Chapter 7

Plate 24PENNSYLVANIANPELECYPODS

Plate 25CRETACEOUSPELECYPODS

Plate 26CRETACEOUSPELECYPODS

Plate 27CRETACEOUSPELECYPODS

Plate 28CRETACEOUSPELECYPODS

Plate 29TERTIARYPELECYPODS

Plate 30TERTIARYPELECYPODS

Plate 31TERTIARYPELECYPODS

Fig. 18.Morphology and principal parts of the pearly nautilus. (a) Exterior view of a Recent shell. (b) Sectioned view of the same shell to show internal structures.

Fig. 19.Characteristic features of the various types ofcephalopodsutures. (a) Nautiloid type. (b)Goniatitetype. (c)Ceratitetype. (d)Ammonitetype.

The ammonoids are a group of extinct cephalopods which are related to the nautiloids but are characterized by more complexsuturepatterns. Members of this subclass have an external partitioned shell which is straight, curved, or spirally coiled (Pl. 33). This group of cephalopods first appeared inDevoniantime, became extremely abundant and varied during theMesozoic, and was extinct by the end of theCretaceousperiod.

MostPaleozoicammonoids are characterized by a combination curved and angularsuturepattern, and this type of suture pattern is referred to asgoniatitic(fig. 19b). Sutures that are curved and crenulated (marked in places by aseriesof tooth-like indentations) are known asceratitic(fig. 19c). Ceratitic sutures first appeared in theMississippian, became increasingly abundant during theTriassicbut were much less abundant during theCretaceous. Theammoniticsuture has a very complexly subdivided pattern (fig. 19d). Cephalopods with ammonitic sutures range fromPennsylvanianto Cretaceous in age and were the most abundant cephalopods of theMesozoic.

Plate 32

PENNSYLVANIANCEPHALOPODS

CRETACEOUSCEPHALOPODS

Plate 33CRETACEOUSCEPHALOPODS

Ammonoids are locally abundant in many of thefossiliferousrocks of Texas and are among the more usefulMesozoicguide fossils. Goniatites may be found in thePennsylvanianof north-central and Trans-Pecos Texas, and ammonoids with the ceratiticsuturepattern can be collected from the LowerCretaceousof many parts of the State. Cephalopods exhibiting the typical ammonitic suture pattern are abundant in many of the Cretaceous rocks of Texas, and these fossils have contributed much toward an understanding of the Cretaceous strata of this State.

These are squid-like cephalopods characterized by an internal shell or no shell at all. Included in this group are the squids, cuttlefish, octopuses, and the extinct belemnoids, but of these only the belemnoids are useful fossils. Members of this subclass range fromMississippianto Recent in age.

The belemnoids appear to be the oldest and most primitive of the coleoid cephalopods. Their earliest known occurrence is in rocks ofMississippianage, and they were particularly abundant during theMesozoic. They became extinct at the end ofCretaceoustime but have left considerable evidence of their existence in the Mesozoic strata of many parts of the world. Certain forms, because of their abundance and relatively shortgeologic range, are excellent guide fossils. Belemnoids have been found in the Upper Cretaceous of Texas (Pl. 33) but in general are rare or unknown in most Texas formations.

Members of thephylumAnnelida include the segmented worms such as the common earthworm. Annelids are marine, fresh water, or terrestrial and have apparently been common through much of geologic time. Because of their lack of hard parts, most of these worms have left little directfossilevidence of their activities in the geologic past. Some annelids secrete straight or coiledcalcareoustubes, and fossil worm tubes of this sort (fig. 20) are commonly found attached to brachiopods, mollusks, and other objects. Tubes of this nature have been reported fromPaleozoic,Mesozoic, andCenozoicrocks in Texas.

Fig. 20.Types of typical annelid worms. (a)Serpula(×1) (b)Hamulus(×2). (c)Spirobis(×15).

Some annelids have smallchitinousjaws and teeth which also may be preserved as fossils. These dental structures are called scolecodonts and are microfossils.

The arthropods are one of the more advanced groups of invertebrates, and they are known from theCambrianto the Recent (Pl. 34). Modern representatives of this group include the crabs, shrimp, crayfish, insects, and spiders. Arthropods vary greatly in size and shape and are among the most abundant of all animals. They have become successfully adapted to a widevarietyof environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by achitinousexoskeletonwhich, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.

Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein.

The members of this subphylum are extinct arthropods which were most abundant during earlyPaleozoictime.

The trilobites are a group of exclusively marine arthropods which derive their name from the typical three-lobed appearance of their bodies (fig. 21a). Thetrilobitebody is divided into acentraloraxiallobeand twolaterallobes. The body of the animal was encased in achitinousexoskeleton. The top part of this exterior covering, thecarapace, is very thick, and it is this part of the trilobite that is usually preserved.

Plate 34FOSSILARTHROPODS

Fig. 21.Morphology and principal parts of trilobites.

The body is also divided into three parts from front to back. Beginning at the front of the animal these divisions are thecephalonor head, thethoraxor abdomen, and thepygidiumor tail (fig. 21a). The body segments of the thorax were arranged in such a manner as to permit the animal to roll up into a ball, and many trilobites are found in this position (fig. 21b).

Trilobites first appeared in theCambrianand were extinct by the end of thePermian. They occur sparingly in certain of thePaleozoicrocks of Texas and when found are likely to be fragmental and in a poor state of preservation.

The crustaceans are the crabs, shrimp, crayfish, and ostracodes. Although not abundant,fossilcrabs have been described from certainCretaceousandTertiaryformations of the State (Notopocorystes,Pl. 34). However, the most useful and abundant crustacean fossils are the members of the class Ostracoda.

The ostracodes are small, bivalved, aquatic crustaceans which have the external appearance of small clams (Pl. 34). The remains of these tiny animals are so small that they are best studied under a low-power microscope, and because of their small size they are particularly useful to the micropaleontologist.

Fossilostracodes range fromOrdovicianto Recent in age and have been recorded in thePaleozoic,Mesozoic, andCenozoicrocks of Texas. Their remains are particularly abundant in certain of theCretaceousandTertiarymarine formations of the State.

The echinoderms are a large group of exclusively marine animals, most of which exhibit a marked five-foldradial symmetry(Pls.35,36). Living echinoderms have well-developed nervous and digestive systems, a distinct body cavity, and are a relatively complex group of organisms.

The typicalechinodermhas a skeleton composed of numerouscalcareousplates which are intricately fitted together and covered by a leathery outer skin (theintegument). The echinoderm body often exhibits a typical star-shaped form, but some types may be heart-shaped, biscuit-shaped, or cucumber-shaped.

Members of thisphylumrange fromCambrianto Recent in age and are abundant as fossils in many of the marine formations of Texas.

ThephylumEchinodermata has been divided into two subphyla, the Pelmatozoa (those forms that were attached to sea floor by a stem or a stalk) and theEleutherozoa (the stemless unattached echinoderms).

These are echinoderms which are more or less permanently attached to the bottom of the sea by means of a stalk which is composed of slightly movable,calcareous, disk-like segments (fig. 23).

Pelmatozoans range fromCambrianto Recent in age, and their fossilized remains are particularly abundant inPaleozoicrocks. The subphylum has been divided into several classes, but only three of these, the Cystoidea, Blastoidea, and Crinoidea, are discussed here. With the exception of the Crinoidea, all of the attached echinoderms are extinct.

These are primitive attached echinoderms which were relatively common in earlyPaleozoictime. The typicalcystoidhas a somewhat globular or sac-likecalyx(the main body skeleton) composed of numerous, irregularly arranged,calcareousplates (fig. 22b). The plates composing the calyx are usually perforated by pores or slits which were probably used in excretion orrespiration. The calyx was attached to the sea bottom by a short stem.

Cystoids range fromCambriantoDevonianin age and were especially abundant duringOrdovicianandSiluriantime. Their remains are rare or absent in the rocks of Texas.

