Million Years Ago in Scotland

One of the most plentiful fossil locations for early terrestrial life is the Rhynie Chert in the north of Scotland. Located near the town of Rhynie, in Aberdeenshire, the site contains fossils from a time when that part of Scotland was located south of the equator, about 400 million years ago. Chert is a quartz-related mineral that occurs naturally from the breakdown of minerals in rocks or decayed organic matter. In the case of the Rhynie Chert, silica deposits literally rained down from geysers or were spread by the action of hot springs to cover and preserve many fine examples of terrestrial plants and arthropods. Most of the plants known from this location probably grew close to the water, where early soils were most common, because the surrounding volcanically active geography was not as welcoming to plant life.

Mining the Rhynie Chert for specimens is quite different from scouting sedimentary rocks for visible signs of fossils. Most of the goodies are locked inside the quartz block and are invisible to the naked eye until examined in a laboratory. The first step in the mining procedure is to use a rock saw to cut a chunk of chert into smaller slices. These slices are then cleaned and polished; this reveals more details within the rock as well as possible signs of organic remains. Because most of the specimens, especially of arthropods, are tiny, a microscope is needed to study them closely. Portions of the larger chert slices that contain specimens are further cut into microscopically thin slices of rock that can be viewed under a microscope. These thin, polished slices of rock are translucent. This allows light to seep through the slices to reveal further detail of the specimens found inside.

Among the many fossils found in the Rhynie Chert are the oldest known insect species, Rhyniella and Rhyniognatha. As many as 15 species of early terrestrial and near-shore arthropods have been identified in the chert as well as seven species of early land plants.

Fossilization by means of a hot-spring bath preserved dead organisms before they had a chance to decay. This resulted in some remarkable specimens, with exquisite detail down to their very cells. In addition to

Rhynie Scotland, 400 million years ago. Fossils from this location today reveal the nature of the earliest land plants and insects. The area was once subject to frequent volcanic activity.

early insects, the Rhynie Chert also preserved spectacular arthropods called trigonotarbids—ancestors of modern spiders. Most chert insect specimens are quite small in size, with body lengths in the range of one-quarter inch (5 mm). Despite their small size, the chert specimens reveal minute details such as mouthparts, muscle tendons, and respiratory elements of early insects.

Plants of the chert were mostly l ow-growing sprigs that rarely grew more than eight to 16 inches (20 to 40 cm) tall. Horneophyton had upright, leafless, water-conducting branches that bore spores in small, protective,

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cuplike chambers on the branch tips. Lycopodium was a short plant with a central trunk covered with scalelike overlapping leaves. One of the first successful upright vascular plants was Rhynia, which had a porous covering on its outer stem that moderated water loss while its inner structure of vascularized tissue gave it strength to hold itself up.

British paleontologist Richard Fortey, who specializes in Paleozoic life, remarks that the community of organisms found in the Rhynie Chert is extraordinary not because of "how different they are from their equivalents in the living world," but because of "how remarkably similar" they are. What can be seen in the chert is evidence that plants and animals were living in symbiosis—close, mutually dependent association—nearly as soon as they both ventured out of the water. This mutual dependency is seen today in natural habitats all over the world.

Fragment of the early insect Rhyniognatha shows the large mandibles of the mouth and other parts

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with the operation of the spider's frontmost legs. As a result, spiders possess excellent mobility. The pedipalps of spiders often are tipped with a hook for holding prey. The chelicerae are the fangs of the spider; they deliver debilitating venom and help to tear apart the prey. Spiders do not have teeth. After the spider's venom has immobilized its prey, the spider liquefies the prey's insides by injecting enzymes from its own digestive track into the prey. The spider then sucks the soupy meal into itself using the pumping action of its stomach.

Terrestrial spiders clearly evolved from a line of marine arthropod species sometime during the Silurian Period, but their ancestors are a matter of speculation. One extinct, ancestral group of arachnids dating from the Late Silurian and terminating in the Early Permian is the Trigonotarbida. These were not true spiders, as they lacked silk glands and chelicerae equipped with venom glands, but they could have been related to true spiders through a common ancestor. The earliest trigonotarbids date from the Early Devonian deposits of Scotland, Wales, Germany, New York, and Canada.

The true spiders are part of the arachnid group Araneae, which includes more than 40,000 extant species. Within the Araneae are the Mygalomorphae, a group that consists of tarantulas and their closest kin, and the Opisthothelae, a group that includes most common spiders.

