CHAPTER IV.. How Fossils Are Found: and the Rocks They Form.

HOW FOSSILS ARE FOUND: AND THE ROCKS THEY FORM.

As already noticed, it is the hard parts of buried animals and plants that are generally preserved. We will now consider the groups of organisms, one by one, and note the particular parts of each which we may reasonably expect to find in the fossil state.

MAMMALS.--The bones and teeth: as the Diprotodon remains of Lake Callabonna in South Australia (Fig. 14), of West Melbourne Swamp, Victoria, and the Darling Downs, Queensland. Rarely the skin, as in the carcases of the frozen Mammoth of the tundras of Northern Siberia; or the dried remains of the Grypotherium of South American caves.

Exposed on Sand-blow at Seal Bay, King Island.

(Photo by C. L. Barrett). ]

About 2/3 nat. size. Of Cainozoic (? Janjukian) Age. Redruth, Victoria.

(Nat. Mus. Coll.) ]

About 1/4 nat. size. A Marine Turtle from the Lower Cretaceous of Flinders River, Queensland.

(Nat. Mus. Coll.) ]

BIRDS:--Bones: as the Moa bones of New Zealand and the Emu bones of the King Island sand-dunes (Fig. 15). Very rarely the impressions of the feathers of birds are found, as in the ironstone occurring in the Wannon district of Victoria (Fig. 16), and others in fine clays and marls on the continent of Europe and in England. Fossil eggs of sea-birds are occasionally found in coastal sand-dunes of Holocene age.

REPTILES.--Skeletons of fossil turtles (Notochelone) are found in Queensland (Fig. 17). Whole skeletons and the dermal armour (spines and bony plates) of the gigantic, specialised reptiles are found in Europe, North America, and in other parts of the world.

FISHES.--Whole skeletons are sometimes found in sand and clay rocks, as in the Trias of Gosford, New South Wales (Fig. 18), and in the Jurassic of South Gippsland. The ganoid or enamel-scaled fishes are common fossils in the Devonian and Jurassic, notably in Germany, Scotland and Canada: and they also occur in the sandy mudstone of the Lower Carboniferous of Mansfield, Victoria.

INSECTS.--Notwithstanding their fragility, insects are often well preserved as fossils, for the reason that their skin and wings consist of the horny substance called chitin. The Tertiary marls of Europe are very prolific in insect remains (Fig. 19). From the Miocene beds of Florissant, Colorado, U.S.A., several hundred species of insects have been described.

=A Fossil Fish with Ganoid Scales (Pristisomus crassus, A.S. Woodw.).=

About 1/2 nat. size. Trias (Hawkesbury Series), of Gosford, New South Wales.

(Nat. Mus. Coll.) ]

Nat. size. Oligocene beds; Baltic Prussia.

(F.C. Coll.) ]

Slightly reduced. From the Pleistocene of Port Darwin, Northern Territory.

(Nat. Mus. Coll.) ]

Half nat. size. Showing complex sutures. L. Cretaceous: Marathon, Flinders River, Queensland.

(Nat. Mus. Coll.) ]

CRUSTACEA.--The outer crust, or exoskeleton, of these animals is often hard, being formed of a compound of carbonate and phosphate of lime on an organic, chitinous base. The earliest forms of this group were the trilobites, commencing in Cambrian times, and of which there is a good representative series in Australian rocks. Remains of crabs and lobsters are found in the various Cainozoic deposits in Australia (Fig. 20), and also in the Jurassic in other parts of the world.

MOLLUSCA.--The Cuttle-fish group (Cephalopoda, "head-footed"), is well represented by the Nautilus-like, but straight Orthoceras shells commencing in Ordovician times, and, in later periods, by the beautiful, coiled Ammonites (Fig. 21). The true cuttle-fishes possess an internal bone, the sepiostaire, which one may see at the present day drifted on to the sand at high-water mark on the sea-shore. The rod-like Belemnites are of this nature, and occur abundantly in the Australian Cretaceous rocks of South Australia and Queensland (Fig. 22).

1/3 nat. size. Lower Cretaceous. Central South Australia.

(Nat. Mus. Coll.) ]

Attached to a Polyzoan.

About 1/3 nat. size. Dredged from Westernport, Victoria.

(C.J. Gabriel Coll.) ]

Elephant-tusk shells (Scaphopoda) are frequent in our Tertiary beds: they are also sparingly found in the Cretaceous, and some doubtful remains occur in the Palaeozoic strata of Australia.

The shells of the ordinary mollusca, such as the snails, whelks, mussels, and scallops, are abundant in almost all geological strata from the earliest periods. Their calcareous shells form a covering which, after the decay of the animal within, are from their nature among the most easily preserved of fossil remains. There is hardly an estuary bed, lake-deposit, or sea-bottom, but contains a more or less abundant assemblage of these shell-fish remains, or testacea as they were formerly called ("testa," a shell or potsherd). We see, therefore, the importance of this group of fossils for purposes of comparison of one fauna with another (antea, Fig. 1).

