On the occurrence of fossil conifers with affinities to Geinitzia in the late Cretaceous

Transcription

1 On the occurrence of fossil conifers with affinities to Geinitzia in the late Cretaceous (Campanian) Mesaverde Group, Williams Fork Formation of northeastern Utah, U.S.A Benjamin J. Burger* and Christopher J. Ward Utah State University Uintah Basin Regional Campus Department of Geology 0 North Aggie Blvd. Vernal, Utah 0 *Corresponding author: benjamin.burger@usu.edu Abstract. In this paper we describe fossil conifer branches discovered in the Mesaverde Group, Williams Fork Formation in northeastern Utah, along Snake John Reef. Fossil conifers from the Campanian of northeastern Utah have not been previously studied, despite their common occurrence in the formation. The recovered fossils closely resemble Geinitzia known from the late Cretaceous of Europe, with several previous reported occurrences in North America, including New Jersey and Southern Utah. The fossils share morphological characteristics with Geinitzia, exhibiting short spirally arranged thin needles, with appressed scale-like leaves along the shoots. They differ from Araucarites in that the appressed needles are more scale like and smaller, and differ from the members of the modern Araucariaceae in lacking broad bases to the needles, although the fossils resemble the modern species Araucaria heterophylla (Norfolk Island Pine) native to the South Pacific. The observed branching pattern in the fossil reflects similarities found in the Cupressaceae Family, and Geinitzia may be regarded as an early member of this group, or having an affinity to fossil Araucariaceae conifers, which despite having a modern southern hemisphere geographic distribution today were widespread during the late Mesozoic, extending across North America and Europe. 1 PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

2 Materials & Methods The fossils described in this report are confined to a single slab of calcareous rich sandstone retrieved from northeastern Utah, near the town of Jensen, Utah (Figure 1). Locally the Mesaverde Group forms a large ridge line known as Snake John Reef where the fossils were discovered near the border with Colorado. The fossils were recovered from 1 meters above the lower contact with the late Cretaceous Mancos Shale, and meters below the contact with the Eocene Colton Formation. In Colorado, the Mesaverde Group is subdivided into the sandstone dominant Iles Formation below, and the coal-rich Williams Fork Formation above. In northeastern Utah the Mesaverde Group is undifferentiated, but based on measured sections the formation can be divided into two formations, the lower sandstone dominated Asphalt Ridge Formation (equivalent to the Iles Formation in Colorado), which differs slightly in Utah to include bituminous calcareous sandstone and an upper siltstone dominated unit that also contains coal, interpreted as belonging to the Williams Fork Formation. The fossils come from the upper siltstone Williams Fork Formation, meters above the contact with the Asphalt Ridge Formation. The fossils were collected at NW Sec. 0 TS RE, latitude N longitude 1 0. W under Department of the Interior Paleontology Permit UT1-001S The recovered fossil described here was found on a single large sandstone block, and shows the branching pattern, with four terminal branches and an isolate branch in the same slab (Figure 1). The fossil conifer branches compare closely with figures in Kunzmann (0) and Halamski (01) of Geinitzia reichenbachii from the Campanian of Europe. The branching PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

3 pattern is less symmetrical and dense, and similar to modern Sequoia in habit. The fossils exhibited on the slab show both curved needlelike leaves extending from a wider scale-like base near the stem. This feature resembles some of the modern Araucaria species which show needles with a wider leaf base. The needles measure about 1 cm in length, with a wider scale-like leaf base making up about mm of the total leaf length. The leaves are spirally arranged on each branch. Discussion 1 1 The age of the Williams Fork Formation is within the ammonite Didymoceras cheyennense biozone, indicating that the age of the formation is late Campanian (about Ma; Diem & Archibald, 00; Foster and Hunt-Foster, 01). Lack of siliceous clasts, ripple marks, presence of Teredo-bored petrified wood and occurrence of several limestone units indicate that the Williams Fork Formation represents a forested marginal marine environment, geographically located on the retreating coast of the Western Interior Seaway Tidwell et al. (00) described a fossil conifer specimen belonging to Geinitzia sp. from the late Cretaceous Blackhawk Formation in the southern Wasatch Plateau in southern Utah, and Richardson () reported on the occurrence of Geinitzia reichenbachii near Thompson, Utah in the Late Cretaceous Farrer Formation. Both units are relatively correlative to the Williams Fork Formation, Mesaverde Group in northeastern Utah (Foster and Hunt-Foster, 01). Specimens identified as Geinitzia gracillima are also known from the late Cretaceous of New Jersey (Jeffrey, 1). None of these specimens of Geinitzia from North America have been well illustrated. PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

