The Elements of Evolution – Amphibians & Reptiles

Amphibians & Reptiles

320 MYA amphibians were the predominant fauna. Their telling weakness was the requisite of a moist habitat, especially in needing to lay their membrane-encased eggs in water to keep them from drying out.

The first frogs retained a tail and lacked the hopping ability of next-generation frogs. Otherwise, the frog body plan has been much the same for almost 200 million years. The last common ancestor of all extant frogs lived 210 MYA. Spurts of diversification coincided with break-ups of major landmasses. ~130 MYA, the modern frog evolved.

The earliest reptiles, small and inconspicuous, evolved from an amphibian ancestor: a monophyletic descent better suited to aridity, thus granting greater territorial range.

Reptiles first distinguished themselves by the innovation of eggs with calcium carbonate shells that resisted desiccation (amniotic eggs). This afforded laying eggs on land.

The most formidable challenge to life on land has been and remains desiccation. Adaptation to aridity has repeatedly driven the evolution of terrestrial animals and plants.

Reptiles branched out early on, acquiring new niches via distinct feeding strategies. The earliest reptiles were piscivores (fish eaters), followed by a craving for the crunch of insects (insectivores), before diversifying both up and down the food chain.

It took extensive adaptation for vertebrates to become herbivores, thereby gaining access to the nutrients locked behind the cellulose walls of plants. The credit goes to the gut flora which perform digestion: a symbiotic advance.

The origin and early evolution of herbivory represents a major evolutionary event in terrestrial vertebrate history because it allowed tetrapods to directly access the vast, largely untapped resources provided by the primary producers on land: plants. ~ Romanian paleontologist Robert Reisz & German paleontologist Jörg Fröbisch

315 MYA, synapsids descended from reptiles. One family, the caseids, were at first carnivorous. Then they took to eating plants, becoming the first fully terrestrial herbivorous vertebrates. Herbivory independently evolved at least 5 times in the different synapsid clades. Edaphosaurs were one of them.

Caseids and edaphosaurs had distinct feeding strategies. Like today’s iguanas, caseids did not chew their food. Conversely, edaphosaurs had massive crushing dentition and strong jaws for grinding.

In all instances of convergent evolution to herbivory, synapsids became much larger than the small carnivores they descended from.

Other synapsids at the time had the tall dorsal sail of edaphosaurs, including the large apex predator Dimetrodon. The sail served a thermoregulatory function: letting an ectothermic synapsid capture or release heat.

◊ ◊ ◊

Rapid climate change 305 MYA crafted a cooler, drier world. This devastated the tropical rainforests that covered the equatorial belt, fragmenting forests. Whence the Carboniferous Rainforest Collapse, an extinction event that decimated amphibian populations.

So began the rise of reptiles: adapting to new niches by acquiring new diets and other adaptations. ~20 orders of reptilian families arose.

Reptiles emerged via better water conservation. It seems paradoxical that reptile success led to their going back to the water. Numerous reptiles became aquatic at various times. Adaptation answers wherever resource opportunity knocks.

Lizards & Snakes

The prevailing theory is that snakes evolved from burrowing lizards 128 MYA, long after lizards 240 MYA descent. Instead, it is more likely that snakes descended from sea serpents: elongating their bodies and losing their stubby legs with flipper feet while slithering ashore.

In contrast, turtles most certainly evolved from lizards, and rather abruptly: trading celerity for a protective shell to facilitate better burrowing. This inclination for subterranean safety may have helped turtles survive the Permian mass extinction 252 MYA.

 Lizard Adaptability

Adaptation can happen quickly: either by need or to take advantage of opportunity.

In 1971, biologists moved 5 adult pairs of ruin lizards from their home island in the south Adriatic Sea to a neighboring island. 35 years later the lizards were quite different: head size and shape, bite strength, and greatly adapted digestive tracts, along with dramatic changes in population density and social structure.

