodira (lateral neck flexion)

    • Cryptodira (vertical flexion).

  • Evolution:

    • Likely derived from diapsids with secondarily closed fenestrae.

    • Placed near euryapsids.

Testudinata (Turtles) Features

  • Fossils appear in the Upper Triassic, around 220 million years ago.

  • Shells consist of a dorsal carapace and a ventral plastron.

    • The outer horny layer is keratinized, while the inner layer is bony, forming from the fusion of ribs, vertebrae, and dermally-ossified elements.

  • Among vertebrates, limbs and limb girdles are located deep to the ribs.

  • Turtles lack teeth and use tough, horny plates for gripping food.

Lepidosauria

  • Order Rhynchocephalia:

    • Primitive but with different types of scales, teeth, and internal morphology.

    • Retains ancestral diapsid skull.

  • Order Squamata:

    • Modern lizards, snakes, and amphisbaenians with a diapsid skull and adaptive modifications.

  • Key Distinctions:

    • Lizards: eyelids, ear openings.

    • Snakes: no eyelids or external ears.

    • Functional Consequences:

    • Loss of temporal bars increases jaw mobility.

Order Rhynchocephalia

  • Only 2 living species of tuatara in New Zealand.

  • Now restricted to small islands due to human introduction of nonnative species that preyed upon them, slow growth, and reproductive rates.

  • Skull similar to diapsid skulls of 200 million years ago, well-developed median parietal eye beneath skin; exemplifies one of the slowest rates of evolutionary change among vertebrates.

Order Squamata

  • Diapsid skulls have lost dermal bone ventral and posterior to the lower temporal opening.

  • Kinetic Skull:

    • Movable joints in palate and across the roof of the skull allow the snout to tilt, enhancing prey manipulation and effective jaw closure.

    • Varied adaptations exist:

    • Some species are blind; others possess spectacles (transparent eyelids).

    • Nictitating membrane is present.

Suborder Sauria: Lizards

  • Examples:

    • Gecko: small, agile, nocturnal.

    • Iguana: includes New World lizards and the marine iguana from the Galápagos.

    • Chameleon: primarily from Africa and Madagascar; many possess an extendable tongue.

  • Degenerate Limbs:

    • Some lizards have reduced limbs; glass lizards are nearly limbless.

Suborder Sauria: Lizards Characteristics

  • Movable eyelids (unlike snakes) and an external ear that snakes lack.

  • Nocturnal species (e.g., geckos) possess retinas with only rods, whereas day-active lizards have both rods and cones.

  • Vocal Communication:

    • Geckos utilize vocal signals to announce territory and deter males.

# Amphisbaenians