Four on the Floor: The Conquest of Land by Vertebrates
Not every organism can tolerate different salinity conditions. Osmoconformers have very little "buffering" between internal physiology and the external environment. Internal salt levels therefor tend to match the ambient conditions. Since organismal physiology requires salts, it is impossible for osmoconformers to survive well in freshwater conditions, because they cannot maintain their internal osmolarity.
In contrast, osmoregulators have mechanisms (normally associated with the excretory system) to control salt levels. Freshwater osmoregulators have higher internal osmolarities than ambient conditions, and so need to excrete excess water to retain their salt levels. In contrast, saltwater osmoregulators, the internal osmolarities are lower than ambient, so they need to retain water but excrete excess salt. Given these different requirements, most animals are stenohaline (have narrow range of salinity tolerances). However, some are euryhaline (have broad range of salinity tolerances). Not surprisingly most groups that make it to freshwater and land tended to be euryhaline.
Furthermore, sarcopterygians themselves had fins with bones running down the body of the fin, making it very strong and muscular. This can be used to dig a hole (as some lungfish use in order to survive the dry season); to push through vegetation; and to crawl out of one pond and move into another. (This latter ability isn't that strange: it is found in some living actinopterygians such as catfish, mudskippers, snakeheads, even freshwater eels).
All the major groups of sarcoptergyians (Tetrapodomorpha) is of particular interest for us. Like the other groups (coelacanths and lungfish), some were marine and some were terrestrial. In fact some of the aquatic tetrapodomorphs were large shark-like predators. Among them were some in which the lobe fin developed wrists and elbows and in which the skull became flatter and wider. These suggest these advanced tetrapodomorphs spent more more in shallow water, even shuffling out of the water in order to catch food on the land. The best known of these "fishapods" (Tiktaalik of the Middle Devonian, shows that it still lacked fingers and toes (digits), but otherwise was very similar to the next phase of vertebrate evolution.
Stegocephalia is the clade of fingered vertebrates. In these forms the lobefin fin lobe became the walking limbs. However, basal stegocephalians like Acanthostega and Ichthyostega were still primarily aquatic.
In fact, they were built primarily for swimming, and could probably only move around on land as well as a modern mudskipper or snakehead fish:
Traditionally the hypothesis was that stegocephalians developed better limbs in the context of freshwater (since ponds are more likely to dry up than the ocean). While this is still pretty reasonable, the oldest stegocephalian tracks (from the earliest Middle Devonian) are on a marine beach, not a lake shore. So it is possible that terrestrialization was straight from the sea, not from freshwater.
Is Romer's Gap real, or is it an artifact of poor sampling during the Mississippian. While it is true that some additional fossils of stegocephalians and arthropods have been found in this Subperiod, the discrepancy between initial appearance and much later diversification seems to hold. Some have suggested low oxygen levels in the atmosphere may have suppressed diversification, and that only with the rise of increased oxygen did the groups increase in specialization.
As possible evidence for the decreased oxygen level during the Mississippian, the size of the spiracle (first gill slit in jawed vertebrates, modified into an opening in the back of the skull used to help flush oxygenated water past the gills) increased in sarcopterygians. Since the oxygen level in the water is controlled by the atmosphere, this may indicate the need to increase the amount of water needed to keep the body oxygenated.
During the Carboniferous arthropods continued to diversify, including new varieties of arachnids and the arthropleurids (or "godzillapedes"), the largest land-dwelling arthropods of all time. But the primary new adaptation of this part of arthropod history is the origin of insect wings. Winged insects--Pterygota--may have actually be present in the Devonian (if Rhyniognatha is in fact a pterygote); however, there are definite winged insects in the mid-Carboniferous (just above "Romer's Gap").
Debate remains whether wings were derived from lateral extensions of the thoracic body surface (paranotal surface) or from the breathing-part of the limbs (exites). Wings may have initially helped with cooling the body and/or sailing across water surfaces, but of course their main function is to turn the world of the insects into a three-dimensional world.