Lizards on barren Pod Kopiste island, the old homestead, were well-suited to snagging mobile prey; feasting mainly on insects. Life next door, on Pod Mrcaru, offered lusher vegetation.

The lizards on Pod Mrcaru adapted larger heads and a stronger bite to tear fibrous plants. Depending on the season, 2/3rds of the new diet was vegetarian. Digestive tracts adapted to slow food passage, thus giving more time for microbial digestion of plants, including creation of fermentation chambers: an extremely novel adaptation for lizards.

This change in diet provided a larger and more predictable food supply. Whence came increased population density. Because foraging became the norm rather than predation, the adapted lizards gave up defending territories.


Most squamates (lizards and snakes) living today are egg layers, but several species birth live young (viviparity) like mammals. Reptilian viviparity evolved to protect offspring in cold climates, separately doing so in squamates over 100 times.

Viviparity makes sense where temperatures dip so low that egg-encased embryos would develop slowly or not at all. A female carrying her young can regulate temperature by moving to a warmer location as necessary, thus enabling embryos to mature faster and at less risk.

Some tropical lizards are viviparous. These lizards adopted live birth when they lived at high elevations, where it got too cold to risk laying eggs. They then came down to enjoy the balmy weather. Their flexibility in being able to mate with mountainous relatives made them more fit than the egg layers confined to the lowlands, with whom the once-mountaineers will not interbreed.

Lizards in temperature regions came from those that evolved in the mountains, then ventured down to the tropics before migrating north.

Live birth is also related to body size. Small squamates must lay eggs, as their bodies cannot support viviparity.

Marine reptiles that live in the sea – ichthyosaurs and plesiosaurs – also evolved viviparity out of adaptive necessity. However intricate the adaptations involved in bearing live young, the basic mechanism is straightforward: viviparity evolves from oviparity via egg retention.

Viviparity independently arose at least 115 times in various lizards and snakes. The squamate genome affords rapid adaptation to the birthing mode most efficacious for lifestyle.



The ginkgo tree is from the era of dinosaurs; but while the dinosaur has been extinguished, the modern ginkgo has not changed. ~ Japanese organic chemist Koji Nakanishi

The ginkgo arose during the Permian (before the dinosaurs). Like the fern, the gingko had staying power. The success of both owe to simplicity.

The gingko is a seed plant closely related to conifers, with 1 major difference: motile sperm. This was a holdover from aquatic algae, and a trait retained in bryophytes, before vascular tissue became all the rage.

Swimming sperm are common in the animal kingdom, but not vascular plants. Ginkgo and the earlier-evolved cycads, which arose in the early Permian, are the only vascular plants with motile sperm.

Though the ginkgo evolved genetically, it never showed much speciation. The ginkgo did tinker with elaboration. But, like the evolutionary trajectory of the horse, the ginkgo winnowed to a single family. The ginkgo survived 270 million years, through 3 major extinction events, much the same as it started.

The ginkgo is the floral equivalent of the shark: admirably adapted in its early form, with an adept portfolio of life-history variables. Its sexual flexibility is exemplary.

Ginkgo are modularly constructed. Their sex is determined at the tissue level. Trees can produce branches of the opposite sex.

Ginkgo have other vegetative reproductive traits, including sprouting. They produce lignotubers, which are a safeguard against destruction by fire.

55 MYA, the ginkgo’s range extended beyond the Arctic circle. Iceland at the time was lush forest of redwoods, oak, swamp cypress, and ginkgo. Adapted to a warm temperate climate, global cooling 15.5 MYA contracted the ginkgo’s presence.


The impressive Devonian reefs were a concerted construction by calcareous algae, coral, bryozoans, and sponges, among others. After their devastation at the end-Devonian Hagenberg extinction event, the knack of communities forming to build robust burghs on the sea floor was lost for almost 100 million years. Only in the late Permian did reefs make a comeback, and their scale paled in comparison.