4 In Europe, the genus Geinitzia is known from the Santonian to Campanian (Kunzmann, 0; Halamski, 01). Recently there has been some debate as to whether the genus should be considered a natural genus, rather than a leaf-form genus based on its unique needles, and lack of well-preserved cones (Zijkstra et al. 0, Herendeen 0). However, based on leaf epidermal cell structure, Kunzmann (0) has argued that Geinitzia reichenbachii from Europe should be considered a natural genus The broad heterogeneous leaf-needle pattern of Geinitzia closely resembles the living species of Araucaria heterophylla which is a native conifer found in the South Pacific. The wide biogeographic distribution of fossils in the Northern Hemisphere of Geinitzia during the late Cretaceous is likely due to the warm climate during the time that supported the warm habitat that these types of conifers require. With the arrival of colder climates during the Cenozoic in the Northern Hemisphere, the broad biogeographic distribution of Geinitzia was significantly reduced, followed by extinction. The disappearance of Geinitzia was succeeded by cold tolerant conifers (such as Pinus, Picea, Pseudotsuga, Sequoia, and even Metasequoia), which went on to dominate the Northern Hemisphere. Kunzmann (0) and Farjon (00) suggest that Geinitzia (placed within the family Geinitziaceae) was ancestral to or an early member of the Cupressaceae family, more closely related to living Sequoia and Metasequoia than to modern araucarian conifers from the Southern Hemisphere. This is despite morphological similarities with modern araucarian conifers in these Mesozoic fossils from Utah. Araucarian fossils are well documented in the earlier Jurassic of Utah from the Morrison Formation (Stockey, 1). It may be that modern araucarian conifers resemble these Mesozoic fossils due to convergence in the development of needle-like leaves, which was independently acquired in araucarian conifers. Conclusion PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

5 Regardless of the systematic relationship, the occurrence of Geinitzia in the late Cretaceous of Utah indicates a warmer climate during this time, and a widespread flora that was similar to that found in Europe. The new fossil further suggests that Geinitzia was a geographically wide spread conifer and a significant component of the forests along the Western Interior Seaway of North America during the late Campanian References Diem, S. and Archibald, J.D. 00. Range extension of southern chasmosaurine ceratopsian dinosaurs into northwestern Colorado. Journal of Paleontology (): 1-. Farjon, S. 00. A natural history of conifers. Timber Press, Portland Oregon, U.S.A p. 0. Foster, J.R. and Foster-Hunt, R.K. 01. First report of a giant neosuchian (Crocodyliformes) in the Williams Fork Formation (Upper Cretaceous: Campanian) of Colorado. Cretaceous Research :-. Halamski, A.T. 01. Latest Cretaceous leaf floras from southern Poland and western Ukraine. Acta Palaeontologica Polonica ():0-. Herendeen, P. 0. Report of the nomenclature committee for fossil plants:. Taxon 0():0-0. Jeffrey, E.C. 1. The affinities of Geinitzia gracillima. Botanical Gazette 1(1):1-. Kunzmann, L. 0. Geinitzia reichenbachii (Geinitz, 1) Hollick and Jeffrey, and Sedites rabenhorstii Geinitz, 1 (Pinopsida; Late Cretaceous) reconsidered and redescribed. Review of Palaeobotany and Palynology. 1:1-. Richardson, G.B.. Reconnaissance of the Book Cliffs coal field between Grand River, Colorado and Sunnyside, Utah. U.S. Geological Survey Bulletin 1, p. Stockey, R.A. 1. Mesozoic Araucariaceae: Morphology and systematic relationships. Journal of Plant Research ():-0. Tidwell, W.D. Britt, B.B., Tidwell, L.S. 00. A review of the Cretaceous floras of east-central Utah and western Colorado. pp. -. in Hylland, M.D., Clark, L. and Chidsey, T.C. Central Utah Diverse Geology of a Dynamic Landscape, Utah Geological Association PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

6 Zijkstra, G., Konignenburg-van Cittert, H., Kunzmann, L., Bosma, H., Kvacek, J. 0. Proposal to conserve the name Geinitzia with a conserved type (fossil Coniferophyta). Taxon (1):1, pp PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

7 Authors comments: Neither author is an expert in fossil conifers, yet we felt that the documentation of fossil conifers in the late Cretaceous of northeastern Utah was warranted, given how little has been published on the fossil plant record of late Cretaceous rocks in northeastern Utah. We gladly solicit comments on these fossil conifer specimens from Utah. PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01

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9 Figure 1. Slab containing fossils with affinity to Geinitzia from the late Cretaceous Williams Fork Formation of Utah. A. Over view of slab containing fossils. B. Close up view of the terminal branch. C. Close up view of terminal branch from isolated branch on same slab. D. Close up view of terminal branch on same slab, showing leaf scars. E. Close up view of terminal branch. PeerJ Preprints CC-BY.0 Open Access rec: 1 Mar 01, publ: 1 Mar 01