The chitons or mail-shells, by their jointed nature, consisting of a series of pent-roof-shaped valves united by ligamental tissue, are nearly always represented in the fossil state by separate valves. Fossil examples of this group occur in Australia both in Palaeozoic rocks and, more numerously, in the Cainozoic series.

2/3 nat. size.

Flinders, Victoria.

(F.C. Coll.) ]

About 1/2 nat. size. Cainozoic (Balcombian). Muddy Creek, Victoria.

(F.C. Coll.) ]

MOLLUSCOIDEA.--The Brachiopods or Lamp-shells consist generally of two calcareous valves as in the true mollusca (Fig. 23), but are sometimes of horny texture. Like the previous class, they are also easily preserved as fossils. They possess bent, loop-like or spiral arms, called brachia, and by the movement of fine ciliated (hair-like) processes on their outer edges conduct small food particles to the mouth. The brachia are supported by shelly processes, to which are attached, in the Spirifers, delicate spirally coiled ribbons. These internal structures are often beautifully preserved, even though they are so delicate, from the fact that on the death of the animal the commissure or opening round the valves is so tightly closed as to prevent the coarse mud from penetrating while permitting the finer silt, and more rarely mineral matter in solution, to pass, and subsequently to be deposited within the cavity. At the Murray River cliffs in South Australia, a bed of Cainozoic limestone contains many of these brachiopod shells in a unique condition, for the hollow valves have been filled in with a clear crystal of selenite or gypsum, through which may be seen the loop or brachial support preserved in its entirety.

The Sea-mats or Polyzoa, represented by Retepora (the Lace-coral) (Fig. 24) and Flustra (the Sea-mat) of the present sea-shore, have a calcareous skeleton, or zoarium, which is easily preserved as a fossil. Polyzoa are very abundant in the Cainozoic beds of Australia, New Zealand, and elsewhere (Fig. 25). In the Mesozoic series, on the other hand, they are not so well represented; but in Europe and North America they play an important part in forming the Cretaceous and some Jurassic strata by the abundance of their remains.

WORMS (VERMES).--The hard, calcareous tubes of Sea-worms, the Polychaeta ("many bristles") are often found in fossiliferous deposits, and sometimes form large masses, due to their gregarious habits of life; they also occur attached to shells such as oysters (Fig. 26). The burrows of the wandering worms are found in Silurian strata in Australia; and the sedentary forms likewise occur from the Devonian upwards.

ECHINODERMATA.--Sea-urchins (Echinoidea) possess a hard, calcareous, many-plated test or covering and, when living are covered with spines (Fig. 27). Both the tests and spines are found fossil, the former sometimes whole when the sediment has been quietly thrown down upon them; but more frequently, as in the Shepherd's crown type (Cidaris), are found in disjointed plates, owing to the fact that current action, going on during entombment has caused the plates to separate. The spines are very rarely found attached to the test, more frequently being scattered through the marl or sandy clay in which the sea-urchins are buried. The best conditions for the preservation of this group is a marly limestone deposit, in which case the process of fossilisation would be tranquil (Fig. 28).

Attached to a Pecten.

Slightly Enlarged. Cainozoic (Balcombian). Muddy Creek, Hamilton, Victoria.

(F. C. Coll.) ]

=A Regular Sea-Urchin (Strongylocentrotus erythrogrammus, Val.)=

About 2/3 nat. size. Showing Spines attached. Living. Victoria.

(F. C. Coll.) ]

Test denuded of Spines.

About 2/3 nat. size. Cainozoic (Janjukian): Curlewis, Victoria.

(Nat. Mus. Coll.) ]

About 1/2 nat. size. A Brittle Star from the Lias of Seaton, Devon, England.

(Nat. Mus. Coll.) ]

The true Starfishes (Asteroidea), are either covered with calcareous plates, or the skin is hardened by rough tubercles; and these more lasting portions are preserved in rocks of all ages. The shape of the animal is also often preserved in an exquisite manner in beds of fine mud or clay.

The Brittle-stars (Ophiuroidea) have their body covered with hard, calcareous plates. Their remains are found in rocks as old as the Ordovician in Bohemia but their history in Australia begins with the Silurian period (Fig. 29). From thence onward they are occasionally found in successive strata in various parts of the world.

The bag-like echinoderms (Cystidea) form a rare group, restricted to Palaeozoic strata. The plates of the sack, or theca, and those of the slender arms are calcareous, and are capable of being preserved in the fossil state. A few doubtful remains of this group occur in Australia.

The bud-shaped echinoderms (Blastoidea) also occur chiefly in Devonian and Carboniferous strata. This is also a rare group, and is represented by several forms found only in New South Wales and Queensland.