Fig. 22.Two extinct attached echinoderms. (a)Pentremites(Mississippian). (b)Caryocrinites(Silurian).

The blastoids are extinct short-stemmed echinoderms with a small, symmetrical, bud-likecalyx. Theblastoidcalyx is composed of 13calcareousplates arranged in a typical five-sided pattern (fig. 22a). Themouthis located in the center of the calyx and is surrounded by five openings calledspiracles. Five distinctambulacralorfood groovesradiate outward from the mouth.

Blastoids range fromOrdoviciantoPermianin age and were especially abundant during theMississippianperiod. No blastoids have been reported from any of the rocks of Texas.

The crinoids are commonly calledsea-liliesbecause of their flower-like appearance. Thecalyxis composed of symmetrically arrangedcalcareousplates, and most crinoids have a long stem. Other crinoids are free-swimming in the adult stage and are attached only during the earlier phases of their development.

The crinoidcalyxis typically cup-shaped (fig. 23) and five grooves radiate out from its center. These grooves continue outward along the complexly segmented arms and are used as channels to convey food to the mouth.

Fig. 23.Typical modern crinoid, or “sea lily,” showing principal parts.

The crinoid stem is attached to the baseof thecalyxand serves for purposes of support and attachment. This stem consists of a relatively long flexible stalk composed of numerouscalcareousdisk-shaped segments calledcolumnals(fig. 23;Pl. 35), each of which contains a round or star-shaped opening in its center. Many crinoids have very long stalks (some are as much as 50 feet in length), and when the animal dies the columnals become separated and are scattered about on the ocean floor. ManyPaleozoiclimestones contain such great numbers of crinoid columnals that they are referred to ascrinoidal limestones(fig. 8). Crinoidal limestones occur in some of theMississippianandPennsylvanianformations of central Texas and in the Pennsylvanian of north-central and Trans-Pecos Texas.

The stalk is attached to the sea floor or some other object by means of a rootsystemcalled theholdfast(fig. 23). This structure commonly branches out into the surrounding sediments, and in this manner the crinoid animal is firmly anchored to the bottom of the sea.

Crinoids, like most echinoderms, are gregarious animals—that is, they commonly live together in large numbers, and for this reason great numbers of crinoid remains are commonly found concentrated in relatively small local areas. Mostfossilcrinoids are found as stem fragments because the more fragilecalyxand rootsystemare less likely to be preserved.

The earliest known crinoids have been found in rocks ofOrdovicianage, and their remains are particularly abundant inPaleozoicrocks. Crinoids are living today but most of them are stemless free-swimming forms called “feather stars,” much less abundant than their Paleozoic ancestors.

The eleutherozoans are free-swimming, bottom-dwelling, echinoderms which have been divided into two classes. The class Asterozoa (star-shaped echinoderms) contains the subclasses Asteroidea (the starfishes) and the Ophiuroidea (the brittle stars). Although they are known as fossils, neither of these groups is of paleontological importance. The class Echinozoa (echinoderms without laterally directed arm-like extensions) contains the subclasses Echinoidea (the sea urchins and sand dollars) and Holothuroidea (the sea cucumbers). Of these two subclasses, only the Echinoidea are useful fossils.

These are typical star-shaped free-moving echinoderms in which the body is divided into a central disk and radiating arms.

This class contains the starfishes which, although not common fossils, illustrate well the typicalechinodermcharacteristics (Pl. 35).Fossilstarfishes have been found sparingly in certain formations in Texas, but well-preserved specimens are quite rare. However, excellently preserved starfishes have been found in slabs ofCretaceouslimestones from central and north-central Texas.

The ophiuroids are echinoderms with a well-defined central disk and five long, slender, whip-like arms. They have been called brittle stars because of their ability to shed their arms when they are disturbed. Their long, slender, snake-like arms have also resulted in their being called serpent stars. Ophiuroids range fromOrdovicianto Recent in age, but because of the delicate nature of their bodies they are seldom found as fossils. Ophiuroid remains have been found in certainMesozoicandCenozoicrocks of Texas, but they consist largely of small segments of the arms or body fragments.

