dry periods and stores urea (up to a level of 300 m-Osmole/I) for excretion during
the rainy season (MCCLANAHAN, 1967). Such tolerance to the presence of urea reflects its high solubility, uniform distribution in the body fluids and low toxicity.
Vertebrates may change modes of nitrogen excretion in different environmental
situations. The lungfish, Protopterus, is ureotelic only while it is aestivating; in its
usual aquatic surrounding it is ammoniotelic. The same sort of transition takes place
in Xenopus. Such changes have been relat ed to changes in the liver enzy mes of the
ornithine-urea cycle (GOLDSTEIN, 1968) and these may possibly be controlled by
adrenocortical hormones (SCHIMKE, 1963).
7. Osmoregulation and the Origin of Vertebrates
Vertebrates originated in early Palaeozoic times, probably in the early part of the
Ordovician age about 400 million years ago . While it is agreed that this took place
in aqueous media, there is a sharp dichotomy of opinion as to whether it was in
fresh water or the sea. An account of this disagreement is germane to our subject
as the evidence is not only palaeontological but draws upon physiological information about the osmoregulation of contemporary species.
In the post-Darwinian period to 1900 it was generally assumed that vertebrates
originated in the sea. At that time the oldest fossil remains of the fishes were considered to be present in deposits of Devonian age, which were thought to be of
marine origin. However, CHAMBERLIN in 1900 and BARREL in 1916 challenged this
interpretation, and showed that some of these deposits had been laid down in fresh
water. It is now accepted that Devonian vertebrate remains are found in deposits
of either marine or freshwater origin (see ROBERTSON, 1957). The earliest vertebrates were jawless fishes called ostracoderms, which have been classified with contemporary agnathans, and have been identified in Ordovician-Silurian deposits
which precede the Devonian remains. The question as to whether these fossils are
marine or freshwater in origin is still hotly contested (ROMER, 1955 and 1967; ROBERTSON, 1957). The arguments revolve largely around questions as to whether the
presence of terrestrial plants and marine invertebrates reflects the true origin of
the deposits, or whether they were washed into the area of the beds from another,
contrasting, osmotic enviroment. HOMER SMITH (1961) considered the Scottish
verdict ' not proven' appropriate to the situation.
The contemporary biological information, relevant to a marine or freshwater
origin for the vert ebrates, provides an interesting int ellectual exercise relevant to
our present topic of osmoregulation, though it should be remembered that such
evidence is indirect and only circumstantial.
It is usually agreed that vertebrates arose from a marine chordate ancestor, possibly a free-swimming tunicate larva (ROMER, 1967). Three contemporary groups
of chordates, the Hemichordata, Cephalochordata and Urochordata are exclusively marine in habitat. The problem is, whether the transition that gave rise
to the fourth group, the Vertebrata, took place in the sea or fresh water.
The sea, in early Palaeozoic times, had a composition similar to that which it
has today, so that the osmotic problems of the prehistoric animals would have been
similar to those of contemporary species. The precise nature of such problems,
35
the rainy season (MCCLANAHAN, 1967). Such tolerance to the presence of urea reflects its high solubility, uniform distribution in the body fluids and low toxicity.
Vertebrates may change modes of nitrogen excretion in different environmental
situations. The lungfish, Protopterus, is ureotelic only while it is aestivating; in its
usual aquatic surrounding it is ammoniotelic. The same sort of transition takes place
in Xenopus. Such changes have been relat ed to changes in the liver enzy mes of the
ornithine-urea cycle (GOLDSTEIN, 1968) and these may possibly be controlled by
adrenocortical hormones (SCHIMKE, 1963).
7. Osmoregulation and the Origin of Vertebrates
Vertebrates originated in early Palaeozoic times, probably in the early part of the
Ordovician age about 400 million years ago . While it is agreed that this took place
in aqueous media, there is a sharp dichotomy of opinion as to whether it was in
fresh water or the sea. An account of this disagreement is germane to our subject
as the evidence is not only palaeontological but draws upon physiological information about the osmoregulation of contemporary species.
In the post-Darwinian period to 1900 it was generally assumed that vertebrates
originated in the sea. At that time the oldest fossil remains of the fishes were considered to be present in deposits of Devonian age, which were thought to be of
marine origin. However, CHAMBERLIN in 1900 and BARREL in 1916 challenged this
interpretation, and showed that some of these deposits had been laid down in fresh
water. It is now accepted that Devonian vertebrate remains are found in deposits
of either marine or freshwater origin (see ROBERTSON, 1957). The earliest vertebrates were jawless fishes called ostracoderms, which have been classified with contemporary agnathans, and have been identified in Ordovician-Silurian deposits
which precede the Devonian remains. The question as to whether these fossils are
marine or freshwater in origin is still hotly contested (ROMER, 1955 and 1967; ROBERTSON, 1957). The arguments revolve largely around questions as to whether the
presence of terrestrial plants and marine invertebrates reflects the true origin of
the deposits, or whether they were washed into the area of the beds from another,
contrasting, osmotic enviroment. HOMER SMITH (1961) considered the Scottish
verdict ' not proven' appropriate to the situation.
The contemporary biological information, relevant to a marine or freshwater
origin for the vert ebrates, provides an interesting int ellectual exercise relevant to
our present topic of osmoregulation, though it should be remembered that such
evidence is indirect and only circumstantial.
It is usually agreed that vertebrates arose from a marine chordate ancestor, possibly a free-swimming tunicate larva (ROMER, 1967). Three contemporary groups
of chordates, the Hemichordata, Cephalochordata and Urochordata are exclusively marine in habitat. The problem is, whether the transition that gave rise
to the fourth group, the Vertebrata, took place in the sea or fresh water.
The sea, in early Palaeozoic times, had a composition similar to that which it
has today, so that the osmotic problems of the prehistoric animals would have been
similar to those of contemporary species. The precise nature of such problems,
35