Fossils of the Carpathian Region

Glossary

431 GLOS SA RY This section contains short explanations, in alphabetical order, of some selected terms and phrases which are not otherwise defined in the main text or in the figure captions of the book. Ammonite. Ammonites are extinct marine invertebrates. From a geological point of view, ammonites are the most important of the lineages of cephalopods—the clade that also include octopus, squid, cuttlefish, and nautiloids. The name derives from the Egyptian god Ammon, who was often depicted ornamented with two coiled ram’s horns behind each of his ears; these resemble the generally spirally coiled shells of ammonites. The smallest adult-sized ammonites were about 1–2 centimeters across the spiral, with the largest on record exceeding 1.5 meters. Most had a planispiral flat coil but others had straight shells or loosely coiled spirals, the most bizarre of the shapes to evolve. The most spectacular, socalled heteromorph (mostly uncoiled) forms were especially frequent in the Cretaceous. Ammonites had an outer shell that began with a minute, bubble-like chamber (called the protoconch) and then continued into a chambered region (the phragmocone). The successive chambers that made up the shell became progressively larger and were separated by walls known as septa. The last, and largest, chamber was the body chamber, where the soft body of the animal was situated ; a thin tube (called the “siphuncle”) passed through the septa, connecting it with the chambers of the phragmocone. Using this tube the ammonite was able to control the relative proportions of liquid and gas inside its shell and, thus, its buoyancy. With this mechanism, ammonites were able to rise and fall in the water column. The intersection of the septum with the outer shell, a structure called the suture line, was often very complicated in these cephalopods, and had many folded, curved saddles and pointed lobes. Indeed, the suture line is characteristic of certain groups of ammonites; their shells were either smooth or, in most cases, slightly or heavily ornamented with simple or branching ribs—which could be either weak or strong—marked by spines or rows of tubercles. The ventral portion of their shells had one or more grooves or was keeled. Although we know a lot about what ammonites looked like, less is known about how they lived. Their closest living analog is the Nautilus, which has a very simple suture line and a gently curved chamber wall, but it is hard to imagine that all ammonites—so different from one another and from the Nautilus—had the same mode of life. The shell of these cephalopods suggests that they could swim, but not very efficiently, and surely they were masters of flotation in the water column. The first ammonites appear in the fossil record in the Early Devonian; they diminished in number around the Permo-Triassic boundary, and nearly went extinct toward the end of the Triassic. Ammonites rebounded in number during the Jurassic and Cretaceous but finally went extinct, along with the dinosaurs , 65 million years ago—at the end of the Mesozoic era. Ammonites are excellent index fossils—in some sequences almost each bed is characterized by a unique ammonite, or set of ammonites. As a group they enjoyed a long time span and their rapid evolution was reflected in their changing ornamentation . They also inhabited the open sea, and so their fossils are found over vast areas. All these features, in combination, make them excellent fossils for sedimentary correlation over space and time. Bedding surface, or bedding plane. In most sedimentary rocks the single beds that make up a succession (a packet of rocks) are bounded by flat, or slightly undulating, surfaces; these are the bedding surfaces, or bedding planes. In most cases, these surfaces have an erosional origin. Unconsolidated sediment was swept away and resulted in a surface that represents a small stratigraphic gap in the succession. Later on this eroded surface might be cemented, bored into, or bioturbated: all these later processes can make observation of the bedding surface problematic. Belemnite. An extinct group of marine invertebrates closely related to modern squid. Within cephalopods, belemnites are the most important group in subclass Coleoidea. Belemnites had an inner shell that had a heavy calcite guard on its rear. The main role of this structure appears to have been to counterbalance the front of the organism—although, in most cases, this guard is all that remains of the animal as a fossil. Belemnites were active swimmers and had an important role in ecological...