The echinozoans are a group of unattached echinoderms whose bodies consist of numerouscalcareousplates and spines, but they do not possess the radiating arm-like extensions which characterize the asterozoans.

Echinoids are free-moving echinoderms with disk-shaped, heart-shaped, biscuit-shaped, or globular exoskeletons (Pl. 36). Modern representatives of this group include the familiar sea urchins, heart urchins, and the sand dollars.

Plate 35

Plate 36CRETACEOUSECHINOIDS

Theechinoidtest(exoskeleton) is composed of many intricately fittingcalcareousplates (Pl. 36) which enclose the animal’s soft parts. The exterior of the test is typically covered with large numbers of movable spines (Pl. 36) which vary greatly in size. These spines are of some aid in locomotion, support the skeleton of the animal, and provide a measure of protection from enemies.

The oldest known echinoids have been recorded from rocks ofOrdovicianage, but it was not until theMesozoicthat the group began to flourish. They were especially abundant during theCretaceousand have been abundant and varied from that time until the present.

Echinoids are particularly numerous in many of the LowerCretaceousformations of Texas where they are commonly found in an excellent state of preservation. Heart urchins and biscuit urchins may be found in large numbers in many areas of the State, and especially in areas where there are good exposures offossiliferousLower Cretaceous rocks.

Members of this class, commonly calledsea cucumbers, have a rather elongate, sac-like, cucumber-shaped body and bear little resemblance to other members of thephylumEchinodermata. The sea cucumbers do not have a well-defined skeleton; rather the body is supported by many small, disconnected,calcareousplates or rods calledossiclesorsclerites(Pl. 35). These minute structures are embedded in the leathery skin which covers the body of the sea cucumber and may be preserved as fossils. Such remains are locally abundant in certain formations in Texas, but because of their small size, scattered occurrence, and problems in classification, this group is of little use to most paleontologists.

Holothuroid body impressions have been reported from the MiddleCambrian, and sclerites from rocks as old asMississippian.

The chordates are the most advanced of all animals and are characterized by the presence of a well-developed nervoussystemand a body supported by a bony or cartilaginousnotochordand/orspinal column. In the higher chordates (the vertebrates) the notochord is normally replaced by bone, but in the lower chordates (for example, the graptolites) it remains in a cartilaginous condition.

ThephylumChordata contains only two subphyla of paleontological significance. These are the subphylum Hemichordata, composed of primitive chordates (including the graptolites which are important fossils), and the Vertebrata, which includes all animals with backbones.

The hemichordates are characterized by a well-defined notochord which runs the length of the body, but they do not possess a true backbone. Only one class, the Graptolithina, is of paleontological importance.

The graptolites are a group of extinctcolonialanimals which were very abundant during earlyPaleozoictime. They are characterized by achitinousexoskeletonconsisting of rows of cups or tubes which housed the living animal. These cups are attached to single or branching stalks (fig. 24) which in some forms were attached to sea weeds, rocks, or other foreign objects where they led a fixed existence. The stalks of the unattached graptolites grew on floats (fig. 24a) and these floating forms attained wide geographic distribution. It is also possible that some of the attached forms were fixed to floating objects, such as sea weed, and thus were distributed in this manner.

Previous classifications have recognized the graptolites as members of thephylumCoelenterata. As coelenterates they were assigned, at various times, to the classes Hydrozoa, Scyphozoa, and Graptozoa. In addition, they were also classified as bryozoans by certain of the early paleontologists. This publication, in keeping with recent changes intaxonomy, considers graptolites to be an extinct group of hemichordates. This classification is based upon research in which uncompressedgraptolites were etched out ofchertand studied in great detail. Information derived from these relatively undistorted specimens indicates a much higher degree of body organization than was previously suspected, and as a result of these studies most paleontologists now consider graptolites to be some form of primitive chordate.

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