PALEONTOLOGY AND GENETICS 



EVALUATION OF THE FOSSIL RECORD 



With the above facts in mind, some evakiation of the fossil record for 

 various groups may be attempted. Characteristics to be treated are the 

 abundance of fossils available for study, their degree of completeness, and 

 the ease with which they can be interpreted. The Protozoa (unicellular, 

 or better, acellular, animals) are generally not fossilized, but those which 

 have calcareous or siliceous shells have been fossilized in immense num- 

 bers. These are chiefly the Foraminifera, the Heliozoa, and the Radiolaria, 

 all of them orders of the class Rhizopoda, which is best known by the 

 ameba. Chalk deposits and some limestones are largely formed from the 

 shells of Foraminifera, while siliceous rocks are largely radiolarian in ori- 

 gin. The shells are often recoverable in a good state of completeness, and 

 they are fairly good as far as ease of interpretation is concerned. The 

 Porifera (sponges) are represented in the fossil record by their spicules, 

 which may be either calcareous or siliceous. They are only fair in abun- 

 dance, but their preservation is often good. However, ease of interpreta- 

 tion is again only fair. 



Most of the Coelenterata ( hydroids, jellyfishes, corals, and their allies ) 

 have left a very scant fossil record of poor quality. But the corals have left 

 numerous fossils, quite complete, and readily identifiable. The Annelida 

 and other worm-like phyla have been fossilized so rarely that such fossils 

 as are available have little value for tracing the history of these groups. 

 Yet a few of the known annelid fossils are surprisingly complete. Because 

 of the importance of several of the worm-like phyla among living animals, 

 this is an especially serious deficiency in the fossil record. Marine Arthrop- 

 oda are abundantly represented in the fossil record, but insects are rather 

 scantily represented. Many of these arthropod fossils are quite complete, 

 and the ease of interpretation is fair, often good. The Brachiopoda, or 

 lampshells, are a minor phylum, at present consisting of only a few spe- 

 cies, mostly confined to tropical seas. In Paleozoic times they were more 

 important. Their fossil record is one of the best in all respects. 



The Mollusca, including such animals as sea cradles, octopuses, clams, 

 and snails, have left a very abundant fossil record. The shells are often 

 in a good state of preservation, and in many lines of descent they are 

 readily interpretable. Fossils of the Echinodermata, a group including 

 the starfishes and their allies, are very numerous. In completeness they are 

 excellent, and in interpretability they are fair. Finally, the vertebrates 

 are present in the fossil record in good numbers. Their completeness and 

 interpretability are excellent. The prochordates, however, having no bony 

 skeleton, are not represented in the fossil record at all unless Jamoytius, 

 a controversial Amphioxus-like fossil from Silurian rocks of England, is 

 a prochordate. 



Yet, fragmentary though the fossil record is, it is a striking thing that 

 it gives clear testimony to the fact of evolution, and considerable detail 

 can be worked out in many lines of descent. The most ancient fossils in- 

 clude only invertebrates. Then primitive fish-like vertebrates appear, and 

 these gradually blend into true fishes, similar to some species now living. 



71 



