The Vertebrate Invasion of the Land
Primitive sarcopterygian diversity:
Eusthenopteron: Represntative of a group of choanate fish ranging from the Devonian to the early Permian. Indicators of what the last fully aquatic ancestors of land vertebrates might have looked like. Fortunately, the anatomy of some, such as Eusthenopteron, is very well know from many excellent fossils.
Panderichthyes: This is an example of a small group of fish-like vertebrates even closer to the common ancestry of land vertebrates than Eusthenopteron:
- Fossils found in fresh water deposits. Their disproportionately small eyes compare well with living fish that inhabit muddy bodies of water. In that realm, they were probably powerful predators.
- It is often suggested that they were able to move on land. Although it is conceivable that they might have dragged themselves to new bodies of water (evidence includes the roughly equal length of radius and ulna in some) it seems clear from their general profile that this would be an emergency procedure.
- Synapomorphies with land vertebrates:
- Complete loss of dorsal fins.
- Frontal bone in dermal skull roof
- Eye sockets located on the dorsal surface of the skull.
- Biological interpretation: Panderichthyes, although still aquatic, was anatomically somewhat specialized for life in very shallow water fresh water, and may have stalked terrestrial prey such as insects.
Stegocephalii - transition to land:
Tiktaalik: (Dev) (As seen on Southpark!)
Biological meaning: Tiktaalik could absolutely support part of its weight on its forelimbs.
- Loss of fin rays from fore and hind limbs.
- Development of mobile wrist joint.
- Reduction of opercular skeleton.
The biological significance of the invasion of land.
III. Roster of transformations.
- Predispositions of Choanata for invasion of the land: The closest living relatives of Stegocephalii are fresh water fish adapted to life in muddy bodies of water. Perhaps some, like Panderichthyes hunted prey by lying just beneath the surface, eyes peeping out of the water, waiting for plump arthropods to fall in. Adaptation to such a mode of life conferred abilities that could be secondarily adapted to coping with the terrestrial realm:
- Air breathing through nares and choanae.
- Possession of muscular fore and hindlimbs capable of a small amount of flexion at elbow and knee.
- Why do it? We enter the realm of speculation here:
- Probably, the ability to cross dry land was first a response to the periodic drying up of bodies of water, an alternative to the lungfish's aestivation strategy. The "drying up" could be either seasonal or in response to the tides.
- Once this ability was present, then passage over land could have become a regular means of dispersing to new aquatic habitats. (Many living salamanders have, in addition to an aquatic water-breathing larva and semiaquatic air-pbreathing adult, a highly terrestrial air-breathing eft phase.)
- Once movement on land became routine, the door would be open to the exploitation of terrestrial food sources. In this regard, it is interesting that the first vaguely terrestrial vertebrates appear at the same time as the first tree-size land plants. Before there were forests with plenty of bugs living in them, movement on land may have been less advantageous.
- Breathing: The problem with life on land wasn't the absence of lungs. Rather it was the presence of gills, which became a true liability on land.
- Mechanical support:
- Paired limbs became larger, more robust, and more flexible.
- Digits: Most members of Stegocephalii lack fin rays on their paired fins and, instead, have, digits - fingers and toes. These appear in animals like Acanthostega (below) that are otherwise clearly aquatic, so they must have arisen first as a menas of walking on the bottom or pushing through aquatic vegetation. Nevertheless, their possessors clearly adapted them to use in terrestrial locomotion.
- Sacrum: A bony attachment was established between the pelvis and vertebral column.
- Vertebral centra: These were small or absent in aquatic sarcopterygians, which retained an exposed notochord and ossified arcualia. Movement on land led to the formation of ossifications of the notochord and distinct intervertebral joints.
- Feeding: A fish does not need to bend its neck to pick up food. It just swims up to it and bites. On land, headstands are not an option, so the head must move relative to the torso. Result is a neck.
- Senses: All of the senses must be retooled to function in air.
- Vision: Air and water have different refractive indices, such that one's lens must be adapted to one or the other. The position ot the eye sockets in Panderichthyes suggest that the ability to see in air could have evolved long before the ability to walk on land.
- Hearing: The density of the body is similar to that of water. Thus, sound can pass directly from water through the body into the otic capsule at full intensity. Air is much less dense than water. Thus, for airborn sound to register in the otic cpasule, it must be collected and amplified. This is accomplished by an impedence matching ear:
- Sound is collected over a broad area by a tympanum or ear drum.
- In land vertebrates, a rod-like bone called the stapes transmits the vibrations of the tympanum to a small opening in the otic capsule, the fenestra ovalis (oval window.) (link to image. Stapes is element marked "ST.") By being concentrated into a small area, the sound is amplified so that it can be detected by the inner ear.
- Where does the stapes come from? It is a modified element of the hyoid arch. The air filled space it occupies is called the middle ear.
- Lateral line sense: Useless out of water. Still, as an ancestral feature, they tend to be retained by adults that spend any significant amount of time in the water.
- Reproduction and life history:
- The reproductive methods of early stegocephalians did not, apparantly, keep pace with other terrestrial adaptations. Thus, gelatinous eggs had to be laid in water or moist environments.
- Consequently, even those animals that spent their adult lives on land had to spend their early life in water as a water-breathing animal. For the very earliest stegocephalians that retained their gills, this required no special modification.
Later groups that lost their gill arches and opercula evolved feathery external larval gills, such as we see in some modern amphibians.
Landmarks in evolution of terrestriality: