November, 1928 



EVOLUTION 



Page Five 



Fossil Footprints 



By Frederick A. Lucas, 



tlunoniry Director, American Museum of Natural History 



EMERSON'S saying that "Everything in Nature is engaged 

 in writing its own history" applies aptly to those 

 animals of yore that left their footprints on the sands of sea- 

 shore, mud-flats of drying lakes or beds of shrunken water- 

 courses. So too, worms burrowing in the sand, shell-fish trailing 

 over mud at low tide, stranded crabs scuttling off to sea, all 

 left their tell-tale records. Even falling rain and blowing wind 

 disclosed the directions whence they came, or we may read in 

 the record how turbid freshets swept down, perhaps after long 

 drouths, when the sun had baked the drying lake bottoms. 



Among the earliest signs of animal life on this globe are 

 some long, dark streaks below the Cambrian in England, thought 

 to be worm burrows filled with fine mud. Above these worm 

 borings, in the middle Cambrian, we find abundant remains of 

 the worms themselves and simple shellfish. From that time on 

 there are tracks aplenty, made whenever conditions favored. We 

 find tracks formed in sands alternately dry and submerged by 

 tide or river, or in soft earth filled with sand or mud. 



First came tracks of invertebrates — those worm burrows; 

 curious, complicated trails of the king-crab kindred; broad- 

 ribbed, ribbon like trilobite paths; even faint scratches of insects. 

 Later came footprints of the back-boned tribe; shallow, palmed 

 prints of salamanders; slender lizard sprawls, real footprints, 

 big and little of the Dinosaur horde; and finally, miles above the 

 Cambrian, marks of mammals. Often the footprints are all we 

 have, but in some cases, as with the dinosaur Iguanodon, we 

 also have the fossil foot to fit the print. 



The now famous three-toed dinosaur tracks in the Connecti- 

 cut valley "brownstone", first seen in 1802, were thought to be 



•bird tracks and popularly called the tracks of Noah's raven. 

 Dr. Deane first guessed the truth that they were due to other 

 animals, partly because some prints showed four and five toes 

 and the texture of the sole of the foot, unlike that of any known 

 bird. Certain long tracks and heart shaped depressions made by 

 hip-bones showed where some dinosaur squatted down to rest. 



Dinusaur luutpriiits on Cuniiccticul \ alley Hiowiistunc. 

 Courtesy Am. Mus. Nat. Hist. 



Dinosaur trjck! Where .i dcnovjur s.it down. 



This part of the Connecticut valley was a river bed or long 

 narrow estuary running southward from Turner's Falls, Massa- 

 chusetts, where the tracks are most clear and abundant, subject 

 to sudden changes of water level, now left dry to bake in the 

 sun and again covered with water depositing a layer of mud. 



The wealth of animal life roaming this estuary may be 

 gathered from the listing of over 150 species. Mere size is 

 however no sure indication of differences in reptiles which grow 

 continuously through their long lives. A single animal may have 

 left his footprints over and over in assorted sizes. 



The fine brownstone slab here illustrated measures three by 

 five feet and shows 48 tracks of Protozoum Sillimanium and 

 6 of a lesser species. Quarried near Middletown in 1778, it did 

 duty as a flagstone for sixty years, fortunately with the face 

 down. When taken up for repairs, the tracks were discovered 

 and it was transferred to Amherst College. 



Footprints began to be noticed about 1830 in both England 

 and America, in each case in Triassic rocks. The English tracks 

 were from both dinosaurs and tortoises. Oddly, in both instances 

 the tracks run from west to east, as along a customary migration 

 route, but the animals themselves have not been found. From 

 strata of similar age in the Rhine valley come marks so like a 

 stubby hand that the animal was christened Cheirotherium, 

 ''beast with a hand" and taken for gigantic oppossum. But mam- 

 mals had not yet arisen. The marks were due to giant sal- 

 amander-like labyrinthodonts, found in the same strata. 



Footprints may tell the attitude assumed by extinct animals. 

 Some fine Iguanodon tracks from England and Belgium furnish 

 conclusive proof that many dinosaurs walked erect. Made in 

 soft soil into which the feet sank deeply, the impressions of the 

 toes show very clearly. Had they walked flat footed as we 

 do, long heel marks would have followed the toes. Their 

 absence shows plainly that Iguanodons walked on their toes like 

 birds. Where crocodilians and some short-limbed dinosaurs have 

 crept along we find a continuous furrow between the foot-prints. 

 Since none was found here, we may conclude that these great 

 creatures carried their tails clear of the ground. 



According to the papers, some footprints in the prison-yard 

 at Carson City, Nevada, were made by primitive men of giant 

 stature. They were such as miglit have been made by huge 

 moccasined feet, the papers therefore concluding that they iDere 

 so made. Similarly, Mammoth and Mastodon bones have been 

 eagerly accepted as those of giants; a salamander was used as 

 proof of the deluge; and the "petrified man" flourishes peren- 

 nially. These prints were, however, those of some great ground 

 sloth, a group ranging from Patagonia to Oregon. They looked 

 like tracks of a bi-ped because the hind footprints usually fell 

 upon and obliterated those of the fore-feet. But a few prints of 

 the fore-feet were found, also indications of a struggle between 

 two of the big beasts, for one set of imprints is deeper at the 

 toes, the other at the heels, as if one animal pushed and the 

 other resisted. Some broad depressions with marks of coarse 

 hair show where one sloth sat on its haunches. This prison-yard 

 also contains a great round "spoor" of a mammoth, the hoof 

 prints of a deer and the paw-marks of a wolf( ?), indicating that 

 hereabouts was a pool where all these creatures came to drink. 



