The Student's Elements of Geology Part 57

Until 1859 there was no example of a fossil fish older than the bone-bed of the Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill, near Leintwardine, in Shropshire, by Mr. J.E. Lee of Caerleon, F.G.S., in shale below the Aymestry limestone, a.s.sociated with fossil sh.e.l.ls of the Lower Ludlow formation-- sh.e.l.ls which differ considerably from those characterising the Upper Ludlow already described. This discovery is of no small interest as bearing on the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine cla.s.s.

It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.

When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite. Yet we must hesitate before we accept, even on such evidence, so sweeping a conclusion, as that the globe, for ages after it was inhabited by all the great cla.s.ses of invertebrata, remained wholly untenanted by vertebrate animals.

TABLE 26.2. DATES OF THE DISCOVERY OF DIFFERENT CLa.s.sES OF FOSSIL VERTEBRATA; SHOWING THE GRADUAL PROGRESS MADE IN TRACING THEM TO ROCKS OF HIGHER ANTIQUITY.

COLUMN 1: YEAR.

COLUMN 2: FORMATIONS.

COLUMN 3: GEOGRAPHICAL LOCALITIES.

MAMMALIA:

1798: Upper Eocene: Paris (Gypsum of Montmartre). (George Cuvier, Bulletin Soc.

Philom. 20.)

1818: Lower Oolite: Stonesfield. (In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also above Chapter 19.)

1847: Upper Trias: Stuttgart. (Professor Plieninger. See above Chapter 21.)

AVES:

1782: Upper Eocene: Paris (Gypsum of Montmartre). (Cuvier, Oss.e.m.e.ns Foss. Art.

"Oiseaux.")

1839: Lower Eocene: Isle of Sheppey (London Clay). (Professor Owen Geological Transactions second series volume 6 page 203 1839.)

1854: Lower Eocene: Woolwich Beds. (Upper part of the Woolwich beds. Prestwich Quarterly Geological Journal volume 10 page 157.)

1855: Lower Eocene: Meudon (Plastic Clay). (Gastornis Parisiensis. Owen Quarterly Geological Journal volume 12 page 204 1856.)

1858: Chloritic Series, or Upper Greensand: Cambridge. (Coprolitic bed, in the Upper Greensand. See above Chapter 17.)

1863: Upper Oolite: Solenhofen. (The Archaeopteryx macrura, Owen. See above Chapter 19.)

REPTILIA:

1710: Permian (or Zechstein): Thuringia. (The fossil monitor of Thuringia (Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810.

(Miscel. Berlin.))

1844: Carboniferous: Saarbruck, near Treves. (See Chapter 23.)

PISCES:

1709: Permian (or Kupferschiefer): Thuringia. (Memorabilia Saxoniae Subterr.

Leipsic 1709.)

1793: Carboniferous (Mountain Limestone): Glasgow. (History of Rutherglen by David Ure, 1793.)

1828: Devonian: Caithness. (Sedgwick and Murchison Geological Transactions second series volume 3 page 141 1828.)

1840: Upper Ludlow: Ludlow. (Sir R. Murchison. See Chapter 26.)

1859: Lower Ludlow: Leintwardine. (See Chapter 26.)

Obs.-- The evidence derived from foot-prints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth.

In Table 26.2 a few dates are set before the reader of the discovery of different cla.s.ses of animals in ancient rocks, to enable him to perceive at a glance how gradual has been our progress in tracing back the signs of vertebrata to formations of high antiquity. Such facts may be useful in warning us not to a.s.sume too hastily that the point which our retrospect may have reached at the present moment can be regarded as fixing the date of the first introduction of any one cla.s.s of beings upon the earth.

2. WENLOCK FORMATION.

We next come to the Wenlock formation, which has been divided into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.

a. WENLOCK LIMESTONE.

