Growth, Morphology, Habit and Habitat of Selected Brachiopod and Mollusc Species from the Meade Peak Member of the Phosphoria Formation, Permian, Northeastern Utah - Southeastern Idaho - Southwestern Wyoming
Date of Award:
Master of Science (MS)
Richard R. Alexander
The Permian Meade Peak Member of the Phosphoria Formation was examined at four localities in this investigation. Fossils were collected at each locality. The four collecting localities visited in this investigation are: Brazier Canyon, Utah, Montpelier Canyon, Idaho, Coal Canyon, Wyoming, and Cokeville, Wyoming.
The environment of deposition of the Meade Peak Member in the study area is inferred to have been an outer shelf or basin characterized by moderately deep and quiet water. This has aided reconstruction of life habits, population dynamics, and growth characteristics of the examined species.
The articulate brachiopod Leiorhynchoidea weeksi is inferred to have attached itself to the substrate by means of a pedicle. The observed variability in the sulcus of this species is assumed to have been influenced by intraspecific competition, which may reflect partitioning of nutrient resources at different levels in the water above the substrate.
The articulate brachiopod Lissochonetes ostiolatus probably assumed an unattached existence, lying free on the substrate. Several population samples exhibited stunted growth relative to the L. ostiolatus population sample from Montpelier Canyon. Two environmental variants possibly contributed to the inhibition of growth. These are negative Eh, inferred from the associated organic matter in the lithologies, and competition for space.
Lingula carbonaria an inarticulate brachiopod, is thought to have had an infaunal mode of life. A population sample of L. carbonaria from Cokeville exhibits substantially larger morphologic mean sizes than two other population samples. These differences can be explained by the fact the sandy lithology from which the larger collection was taken was associated with conditions which facilitated growth. Also, availability of phosphate, inferred from the P2O5 content of the lithology, probably accelerated growth because inarticulates utilize calcium phosphate as shell material.
Orbiculoidea missouriensis is an inarticulate brachiopod. It is inferred to have been attached to the substrate by a pedicle. OF the three population samples of O. missouriensis analyzed, the sample from Brazier Canyon displays larger morphologic mean sizes. This is interpreted as indicating that reducing conditions, inferred from associated organic matter, were milder in that environment.
The gastropod bablyonites ferrieri displaysa a low, expanded from, and is thought to have crawled over the surface of the sediment. The food source of this gastropod is not known, but may have been algae, carrion, detritus, or soft-bodied invertebrates.
Two bivalves collected are assumed to have been shallow burrowing, labial palp feeders. Both Nuculopsis montpelierensis and polidevcia obesa represent this mode of life. Of the two population samples of P. obesa examined, the one from Cokeville displays larger morphologic means. The environmental stimuli proposed for this difference is competition and an inferred softer substrate in the montpelier assemblage.
The bivalve Edmondia phosphatica is inferred to have been a shallow burrowing, siphonate species.
Streblochondria montpelierensis and Aviculopecten phosphaticus are bivalves related to modern pectenoids. S. montpelierensis is assumed to have been a byssally attached epifaunal bivalve.
The environmental factors influencing the morphologic variation are also inferred to have affected size-frequency distributions and survivorship curves. The effects of reducing conditions have resulted in higher juvenille mortality and/or inhibited growth. Coarser substrates have yielded population samples that exhibit negatively skewed distributions, whereas finer-grained substrates have yielded positively skewed distributions.
Growth curves constructed for each population sample generally show a decline in growth rate with age. Some linear trends are noted.
Growth lines were used to infer an approximate life span for each species. Conservative approximations of the life spans of the examined species are: L. weeksi, 9 years; L. ostiolatus, 5 years; L. carbonaria, 8 years; A. phosphaticus, 4 years; O. missouriensis, 2.3 years; P. obesa, 5 years; and B. ferrieri, 3.4 years.
Russell, Scott Lewis, "Growth, Morphology, Habit and Habitat of Selected Brachiopod and Mollusc Species from the Meade Peak Member of the Phosphoria Formation, Permian, Northeastern Utah - Southeastern Idaho - Southwestern Wyoming" (1980). All Graduate Theses and Dissertations. 3770.
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