Earth Science Student Research Grants Recipients 2005

Osteolepiforms from the Middle Devonian, Water Canyon Formation of northern Utah

PATRICIA J. HALL
Graduate Studies, Northern Arizona University
Spring 2005

STATEMENT

Periodically in all scientific disciplines a particular area becomes a focus of interest. These "hot spot" topics are international in their importance, and within paleontology a current focus is centered on those vertebrates that were involved in the evolutionary transition from the entirely aquatic lifestyle of fishes to the later terrestrial tetrapods (vertebrates with limbs). A better understanding of these transitional forms can help to unravel the stepwise account of particular characteristics that evolved during this transition in the Middle and Late Devonian.

The group of bony fishes that is known to have given rise to the tetrapods is the sarcopterygians, or lobe-finned fish. These include the modern lungfish and coelacanths, and the extinct rhipidistians. The rhipidistians, which include the osteolepiforms, eusthenopterids, rhizodonts, and panderichthyids, are thought to be the group from which tetrapods arose. The panderichthyids are generally accepted as being closest to the early tetrapods due to their more evolved state; however there is still a large area of disagreement regarding the relationships of the Osteolepiformes. This disagreement exists for osteolepiform interrelationships as well as their relationship with the Tetrapoda. For example, it has still not been agreed upon as to whether or not the osteolepiforms are paraphyletic or monophyletic with respect to the tetrapods, while concurrently there is disagreement regarding the relationships of various genera within the Osteolepiformes (Cloutier, 1996) (Ahlberg and Johanson, 1998). In part, these disagreements stem from a lack of well-preserved material, and therefore thorough descriptions of species within the order. However, despite such arguments, previous studies have conclusively shown that the defining characteristics of Osteolepiformes directly relate to the ancestral lineage of the tetrapods, and are therefore evolutionarily significant

Fig. 1. One slab showing the pristine preservation of two osteolepidids from the Water Canyon Formation, Northern Utah.

A new species of osteolepidid (Osteolepis, Thursius) has now been discovered in the Water Canyon Formation, northern Utah. Much of the material is fully articulated, including some complete animals with articulated skulls (Fig. 1). Such complete specimens in excellent condition are extremely rare; most fish are described from fragmentary material. Not only will a thorough analysis of these specimens augment the biological record and help to clarify their evolutionary standing, but it will also be the first osteolepid material described from the Western United States. Although osteolepiform material was excavated from the Middle Devonian Red Hill site in Nevada, it has never been described or clearly identified (ers. comm. Reed, 2004). This find represents an exciting opportunity to work on exceedingly rare material from an entirely new locality that will add new information to a worldwide debate.

The Water Canyon Formation of Northern Utah was deposited on a miogeocline in a coastal embayment and is composed of the Lower or Card Member which is probably Pragian in age and the Grassy Flat Member which is Emsian and Eifelian in age. During this time, Utah was positioned approximately 20° south of the equator, and the local embayment prevented depositional influence from the equatorial currents. The members record a change in depositional environments from a dolomite-dominated tidal flat to a tide-dominated deltaic setting characterized by dolomicritic sands. Overall, the formation is dominated by the presence of dolomite, although siliciclastic material becomes more abundant up-section with the appearance of deltaic channels. The sequence stratigraphy of the formation, as outlined by Reed (1997), shows an overall transgressive depositional pattern that is punctuated by small-scale regressive events.

Previous to the discovery of these osteolepiforms, the biostratigraphy and age constraints on the Water Canyon were uncertain (Reed, 1997). With this major find the upper part of the Grassy Flat Member has been constrained to the Middle Devonian, which allows for tentative regional correlation of the unit with those of the same age. Further sedimentary analysis, correlation, and paleoenvironmental interpretation can only lead to a better understanding of the strata both locally and regionally.