A part of the fantastic living world of 200,000,000 years ago has been dissolved out of about thirty tons of yellowish-brown limestone by a Smithsonian paleontologist.
The rock comes from a low mountain range in southwestern Texas—the Glass Mountains, about 250 miles east of El Paso. During the Permean geological period, when some of the earliest known forms of animal life appeared on land, the site of the Glass Mountains was a muddy bottom, probably close to the shore of a warm sea. A bewildering array of animals lived in that sea. They died and eventually were buried in the mud. In some cases their bodies were covered with silica. In others silica replaced the shells. When these rocks are placed in hydrochloric acid the limestone is eaten away but the silica shells remain. Years of skilled labor would be required to chip out of the rock what is obtained in a few days in the acid bath.
Most abundant animals of the ancient Texas sea were the brachiopods or lampshells—essentially shelled worms. The broad road of life is strewn with derelicts, stragglers and deserters. Among the most notable among them are these obscure creatures which, in numbers and apparent prosperity, seem to have been close to the dominant animals in the world in the days when giant amphibians, remotely related to present frogs and toads, and monster scorpions were establishing themselves on dry land.
Brachiopods were among the first animals to leave any traces on earth a half billion years ago. Even at that time they were complex creatures, with nerves and stomachs, which indicate a long ancestry before they left any fossil remains. In the tepid Permian seas they reached their climax in numbers and variety. They survive today, but only in a few places. For all practical purposes they are now among the most obscure animals in existence. In the whole world there are about 110 extant species compared to nearly 500 which Dr. G. Arthur Cooper, Smithsonian Institution curator of invertebrate paleontology, and his associates have obtained from one small area of the Glass Mountain limestones.
The existing brachiopod might be mistaken for a small clam. Zoologically, it is an intermediate form between mollusks and annelid worms, and somewhat closer to the latter than the former. Its way of life actually is nearer to that of an oyster than to that of most worms. It now is believed to be most closely related, through some unknown common ancestor, to the bryozoa or lace weavers. In the past both were classified together. The brachiopod never has become a colonial animal.
Its body is enclosed completely in a shell, secreted by the skin or “mantle”, except for a muscular, stalk-like extension, the peduncle, by which it attaches itself to the sea bottom. Inside the shell, folded around the mouth when the animal is at rest, are two arms or tentacles with which it can probe the water and obtain minute food particles. It also apparently breathes through these tentacles, which have a rapid blood circulation.
Most numerous of the extant brachiopods is a curious animal, the lingula, which is nearly world-wide in distribution and whose peduncle is used for food in both Japan and the Philippine Islands. Along the Atlantic coast it is present from Chesapeake Bay to Florida. It makes a nearly vertical burrow in mud or sand from two to twelve inches deep—within which it lives, attached to the bottom by the peduncle. On this footlike appendage it can lift itself until the front part of the shell-enclosed portion of the body is above the surface. This is withdrawn into the burrow instantly on the slightest alarm. The animal apparently has a quite sensitive, although very primitive, nervous system.
The extant brachiopods are usually small animals but in their Permian heyday some attained a length of more than six inches. For essentially 200,000,000 years they were without much competition in the mud burrows to which they had resorted. During this time arose clams, sea snails, and other mollusks which were free to move about and competed with them for the available food supply. The brachiopod was unable to meet this vigorous competition and in a few million years the race was well on its way towards extinction. Most species disappeared. A few, including the Lingula, survived into the age of the great dinosaurs, and their descendants constitute the species living today. They are now obscure creatures and a poverty-stricken group compared to their ancestors.
In the Permian seas they had surplus energy to expend not only in variation of form and habit—but in shell artistry. Some of the specimens obtained by the Smithsonian paleontologists are like glittering gems surrounded by silvery, hair-like spines.
These spiny brachiopods constitute about two-thirds of all the fossils obtained from the Glass Mountain rocks. Although the most abundant they were far from the dominant animals of the Permian sea. They always were defenseless little creatures, dependent on their hard, spiny shells for protection. The sea monsters of the day, creatures related to the present chambered nautilus and some of which were nearly two feet in diameter, unquestionably were the lords of this marine creation. But they were free-swimming predators who had little reason for concern with the humble mud-dwellers. Next to the brachiopods in numbers and variety, and probably their chief competitors, were the ancient lace weavers. Both shared forests of sponges which grew like small trees, up to heights of four feet and four to six inches across. Clams, some of which reached the size of giants, were beginning to claim dominion of the offshore mud and the brachiopods were near the end of their prosperous days.