however, depends on the osmotic concentration of the body fluids and we have
no direct information about this in the fossil species . Contemporary lower chordates are essentially isoosmotic with their marine environment, whereas marinevertebrates may be either slightly hyperosmotic, as in the Chondrichthyes or hypoosmotic, as in the Osteiichthyes. The ostracoderms are classified with the Agnatha,
in which one contemporary group, the Myxinoidea, is iso-or slightly hyperosmotic
to its marine environment, while the other, the Petromyzontoidea, is hypoosmotic
to sea-water. It is uncertain which, if either, of these groups represents the primary
vertebrate condition. If, as suggested by ROMER and HOMER SMITH, the evolution
to a vertebrate took place in fresh water, then osmotic uptake in a myxinoid type
would be much greater than in a petromyzontid, though it is easy to envisage the
presence of compensatory factors, such as a relatively impermeable integument. If,
on the other hand, the evolution occurred in the sea then, as ROBERTSON suggests,
the myxinoid pattern 'may well be the primary one', the advantages and osmotic
simplicity of this system making it the most logical choice. Nevertheless, despite
popular opinion to the contrary, we cannot assume that nature is logical.
Whatever the primary osmotic pattern may have been in the first vertebrate,
if the transition occurred in fresh water, novel osmotic stresses not apparent in its
marine antecedents would arise. The animal would tend to gain water by osmosis
and lose salts by diffusion. This could be compensated for by reducing the permeability of the animals' surface to water and solutes and by excreting the excess
water. Such mechanisms exist in nature and presumably would have been apparent
early in vertebrate evolution, though whether such changes took place in a marine
ancestor and constitute preadaptation for life in hypoosmotic environments (ROBERTSON, 1957), or occurred under the selective pressures in a hypoosmotic environment (H. SMITH, 1961), is not yet apparent.
Early ostracoderms had characteristically heavy dermal armour. ROMER considers this to have evolved as a protection against attacks by eurypterids, which were
arthropods contemporary with the ostracoderms. HOMER SMITH on the other hand
considers this armour to be an adaptation to fresh water, resulting in reduced osmotic water uptake. ROBERTSON rejects both of these views and cites evidence suggesting that ostracoderms fed on eurypterids rather than the reverse. If the early
ostracoderms were marine, as considered by ROBERTSON, then the dermal armour
may have constituted a preadaptation to a subsequent move into fresh water. A
comparable situation can be demonstrated in two species of turtles, the red-eared
turtle, Pseudemys scripta, which has the usual chelonian armour, and the softshell
turtle, Trionyx spinifer, which lacks these hard plates on its carapace. KNUT
SCHMIDT-NIELSEN and I (BENTLEY and SCHMIDT-NIELSEN, 1970) have compared
the osmotic permeability of these two species in fresh water and in a saline equivalent in concentration to the sea. Water exchange in either solution was four to five
times as great in the softshell turtle as in Pseudemys. While the hard dermal plates
in turtles probably serve the purpose of protection against predators, they could
also act as a preadaptation to life in a different osmotic medium, as could have occurred if marine ostradoderms moved into fresh water.
The principal contemporary biological evidence used to support a freshwater
origin for vertebrates, is the comparative anatomy and physiology of the vertebrate
kidney. As we have seen, the renal glomerulus is a structure through which large
36
no direct information about this in the fossil species . Contemporary lower chordates are essentially isoosmotic with their marine environment, whereas marinevertebrates may be either slightly hyperosmotic, as in the Chondrichthyes or hypoosmotic, as in the Osteiichthyes. The ostracoderms are classified with the Agnatha,
in which one contemporary group, the Myxinoidea, is iso-or slightly hyperosmotic
to its marine environment, while the other, the Petromyzontoidea, is hypoosmotic
to sea-water. It is uncertain which, if either, of these groups represents the primary
vertebrate condition. If, as suggested by ROMER and HOMER SMITH, the evolution
to a vertebrate took place in fresh water, then osmotic uptake in a myxinoid type
would be much greater than in a petromyzontid, though it is easy to envisage the
presence of compensatory factors, such as a relatively impermeable integument. If,
on the other hand, the evolution occurred in the sea then, as ROBERTSON suggests,
the myxinoid pattern 'may well be the primary one', the advantages and osmotic
simplicity of this system making it the most logical choice. Nevertheless, despite
popular opinion to the contrary, we cannot assume that nature is logical.
Whatever the primary osmotic pattern may have been in the first vertebrate,
if the transition occurred in fresh water, novel osmotic stresses not apparent in its
marine antecedents would arise. The animal would tend to gain water by osmosis
and lose salts by diffusion. This could be compensated for by reducing the permeability of the animals' surface to water and solutes and by excreting the excess
water. Such mechanisms exist in nature and presumably would have been apparent
early in vertebrate evolution, though whether such changes took place in a marine
ancestor and constitute preadaptation for life in hypoosmotic environments (ROBERTSON, 1957), or occurred under the selective pressures in a hypoosmotic environment (H. SMITH, 1961), is not yet apparent.
Early ostracoderms had characteristically heavy dermal armour. ROMER considers this to have evolved as a protection against attacks by eurypterids, which were
arthropods contemporary with the ostracoderms. HOMER SMITH on the other hand
considers this armour to be an adaptation to fresh water, resulting in reduced osmotic water uptake. ROBERTSON rejects both of these views and cites evidence suggesting that ostracoderms fed on eurypterids rather than the reverse. If the early
ostracoderms were marine, as considered by ROBERTSON, then the dermal armour
may have constituted a preadaptation to a subsequent move into fresh water. A
comparable situation can be demonstrated in two species of turtles, the red-eared
turtle, Pseudemys scripta, which has the usual chelonian armour, and the softshell
turtle, Trionyx spinifer, which lacks these hard plates on its carapace. KNUT
SCHMIDT-NIELSEN and I (BENTLEY and SCHMIDT-NIELSEN, 1970) have compared
the osmotic permeability of these two species in fresh water and in a saline equivalent in concentration to the sea. Water exchange in either solution was four to five
times as great in the softshell turtle as in Pseudemys. While the hard dermal plates
in turtles probably serve the purpose of protection against predators, they could
also act as a preadaptation to life in a different osmotic medium, as could have occurred if marine ostradoderms moved into fresh water.
The principal contemporary biological evidence used to support a freshwater
origin for vertebrates, is the comparative anatomy and physiology of the vertebrate
kidney. As we have seen, the renal glomerulus is a structure through which large
36