This limestone, otherwise well known to collectors by the name of the Dudley Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. This ridgy prominence is due to the solidity of the rock, and to the softness of the shales above and below it. Near Wenlock it consists of thick ma.s.ses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed "ball-stones" in Shropshire, are often enormous, even 80 feet in diameter. They are of pure carbonate of lime, the surrounding rock being more or less argillaceous (Murchison's Siluria chapter 6.) Sometimes in the Malvern Hills this limestone, according to Professor Phillips, is oolitic.

(FIGURE 536. Halysites catenularius, Linn. sp. Upper and Lower Silurian.)

(FIGURE 537. Favosites Gothlandica, Lam. Dudley.

a. Portion of a large ma.s.s; less than the natural size.

b. Magnified portion, to show the pores and the part.i.tions in the tubes.)

(FIGURE 538. Omphyma turbinatum, Linn. Sp. (Cyathophyllum, Goldfuss) Wenlock Limestone, Shropshire.)

Among the corals, in which this formation is so rich, 53 species being known, the "chain-coral," Halysites catenularius (Figure 536), may be pointed out as one very easily recognised, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to near the bottom of the Llandeilo rocks. Another coral, the Favosites Gothlandica (Figure 537), is also met with in profusion in large hemispherical ma.s.ses, which break up into columnar and prismatic fragments, like that here figured (Figure 537, b).

Another common form in the Wenlock limestone is the Omphyma turbinatum (Figure 538), which, like many of its modern companions, reminds us of some cup-corals; but all the Silurian genera belong to the palaeozoic type before mentioned (Chapter 24), exhibiting the quadripart.i.te arrangement of the septalamellae within the cup.

(FIGURE 539. Pseudocrintes bifasciatus, Pearce. Wenlock Limestone, Dudley.)

Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus see Figures 478, 479) contribute their calcareous stems, arms, and cups towards the composition of the Wenlock limestone. Of Cystideans there are a few very remarkable forms, most of them peculiar to the Upper Silurian formation, as, for example, the Pseudocrinites, which was furnished with pinnated fixed arms, as represented in Figure 539. (E. Forbes Mem. Geological Survey volume 2 page 496.)

(FIGURE 540. Strophomena (Leptaena) depressa, Sowerby. Wenlock and Ludlow Rocks.)

The Brachiopoda are, many of them, of the same species as those of the Aymestry limestone; as, for example, Atrypa reticularis (Figure 532), and Strophomena depressa (Figure 540); but the latter species ranges also from the Ludlow rocks, through the Wenlock shale, to the Caradoc Sandstone.

(FIGURE 541. Calymene Blumenbachii, Brong. Ludlow, Wenlock, and Bala beds.)

(FIGURE 542. Phacops (Asaphus) caudatus, Brong. Wenlock and Ludlow Rocks.)

(FIGURE 543. Sphaerexochus mirus, Beyrich; coiled up. Wenlock Limestone, Dudley; also found in Ohio, North America.)

(FIGURE 544. Homalonotus delphinocephalus, Konig. Wenlock Limestone, Dudley Castle.)

The crustaceans are represented almost exclusively by Trilobites, which are very conspicuous, 22 being peculiar. The Calymene Blumenbachii (Figure 541), called the "Dudley Trilobite," was known to collectors long before its true place in the animal kingdom was ascertained. It is often found coiled up like the common Oniscus or wood-louse, and this is so usual a circ.u.mstance among certain genera of trilobites as to lead us to conclude that they must have habitually resorted to this mode of protecting themselves when alarmed. The other common species is the Phacops caudatus (Asaphus caudatus), Brong. (see Figure 542), and this is conspicuous for its large size and flattened form. Sphaerexochus mirus (Figure 543) is almost a globe when rolled up, the forehead or glabellum of this species being extremely inflated. The Homalonotus, a form of Trilobite in which the tripart.i.te division of the dorsal crust is almost lost (see Figure 544), is very characteristic of this division of the Silurian series.

WENLOCK SHALE.