The well known and beautiful fossil forms, the Stone-lilies (Crinoidea) have a very extended geological history, beginning in the Cambrian; whilst a few species are living in the ocean at the present day. The many-jointed skeleton lends itself well to fossilisation, and remains of the crinoids are common in Australia mainly in Palaeozoic strata (Fig. 30). In Europe they are found abundantly also in Jurassic strata, especially in the Lias.

=A Fossil Crinoid (Taxocrinus simplex, Phillips sp.)=

About 1/2 nat. size.

Wenlock Limestone (Silurian), Dudley, England.

(Nat. Mus. Coll.) ]

Nat. Size. Lower Ordovician. Bendigo, Victoria.

(Nat. Mus. Coll.) ]

=Polished Vertical Section of a Stromatoporoid. (Actinostroma).=

Nat. size. Middle Devonian. South Devon, England.

(F.C. Coll.) ]

HYDROZOA.--The Graptolites ("stone-writing") have a chitinous skin (periderm) to the body or hydrosome, which is capable of preservation to a remarkable degree; for their most delicate structures are preserved on the surfaces of the fine black mud deposits which subsequently became hardened into slates. In Australia graptolites occur from the base of the Ordovician to the top of the Silurian (Fig. 31).

Another section of the Hydrozoa is the Stromatoporoidea. These are essentially calcareous, and their structure reminds one of a dense coral. The polyps build their tiers of cells (coenosteum) in a regular manner, and seem to have played the same part in the building of ancient reefs in Silurian, Devonian and Carboniferous times as the Millepora at the present day (Fig. 32).

Photograph of a Polished Slab, 2/3 nat. size. In Devonian Limestone, Buchan, Victoria.]

Probably Chonelasma.

× 4. Mauritius. (Viewed in Two Directions.)

(F.C. Coll.) ]

ANTHOZOA.--The true Corals have a stony skeleton, and this is capable of easy preservation as a fossil. There is hardly any fossiliferous stratum of importance which has not its representative corals. In Australia their remains are especially abundant in the Silurian, Devonian (Fig. 33), and Carboniferous formations, and again in the Oligocene and Miocene.

SPONGES.--The framework of the sponge may consist either of flinty, calcareous, or horny material (Fig. 34). The two former kinds are well represented in our Australian rocks, the first appearing in the Lower Ordovician associated with graptolites, and again in the Cretaceous and Tertiary rocks (Fig. 35); whilst the calcareous sponges are found in Silurian strata, near Yass, and again in the Cainozoic beds of Flinders, Curlewis and Mornington in Victoria.

=Spicules of a Siliceous Sponge (Ecionema newberyi, McCoy sp.)=

Highly magnified. Cainozoic Shell-Marl.

Altona Bay Coal-Shaft.]

=Nummulites (N. gizehensis Ehr. var. champollioni, de la Harpe).=

About nat. size. Middle Eocene Limestone. Cyrene, Northern Africa.

(Coll. by Dr. J. W. Gregory). ]

PROTOZOA.--The important and widely-distributed group of the Foraminifera ("hole-bearers") belonging to the lowest phylum, the Protozoa, generally possess a calcareous shell. The tests range in size from tiny specks of the fiftieth of an inch in diameter, to the giant Nummulite, equalling a five shilling piece in size (Fig. 36). Their varied and beautiful forms are very attractive, but their great interest lies in their multifarious distribution in all kinds of sediments: they are also of importance because certain of the more complex forms indicate distinct life zones, being restricted to particular strata occurring in widely-separated areas.

× 58. Late Cainozoic Age. Bissex Hill, Barbados, West Indies.

(F.C. Coll.) ]

Members of the allied order of the Radiolaria have a flinty shell (Fig. 37); and these organisms are often found building up siliceous rocks such as cherts (Fig. 38).

PLANTS.--The harder portions of plants which are found in the fossil state are,--the wood, the coarser vascular (vessel-bearing) tissue of the leaves, and the harder parts of fruits and seeds.

Fossil wood is of frequent occurrence in Palaeozoic, Mesozoic and Cainozoic strata in Australia, as, for instance, the wood of the trees called Araucarioxylon and Dadoxylon in the Coal measures of New South Wales (see antea, Fig. 3).

× 40. Middle Devonian: Tamworth, New South Wales.

(From Prof. David's Collection). ]

Nat. size. Pleistocene: near Hobart, Tasmania.

(Nat. Mus. Coll.) ]

Fossil leaves frequently occur in pipe-clay beds, as at Berwick, Victoria, and in travertine from near Hobart, Tasmania (Fig. 39). Fossil fruits are found in abundance in the ancient river gravels at several hundreds of feet below the surface, in the "deep leads" of Haddon, Victoria, and other localities in New South Wales, Queensland and Tasmania.

About 4/5 nat. size. Mount Arapiles, Western Victoria.

(Nat. Mus. Coll.) ]

With Brachiopods. About 2/3 nat. size. Near Lilydale, Victoria.

(F.C. Coll.) ]

FOSSILIFEROUS ROCKS.