trilobites

59:1.1By the dawn of this pe­riod of rel­a­tive quiet on the earth’s sur­face, life is con­fined to the var­i­ous in­land seas and the oceanic shore line; as yet no form of land or­gan­ism has evolved. Prim­i­tive ma­rine an­i­mals are well es­tab­lished and are pre­pared for the next evo­lu­tion­ary de­vel­op­ment. Ame­bas are typ­i­cal sur­vivors of this ini­tial stage of an­i­mal life, hav­ing made their ap­pear­ance to­ward the close of the pre­ced­ing tran­si­tion pe­riod.

59:1.2400,000,000 years ago ma­rine life, both veg­etable and an­i­mal, is fairly well dis­tributed over the whole world. The world cli­mate grows slightly warmer and be­comes more equable. There is a gen­eral in­un­da­tion of the seashores of the var­i­ous con­ti­nents, par­tic­u­larly of North and South Amer­ica. New oceans ap­pear, and the older bod­ies of wa­ter are greatly en­larged.

59:1.3Veg­e­ta­tion now for the first time crawls out upon the land and soon makes con­sid­er­able prog­ress in adap­ta­tion to a non­ma­rine habi­tat.

59:1.4Sud­denly and with­out gra­da­tion an­ces­try the first mul­ti­cel­lu­lar an­i­mals make their ap­pear­ance. The trilo­bites have evolved, and for ages they dom­i­nate the seas. From the stand­point of ma­rine life this is the trilo­bite age.

59:1.5In the later por­tion of this time seg­ment much of North Amer­ica and Eu­rope emerged from the sea. The crust of the earth was tem­porar­ily sta­bi­lized; moun­tains, or rather high el­e­va­tions of land, rose along the At­lantic and Pa­cific coasts, over the West In­dies, and in south­ern Eu­rope. The en­tire Caribbean re­gion was highly el­e­vated.

59:1.6390,000,000 years ago the land was still el­e­vated. Over parts of east­ern and west­ern Amer­ica and west­ern Eu­rope may be found the stone strata laid down dur­ing these times, and these are the old­est rocks which con­tain trilo­bite fos­sils. There were many long fin­ger­like gulfs pro­ject­ing into the land masses in which were de­posited these fos­sil-bear­ing rocks.

59:1.7Within a few mil­lion years the Pa­cific Ocean be­gan to in­vade the Amer­i­can con­ti­nents. The sink­ing of the land was prin­ci­pally due to crustal ad­just­ment, al­though the lat­eral land spread, or con­ti­nen­tal creep, was also a fac­tor.

59:1.8380,000,000 years ago Asia was sub­sid­ing, and all other con­ti­nents were ex­pe­ri­enc­ing a short-lived emer­gence. But as this epoch pro­gressed, the newly ap­pear­ing At­lantic Ocean made ex­ten­sive in­roads on all ad­ja­cent coast lines. The north­ern At­lantic or Arc­tic seas were then con­nected with the south­ern Gulf wa­ters. When this south­ern sea en­tered the Ap­palachian trough, its waves broke upon the east against moun­tains as high as the Alps, but in gen­eral the con­ti­nents were un­in­ter­est­ing low­lands, ut­terly de­void of scenic beauty.

59:1.9The sed­i­men­tary de­posits of these ages are of four sorts:

59:1.101. Con­glom­er­ates—mat­ter de­posited near the shore lines.

59:1.112. Sand­stones—de­posits made in shal­low wa­ter but where the waves were suf­fi­cient to pre­vent mud set­tling.

59:1.123. Shales—de­posits made in the deeper and more quiet wa­ter.

59:1.134. Lime­stone—in­clud­ing the de­posits of trilo­bite shells in deep wa­ter.

59:1.14The trilo­bite fos­sils of these times pre­sent cer­tain ba­sic uni­for­mi­ties cou­pled with cer­tain well-marked vari­a­tions. The early an­i­mals de­vel­op­ing from the three orig­i­nal life im­plan­ta­tions were char­ac­ter­is­tic; those ap­pear­ing in the West­ern Hemi­sphere were slightly dif­fer­ent from those of the Eurasian group and from the Aus­tralasian or Aus­tralian-Antarc­tic type.

59:1.15370,000,000 years ago the great and al­most to­tal sub­mer­gence of North and South Amer­ica oc­curred, fol­lowed by the sink­ing of Africa and Aus­tralia. Only cer­tain parts of North Amer­ica re­mained above these shal­low Cam­brian seas. Five mil­lion years later the seas were re­treat­ing be­fore the ris­ing land. And all of these phe­nom­ena of land sink­ing and land ris­ing were un­dra­matic, tak­ing place slowly over mil­lions of years.

