studied in detail in 1903 by Professor and Miss Sollas, who succeeded in making enlarged models of the fossil in wax.
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| From British Museum Guide to Fossil Reptiles and Fishes, by permission of the Trustees. |
| Palaeospondylus gunni, restored by Dr R. H. Traquair. (Nearly twice nat. size.) |
The skeleton as preserved is carbonized, and indicates an eel shaped animal from 3 to 5 cm., in length. The skull, which must have consisted of hardened cartilage, exhibits pairs of nasal and auditory capsules, with a gill-apparatus below its hinder part, but no indications of ordinary jaws. The anterior opening of the brain-case is surrounded by a ring of hard cirri. A pair of “post-branchial plates” projects backwards from the head. The vertebral axis shows a series of broad rings, with distinct neural arches, but no ribs. Towards the end of the body both neural and haemal arches are continued into forked radial cartilages, which support a median fin. There are no traces either of paired fins or of dermal armour. The affinities of Palaeospondylus are doubtful, but it is probably related to the contemporaneous armoured Ostracoderms.
References.—R. H. Traquair, paper in Proc. Roy. Phys. Soc. Edin., xii. 312, (1894); W. J. Sollas and I. B. J. Sollas, paper in Phil. Trans. Roy. Soc. (1903 B.). (A. S. Wo.)
PALAEOTHERIUM (i.e. ancient animal), a name applied by
Cuvier to the remains of ungulate mammals recalling tapirs
in general appearance, from the Lower Oligocene gypsum
quarries of Paris. These were the first indications of the
occurrence in the fossil state of perissodactyle ungulates allied
to the horse, although it was long before the relationship was
recognized. The palaeotheres, which range in size from that
of a pig to that of a small rhinoceros, are now regarded as representing
a family, Palaeotheriidae, nearly related to the horse-tribe,
and having, in fact, probably originated from the same
ancestral stock, namely, Hyracotherium of the Lower Eocene
(see Equidae). The connecting link with Hyracotherium was
formed by Pachynolophus (Propalaeotherium), and the line
apparently terminated in Paloplotherium, which is also Oligocene.
Representatives of the family occur in many parts of Europe,
but the typical genus is unknown in North America, where,
however, other forms occur.
|
| (From the Paris gypsum.) |
| Restoration of Palaeotherium magnum. (About 13 nat. size.) |
Although palaeotheres resemble tapirs in general appearance, they differ in having only three toes on the fore as well as on the hind foot. The dentition normally comprises the typical series of 44 teeth, although in some instances the first premolar is wanting. The cheek-teeth are short-crowned, generally with no cement, the upper molars having a W-shaped outer wall, from which proceed two oblique transverse crests, while the lower ones carry two crescents. Unlike the early horses, the later premolars are as complex as the molars; and although there is a well-marked gap between the canine and the premolars, there is only a very short one between the former and the incisors. The orbit is completely open behind. In other respects the palaeotheres resemble the ancestral horses. They were, however, essentially marsh-dwelling animals, and exhibit no tendency to the cursorial type of limb so characteristic of the horse-line. They were, in fact, essentially inadaptive creatures, and hence rapidly died out. (R. L.*)
PALAEOZOIC ERA, in geology, the oldest of the great time
divisions in which organic remains have left any clear record.
The three broad divisions—Palaeozoic, Mesozoic,
Cainozoic—which
are employed by geologists to mark three stages in the
development of life on the earth, are based primarily upon the
fossil contents of the strata which, at one point or another, have
been continuously forming since the earliest times. The
precise line in the “record of the rocks” where the chronicle
of the Palaeozoic era and that of the Mesozoic era opens—as
in more recent historical documents—is a matter for editorial
caprice. The early geologists took the most natural dividing
lines that came within their knowledge, namely, the line of change
in general petrological characters, e.g. the “Transition Series”
(Übergangsgebirge), the name given to rocks approximately of
Palaeozoic age by A. G. Werner because they exhibited a transitional
stage between the older crystalline rocks and the younger
non-crystalline; later in Germany these same rocks were said to
have been formed in the “Kohlenperiode” by H. G. Bronn and
others, while in England H. T. de la Beche classed them as a
Carbonaceous and Greywacke group. Finally, the divisional time
separating the Palaeozoic record from that of the Mesozoic was
made to coincide with a great natural break or unconformity of
the strata. This was the most obvious course, for where such
a break occurred there would be the most marked differences
between the fossils found below and those found above the
physical discordance. The divisions in the fossil record having
been thus established, they must for convenience remain, but
their artificiality cannot be too strongly emphasized, for the
broad stratigraphical gaps and lithological groups which made
the divisions sharp and clear to the earlier geologists are proved
to be absent in other regions, and fossils which were formerly
deemed characteristic of the Palaeozoic era are found in some
places to commingle with forms of strongly marked Mesozoic
type. In short, the record is more nearly complete than was
originally supposed.
The Palaeozoic or Primary era is divided into the following periods or epochs: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. The fact that fossils found in the rocks of the three earlier epochs—Cambrian, Ordovician, Silurian—have features in common, as distinguished from those in the three later epochs has led certain authors to divide this era into an earlier, Protozoic (Proterozoic) and a later Deuterozoic time. The rocks of Palaeozoic age are mainly sandy and muddy sediments with a considerable development of limestone in places. These sediments have been altered to shales, slates, quartzites, &c., and frequently they are found in a highly metamorphosed condition; in eastern North America, however, and in north-east Europe they still maintain their horizontality and primitive texture over large areas. The fossils of the earlier Palaeozoic rocks are characterized by the abundance of trilobites, graptolites, brachiopods, and the absence of all vertebrates except in the upper strata; the later rocks of the era are distinguished by the absence of graptolites, the gradual failing of the trilobites, the continued predominance of brachiopods and tabulate corals, the abundance of crinoids and the rapid development of placoderm and heterocercal ganoid fishes and amphibians. The land plants were all cryptogams, Lepidodendron, Sigillaria, followed by Conifers and Cycads. It is obvious from the advanced stage of development of the organisms found in the earliest of these Palaeozoic rocks that the beginnings of life must go farther back, and indeed organic remains have been found in rocks older than the Cambrian; for convenience, therefore, the base of the Cambrian is usually placed at the zone of the trilobite Olenellus. (J. A. H.)
PALAEPHATUS, the author of a small extant treatise, entitled
Περὶ Ἀπίστων (On “Incredible Things”). It consists of a series
of rationalizing explanations of Greek legends, without any
attempt at arrangement or plan, and is probably an epitome,
composed in the Byzantine age, of some larger work, perhaps the
Λύσεις τῶν μυθικῶς εἰρημένων, mentioned by Suïdas as the
work of a grammarian of Egypt or Athens. Suïdas himself
ascribes a Περὶ Ἀπίστων, in five books, to Palaephatus of Paros or
Priene. The author was perhaps a contemporary of Euhemerus
(3rd century B.C.). Suïdas mentions two other writers of the
name: (1) an epic poet of Athens, who lived before the time of
