Miscellaneous Fossil Groups

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From Eldredge (1991)


INTRODUCTION

This laboratory considers several different fossil groups who's biologic affinities to other modern groups is in doubt. Most of these fossil groups occur as Paleozoic plankton, and their utility (especially the graptolites and conodonts) are in their biostratigraphic resolution which is on the order of 2.5 MY or less.


CLASSIFICATION & GEOLOGIC RANGES

Phylum Hemichordata

Class Graptolithina (Cambrian - Pennslyvianian)

Order Dendroidea (Cambrian - Pennslyvianian)

Order Graptoloidea (Ordovician - Devonian)

Phylum uncertain

Conodontomorphia (Cambrian - Triassic)

Hyolithia (Cambrian - Permian)

Tentaculitida (Ordovician - Devonian)

Conularia (Cambrian-Triassic)

 

 


GRAPTOLITES

The graptolites are considered by many to belong to the phylum Hemicordata, the phylum which also includes the Recent Rhabdopluerids. Assignment to the Hemichordata is based upon the presence of stolon, a small tube similar to a notochord in chordates which transverses the length of the noncolonial forms and connects the individuals of the colonial forms. Graptolites are exclusively colonial organisms in which each individual is called a zooid. The zooids may serve different functions with regards to reproduction and feeding.


Graptolite Morphology

Graptolite morphology is highly varied. Two basic morphologies include the dendroid type and graptoloid type. Both types consist of small conical cups called thecae in which each zooid is housed and is arranged along a linear series. Each linear series of connected thecae forms a stipe. Together, the skeleton comprising the stipe and thecae is called the rhabdosome. The first or initial theca from which all others bud is called the sicula. The dendroid graptolites are constructed by numerous branching stipes which are sometimes connected by rod-like structures called dissepiments. The graptoloid type consists of one to four stipes and are characterized by small thread-like structure connected to the sicula called the nema. See the excellent example showing this morphology in three dimensions (§12.1).


Graptolite Classification

Classification of graptolites is based on many characters such as the number of stipes, arrangement and orientation of thecae and any specilazed structures. For the Dendroids (§12.2-§12.4) classification is based upon branching arrangement and colony morphology. For the Graptoloids, the rhabdosome may take on a number of different morphologies. Pendant forms are the most primitive where the stipes hang downward from the nema such as Didymograptus (§12.5). Horizontal forms such as Monograptus (§12.6) are when the stapes stick laterally away from the nema. Reclining forms such as Dicellograptus (§12.7) are more advanced and are characterized by the stipes reclined upwards from the nema. The reclining condition may be taken to the extreme in the Scandent form where two or more stipes may fuse to produce biserial or tetraserial rhabdosomes with the theca pointing upwards (§12.8). You should examine the additional graptolites provided (§12.9-§12.14) and make sure you can identify all the morphologic features discussed above.


Graptolite Paleoecology

Graptolites are exclusively marine organisms which occurred from the Middle Cambrian through Upper Carboniferous. They are particulary abundant and are excellent biostratigraphic indices of the Ordovician and Silurian. Although they occur in a variety of lithofacies suggesting a pelagic planktonic life-mode, they are most commonly found in deep water black shale facies. In these environments, graptolites are preserved as compressed thin carbon films.


CONODONTS

Conodonts are small microfossils made of calcium phosphate which usually occur as disarticulated elements scattered throughout sedimentary rocks of the Paleozoic and early Mesozoic. Because they are found as disassociated elements, and are all extinct their taxonomic affinity is in question; yet they may belong to extinct group of organisms similar to some chordate bearing worms. Individual conodont elements belong to natural groupings called assemblages which are often found together.


Conodont Morphology

The individual elements are composed from a variety of small cones, bars, and blades or platforms some of which bear small teeth-like structures called denticles. Generally, conodonts can be grouped into three main morphotypes that are listed below (see also accompanying figure) and can be found on the slide (§12.13).

  • Coniform elements: consist of a single cone or cusp which has a small basal cavity. (§12.15)
  • Ramiform elements: consist of bars with a central cusp and denticles extending on either side of cusp. Usually, a basal cavity can be recognized. (§12.16)
  • Pectiniform elements: includes diverse forms bearing a platform, and numerous denticles, one of which may be an anterior cusp. Usually, a basal cavity can be recognized. (§12.17 and §12.18)

Conodont Paleoecology

Conodonts are exclusivly marine and have been recovered from a variety of paleonvironments. Their presence in near shore to relatively deep environments suggests that they were pelagic (either nektic or planktic). Other examples of conodonts can be found in the additional slide (§12.19).


CONULARIDS

Although there is some controversy in regards to the systematic position of the conularids, most workers today place them with the scyphozoan cnidarians rather than the molluscs. The exoskeleton of conularids is composed of chitin and in outline is pyramidal with four sides. The exterior surfaces of conularids normally have finely spaced longitudinal ribs. The interior of conularids may have thickenings or septa developed. In life, conularids lived attached apical end down to the substrate. A couple of examples are provided (§12.20).


HYOLITHIDS

It is uncertain as to what phylum the hyoliths belong. Until recently they have been classified as mollusks or worms, some prefer toconsider them as a separate phylum, and your text chose to ignore the group altogether. Hyoliths have a bilaterally symmetrical exoskeleton composed of a tapering conical (or pyramid) conch which is closed at one end (posterior) and has an open end (anterior) which may be closed by an operculum. In cross section, the cone of hyoliths may be either triangular or semicircular. The skeleton of hyoliths is composed of calcium carbonate. One group of hyoliths has two anterior bar-like protrusions called helens. Several examples are provided (§12.21).


TENTACULITIDS

Tentaculitids are a group of small animals which lived in a conical shaped exoskeleton composed of calcium carbonate. Like the hyoliths, they have been variously classified as mollusks or worms; unfortunately little is known about this extinct group which may belong to its own phylum. The exoskeleton of the tentaculitids may be relatively smooth as in the genus Styliolina (§12.22) or may have regularly spaced ribs or ridges as in the genus Tentaculites (§12.23-§12.25)

 


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