The most primitive wings were fixed-wings: not fixed like those of an airplane, but ones that at rest are either stuck out to the sides or held largely vertically over the body (as in modern dragonflies in the first case, and damselflies and mayflies in the next). They could do lots of flying and maneuvering around, but fixed-wing insects are limited in moving through dense vegetation, burrowing, and so forth.
Folded wings are characteristic of the Neoptera (the most diverse group of animals of all time). Folding wings allowed insects to colonize many more habitats than basal pterygotes.
An additional set of changes associated with insect evolution is the transformations during ontogeny. Basal pterygotes have nymphs that are wingless and don't look very much like the adults. It is tempting to think that because most dragonflies, damselflies, and mayflies have aquatic nymphs and terrestrial flying adults that the ancestral flying insect condition was an amphibious form. But the breathing structures of the different nymphs are non-homologous, and the even more basal non-flying insects and non-insect hexapod (and even some primitive dragonflies) is to lay eggs in moist soil rather than water.
Neopterans almost all have terrestrial (or burrowing) young. Basal neopterans have nymphs that look like non-flying adults (no major metamorphosis). Derived neopterans (Endopterygota) have are holometabolous (have complete metamorphosis): their larvae (maggots, caterpillars, grubs, etc.) look nothing like (and have ecologies nothing like) the adults.
Completing the Conquest: Tetrapods, Including Amniotes
During "Romer's Gap" there are a few new branches of stegocephalian, including 2 m or longer eel-like aquatic Crassigyrinus. But the most successful group of tetrapods to appear during this time are the Tetrapoda (the crown-group of Tetrapodomorpha). Tetrapods are distinguished from other tetrapodomorphs by lacking gills in the adult, having a five-fingered hand, and a distinct neck (complete separation of the skull from the shoulder blade.
Todays living tetrapods are the Lissamphibia (frogs, salamanders, caecillians) and the Amniota (mammals and sauropsids). In the Paleozoic, however, there were many other branches; some (Batrachomorpha, the Lissamphibia total-group) closer to the living amphibians, and the others (Reptilomorpha, the Amniota total-group) closer to the amniotes. Most of the batrachomorphs and reptilomorphs (and a few Carboniferous stegocephalians outside Tetrapoda) were "amphibians" in the broad sense: that is, they had aquatic "tadpole" larvae with external gills (non-homologous to the internal gills of "fish"), but terrestrial adults.
There was a tremendous diversity of Paleozoic "amphibians": alligator-like forms, boomerang-heads, snake-like forms, armor-plated forms, and more. There were even scaled, marine amphibians (totally distinct from the naked-skinned, salt-intolerant lissamphibians).
All these forms, however terrestrialized as adults, were still stuck living near water in order to reproduce. Tetrapods were capable of the final stages of terrestrialization through the amniotic egg. These eggs have:
Amniotic eggs thus freed amniotes from the pond, and allowed them to colonize all terrestrial habitats. In this way they are exactly analogous to the seeds of spermatophytes.
There were additional traits that unite the amniote groups, such as:
For all their diversity, there is no evidence that any of the other groups of tetrapodomorphs included herbivores. In contrast, many amniote groups (and the immediate outgroups to crown-group Amniota, which may have had amniotic eggs) evolved herbivory in the Permian. From this point on, amniotes joined arthropods as major feeders on land plants.
One of the main changes in anatomy of the amniotes (and other tetrapod) was the development of hearing adapted to the air: impedance matching ears, in which a large tympanum ("eardrum") collects sounds that get delivered to inner ear by one or more bones of the middle ear. Some modern tetrapods lack an eardrum, like salamanders, and the fossilized skulls of basal amniotes and indeed most basal tetrapods show that they lacked these structures. But frogs, mammals, turtles, and eureptiles (lepidosaurs, archosaurs, and their extinct kin). Based on fossil evidence, these groups (and an extinct group of parareptiles) evolved this trait only in the latest Pennsylvanian or early Permian
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