Abstract

The origin and relationships concerned with the development of the biohermal mounds in the El Paso Group have long been a subject of speculation. The determination of precisely what taxon is involved in the stromatolitic-like, climax boundstone binder of the McKelligon Formation has been a problem. An answer has been postulated that seems both reasonable and acceptable (webby, 1986). In view of the fact that these mounds are important to the interpretation of sequence stratigraphy of the El Paso Group, they need to be reexamined (LeMone, 1988).

The El Paso Group Pulchrilamina mounds were first recognized by Cloud and Barnes (1948) who regarded them as being stromatolitic units. Ham and Toomey (1966) subsequently recognized these “stromatolitic structures” as being bioherms and designated the dominant organism as being Pulchrilamina spinosa, which they recognized as a primary mound contributor. Toomey and Ham (1967) described the organism as having unknown affinities, possibly related to a primitive hydrozoan of the Coelenterata. The reason for this assignment was that, at that time, Pulchrilamina had been rejected by both stromatoporoid and stromatolite workers as not having affinities to their respective areas of specialty. The cnidarian expert from Australia, Professor B. D. Webby, after presenting papers at the Cnidarian International Symposium in Washington in the summer of 1983, visited the mound where the type Pulchrilamina had been recovered by Donald Toomey. In addition, he sampled with me a number of other McKelligon Formation biohermal mounds for comparative material.

Webby (1986), in an excellent analysis, has reclaimed the organism into the stromatoporoid taxa. Excluding stromatolitic and thrombolitic bioherms, a number of workers would consider the Pulchrilamina mounds to be the first true expression of a boundstone in that they are composed of a succession of metazoans. Most workers, likewise, would exclude the archaeocyathids as the first metazoan boundstones because they interpret their development as stacked biostromes rather than true biohermal boundstones despite the fact that there appears to be evidence of some algal binding. The Pulchrilamina mounds have been noted in such widespread areas as West Texas and Oklahoma (Toomey and Ham, 1967) and Newfoundland, Canada (Pratt and James, 1982; Klappa and James, 1980).