Along with the scorpions, spiders and the now-extinct trigono-tarbids were among the first land animals. The earliest true spider— with silk glands—is Attercopus, found in rocks of Middle Devonian age in Gilboa, New York. This taxon and other early spiders had the silk-spinning spinneret in the middle of the abdomen; this probably limited the use of the silk to such chores as covering a nest or providing a crude, ground-based trap for passing insects. Spiders with a spinneret positioned on the lower abdomen are placed in a special group called the Mesothelae. The Mesothelae were the most common type of spider during the Paleozoic, but only a few species of mesothelids are still living.

The Opisthothelae include the most common types of spiders, with the spinneret placed at the posterior tip of the abdomen. This type of spider first appears in the fossil record about 250 million years ago, at the end of the Permian Period. Having the silk gland positioned at the spider's rear end made it possible for the creature

MAJOR ORDERS OF LAND ARTHROPODS AND THEIR ORIGINS

Arthropod Group

Characteristics

Members

Early Fossil Evidence for Terrestrial Arthropods

Crustaceans

Biting jaws; two-branched appendages; two pairs of antennae

Lobsters, crabs, shrimp, isopods (pill bugs), barnacles

Marine species (Early Cambrian); there were no terrestrial forms during the Paleozoic; pill bugs evolved later, during the Mesozoic Era.

Centipedes and

Millipedes

Biting jaws; body consisting of head and numerous body segments; 30-plus single-branched legs

Centipedes, millipedes

Trace fossils of millipede burrows (Late

Ordovician)

Specimens such as Casiogrammus (Middle Silurian, Scotland)

Arachnids

Pincers or fangs for mouthparts; eight legs

Spiders, mites, ticks, scorpions, daddy longlegs

Scorpions

Branchioscorpio, an Early Devonian transitional form with legs for walking on land but aquatic lungs

Archaeophonus and Proscorpius (Late Silurian,

New York)

Spiders

Evidence of Middle Devonian spiders exists in the form of a fossil spinneret, the silk-making part of a spider (New York). Gigantocharinus (Late Devonian, Pennsylvania)

Eophrynus (Late Carboniferous, England) (Anthracomartus (Late Carboniferous, Poland)

Insects

Biting jaws; six single-branched legs; three body parts; one pair of antennae

Beetles, flies, bees, fleas, true bugs, grasshoppers, butterflies, termites

Proto-insects (Early Devonian)

First true insect: Rhyniella (Early Devonian,

Scotland)

to weave fancy webs and elevate its habitat to the upper branches of ferns and trees, where there were many flying insects to be captured. The posterior spinneret also provided the safety line that a spider uses when it hangs or falls from a high spot.

The tarantulas, or mygalomorphids, also trace their origins to the Late Paleozoic. These are not web-spinning members of the

Araneae. In contrast to the Opisthothelae, mygalomorphids stalk and attack their prey on the ground, sometimes ambushing them. Fossil specimens include probable mygalomorphid trackways from Early Permian sandstone deposits in Arizona.

In the annals of fossil spiders, one might expect there to be a giant or two. In 1980, a fossil arachnid from Late Carboniferous deposits in Argentina temporarily held the world record for the largest spider. The fossil revealed a large, 16-inch (40 cm) body and a leg span of 20 inches (50 cm). This all seemed clear enough to the team of scientists that first examined the specimen, and the creature, dubbed Megarachne, or mega-spider, was even given a dubious place of honor in the Guinness Book of World Records. Twenty-five years after its discovery, however, arachnid expert Paul Selden of Manchester University in England had an opportunity to look over the spectacular fossil. To his surprise, he found that it was not a spider at all, but an aquatic animal, probably a euryptid. Selden's certainty was assured. "It has large claws and two big compound eyes," he explained, "whereas spiders normally have eight small eyes. It also appears to have a very robust body or shell with ridges across its back which is not found in any spider known to man." What was once thought to be the largest spider ever known turned out to be a crusty sea scorpion, but an odd one. For the record—that is, the Guinness Book of World Records—the honor of being the world's largest spider fell back to the previous titleholder, a specimen of male goliath bird-eating spider (Thera-phosa) found in Venezuela in 1965. That specimen had a leg span of 11 inches (28 cm).

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