Lab 6 Fossils and Phylogeny
Introduction
Biologists have for centuries been ehgaged in the process of sorting and classifting to vast array of morphological diversity seen in the world¹s fauna and flora. The process of classification, ultimately provides a window on the evolutionary, i.e. ancestor-descendant, relationship among groups of organisms, because mrophology often reflects the underlying genetic changes that have determined evolutionary history. The same situation orccurs with the fossil record, the sorting of fossils into groups of related types, ultimately leads to an understanding of their evolutionary relationships.
Taxonomists today use a system of classification that is based on the original notion of binomial nomenclature, developed in the 1700¹s by Linnaeus. This system assigns a genus name followed by a species name (epithet) to each known species. The two terms are always written in italics, hence Sphaerophycus parvum, refers to the specific cyanobacterial fossil which is classified in the genus, Sphaerophycus. The rules for establishing names are agreed upon by international committees who produce a guidline called the ³code.² (The exact code varies for plants, animals fossils, and microbes.) There is a taxonomic hierarchy, a convention of categories of names of groups that fall above the level of genus, but these are not set by agreement, and you are free to assign or create any new categories as you see fit, although the stand set is usually followed:
Domain
Kingdom
Phylum
Class
Order
Family
Genus
species
Fossils, particularly the invetebrates, are classified and placed ito ordered taxonomic groups in the same manner as living organisms. In this lab exercise, we look at the commonly found invertebrate fossils and attempt to classify them into a scheme that reflects their evolutionary relations, or, phylogeny. The invertebrate phyla are grouped into two lineages according to characteristics found in their embryonic development from a blastula. Since these characters are based on examination of the living organisms as they grow, we cannot see them directly in the adult forms. However, the adult morphology and even fossil shells, usually give enough clues to enable us to recognize to which phyla any individual specimen belongs.
Common Marine Invertebrate Phyla
The animal phyla typivcally preserved in the fossil record include,
Chordata.‹ This includes the vertebrates which are the largest subgroup, along with the sharks and other fish with cartilaginous, rather than bony, skeletons.
Hemichordata.‹The only common fossils of this phylum, related to the Chordata, are the graptolites, an extinct group of pelagic, planktonic colonial organisms, common in the lower Paleozoic.
Echinodermata.‹Starfish, crinoids, cystoids, blastoids, corals a great variety of marine organism belong to this phylum.
Arthropoda.‹The insects are the largest class belonging to the arthrppods, but there are many other classes, including the trilobites, arachnids (spiders, scorpions, eurypterids, among others), crustaceans, barnacles.
Annelida.‹The annelids are worms and worm-like organisms typified by segmentation into repeated similar parts. They typically do not form hard parts, but are common in the fossil record in the form of traces.
Mollusca.‹Another very successful fossil group ncludes clams & snails.
Brachiopoda. Brachiopods are a phylum belonging to one of two of the lophophorates (organisms that use a structure called a lophophore to filter and trap food particles from the water). They were a dominant critter on the Paleozoic sea botom. Although they still exist tiday, their biological niche was mostly replaced by pelecypods (clams) at the end of the Paleozoic.
Bryozoans. These colonial forms were also important during the Paleozoic. Like corals, the animial itself is a small zooid or filter-feeding polyp which is housed in a carbonate superestructure secreted by the colony at large.
Porifera (sponges).‹Sponges are not true anomals, although they are usually studied by paleontologists as part of the ³marine invertebrate² fauna.
Evolutionary (Phylogenetic) Relations among Marine
Invertebrate Groups
The basic evolutionary relationships between the various marine invertbrate groups were determined in the 19th century by compariing the developmental (gropwth) stages of individual organisms. On the basis of the patterns of cell division and early structural development of the bastula[*] stage in development, two major evolutionary lines were seen, the Deuterostomes and the Proterostomes. (The Lophophorate groups fall in between these two in terms of development, since their embryonic development contains aspect sof both the Deuterostomes and Proterostomes.)
This poses somewhat of a dilema for the paleontologist, since we only see the adult (mature) forms. But, typically, at the phylum level, invertebrate groups are recognized more on the basis of their fundamental morphology, and ther eis no need to rely on ³living² characters.
In order to recognize the common groups of invertebrate fossils at the phylum level, it is only necessary to recognize the basic symmetry and shape of the fossil organism. So, descriptions of symmetry, such as radial, three-fold, or bilaterial (with a mirror plane) are useful differentiating between different invertebrate phyla.
Exercise
Review the set of fossils on hand. Make a quick sketch of each basic type and write a one sentence ³description² for each phylum represented ion the set. Focus on the basic shape (morphology ) fopr each group.