59:1.16The trilo­bite fos­sil-bear­ing strata of this epoch out­crop here and there through­out all the con­ti­nents ex­cept in cen­tral Asia. In many re­gions these rocks are hor­i­zon­tal, but in the moun­tains they are tilted and dis­torted be­cause of pres­sure and fold­ing. And such pres­sure has, in many places, changed the orig­i­nal char­ac­ter of these de­posits. Sand­stone has been turned into quartz, shale has been changed to slate, while lime­stone has been con­verted into mar­ble.

59:1.17360,000,000 years ago the land was still ris­ing. North and South Amer­ica were well up. West­ern Eu­rope and the British Isles were emerg­ing, ex­cept parts of Wales, which were deeply sub­merged. There were no great ice sheets dur­ing these ages. The sup­posed glacial de­posits ap­pear­ing in con­nec­tion with these strata in Eu­rope, Africa, China, and Aus­tralia are due to iso­lated moun­tain glaciers or to the dis­place­ment of glacial de­bris of later ori­gin. The world cli­mate was oceanic, not con­ti­nen­tal. The south­ern seas were warmer then than now, and they ex­tended north­ward over North Amer­ica up to the po­lar re­gions. The Gulf Stream coursed over the cen­tral por­tion of North Amer­ica, be­ing de­flected east­ward to bathe and warm the shores of Green­land, mak­ing that now ice-man­tled con­ti­nent a ver­i­ta­ble tropic par­a­dise.

59:1.18The ma­rine life was much alike the world over and con­sisted of the sea­weeds, one-celled or­gan­isms, sim­ple sponges, trilo­bites, and other crus­taceans—shrimps, crabs, and lob­sters. Three thou­sand va­ri­eties of bra­chiopods ap­peared at the close of this pe­riod, only two hun­dred of which have sur­vived. These an­i­mals rep­re­sent a va­ri­ety of early life which has come down to the pres­ent time prac­ti­cally un­changed.

59:1.19But the trilo­bites were the dom­i­nant liv­ing crea­tures. They were sexed an­i­mals and ex­isted in many forms; be­ing poor swim­mers, they slug­gishly floated in the wa­ter or crawled along the sea bot­toms, curl­ing up in self-pro­tec­tion when at­tacked by their later ap­pear­ing en­e­mies. They grew in length from two inches to one foot and de­vel­oped into four dis­tinct groups: car­niv­o­rous, her­biv­o­rous, om­niv­o­rous, and “mud eaters.” The abil­ity of the lat­ter group largely to sub­sist on in­or­ganic mat­ter—be­ing the last mul­ti­celled an­i­mal that could—ex­plains their great in­crease and long sur­vival.

59:1.20This was the bio­geo­logic pic­ture of Uran­tia at the end of that long pe­riod of the world’s his­tory, em­brac­ing fifty mil­lion years, des­ig­nated by your ge­ol­o­gists as the Cam­brian.

59:2.10The trilo­bites were still promi­nent. These lit­tle an­i­mals ex­isted in tens of thou­sands of pat­terns and were the pre­de­ces­sors of mod­ern crus­taceans. Some of the trilo­bites had from twenty-five to four thou­sand tiny eye­lets; oth­ers had aborted eyes. As this pe­riod closed, the trilo­bites shared dom­i­na­tion of the seas with sev­eral other forms of in­ver­te­brate life. But they ut­terly per­ished dur­ing the be­gin­ning of the next pe­riod.

59:3.5The trilo­bites rapidly de­clined, and the cen­ter of the stage was oc­cu­pied by the larger mol­lusks, or ceph­a­lop­ods. These an­i­mals grew to be fif­teen feet long and one foot in di­am­e­ter and be­came mas­ters of the seas. This species of an­i­mal ap­peared sud­denly and as­sumed dom­i­nance of sea life.

59:3.11To­ward the close of the fi­nal Sil­urian sub­mer­gence there is a great in­crease in the echin­o­derms—the stone lilies—as is ev­i­denced by the crinoid lime­stone de­posits. The trilo­bites have nearly dis­ap­peared, and the mol­lusks con­tinue monar­chs of the seas; coral-reef for­ma­tion in­creases greatly. Dur­ing this age, in the more fa­vor­able lo­ca­tions the prim­i­tive wa­ter scor­pi­ons first evolve. Soon there­after, and sud­denly, the true scor­pi­ons—ac­tual air breathers—make their ap­pear­ance.

59:5.8Two groups of echin­o­derms be­came es­pe­cially well de­vel­oped, and they are in re­al­ity the guide fos­sils of this epoch. The large shell-feed­ing sharks were also highly evolved, and for more than five mil­lion years they dom­i­nated the oceans. The cli­mate was still mild and equable; the ma­rine life was lit­tle changed. Fresh-wa­ter fish were de­vel­op­ing and the trilo­bites were near­ing ex­tinc­tion. Corals were scarce, and much of the lime­stone was be­ing made by the crinoids. The finer build­ing lime­stones were laid down dur­ing this epoch.