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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Review of Palaeobotany and Palynology 153 (2009) 296–309 Contents lists available at ScienceDirect Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo Ultrastructure, morphology, and topology of Cambrian palynomorphs from the Lone Rock Formation, Wisconsin, USA Wilson A. Taylor a,⁎, Paul K. Strother b a Department of Biology, University of Wisconsin - Eau Claire, 105 Garfield Avenue, P.O. Box 4004, Eau Claire, WI 54701-4004, USA b Weston Observatory of Boston College, Department of Geology & Geophysics, 381 Concord Road, Weston, MA 02493, USA article info abstract Article history: A combination of white light with scanning and transmission electron microscopic analysis has enabled the Received 8 May 2008 detailed characterization of the morphology and topology of problematic spore-like palynomorphs recovered Received in revised form 8 August 2008 from Upper Cambrian near-shore deposits in Wisconsin, U.S.A. Members of the new taxon, Agamachates casearius Accepted 7 September 2008 gen. et sp. nov., are smooth, thick-walled, synoecosporal (within a common wall) packets containing up to four Available online 23 September 2008 spore dyads. The synoecosporal packets themselves may be aggregated into clusters of two or more packets. The discovery that the smallest purported meiotic units are dyads is supportive of prior hypotheses that attempted to Keywords: Cambrian explain the abundance of dyads in the lower Paleozoic fossil record. Their abundance has been especially Cryptospores perplexing given the absence of any modern plants that produce dyads via normal sporogenesis. Dyads appear to Early land plants precede tetrads as the fundamental resistant-walled propagule in the spore record, indicating a transitional stage Dyads in the evolution of sporogenesis in plants prior to the canalization of meiosis into a single coördinated process. The Sporogenesis variation in spore number per synoecosporal packet could be due to endoduplication of zygote DNA prior to Algae cytokinesis during sporogenesis–paralleling a process that occurs in Coleochaete today. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In this study, we present a detailed morphologic examination of what initially appeared to be a planar tetrad from a population of Over the past 35 years, our picture of the terrestrialization of the land palynomorphs extracted from strata of Late Cambrian age. These spore- surface has undergone a series of shifts in perspective. What began as a like palynomorphs possess some morphological and topological debate on the taxonomic status of isolated Silurian trilete spores (Gray qualities that ally them to younger Ordovician cryptospores, but, like and Boucot, 1971; Banks, 1975; Gray and Boucot, 1977; Gray, 1985), their Middle Cambrian counterparts, their morphology and topology is resulted in the recognition that permanent dyads and tetrads of perhaps more varied that seen in younger cryptospores. The current inaperturate spores were shed as diaspores by Early Silurian land plants report is part of a larger study that was undertaken to provide detailed (Pratt et al., 1978, Strother and Traverse, 1979). Richardson et al. (1984) structural information on selected, well-preserved Cambrian spore-like soon proposed a new palynomorph class, the cryptospores, to microfossils using combined light (LM), scanning electron (SEM), and accommodate this pre-trilete spore phase of plant evolution (Richard- transmission electron microscopy (TEM). A complete serial set of nearly son, 1996). Paleobotanists have been generally accepting of the 250 sections cut from one specimen has provided new information on cryptospore record acting as a proxy for the existence of embryophytes the number of spores present in each specimen, the nature of the as far back as the lower Middle Ordovician (Darriwilian) (Wellman and smallest units, and possibly the spore-spore symmetry. We continue to Gray, 2000; Edwards and Wellman, 2001; Steemans and Wellman, argue that the source of these problematic spore-like palynomorphs is 2003). But reports of cryptospores of Middle Cambrian age (Strother, closer to land-derived sporomorphs of plant origin than it is to either 2000; Strother and Beck, 2000; Strother et al., 2004)havemetwith freshwater green algae or acritarchs (which carry the implication of a some degree of skepticism (Wellman, 2003; Steemans and Wellman, marine origin). These microfossils fit clearly within the palynomorph 2003). Chief among the objections to the inclusion of these Cambrian group, cryptospores, as defined by Strother and Beck (2000),sincethey palynomorphs with the cryptospores is their lack of clear tetrahedral appear to represent forms more advanced than the algae, yet less symmetry, the variable numbers of spores in some polyads, and the huge uniformly geometric than spores characteristic of extant tracheophytes. time gap (ca. 45 Myr) between the Middle Cambrian material and the accepted cryptospores of Darriwilian age. 2. Materials and methods ⁎ Corresponding author. Tel.: +1 715 836 3176; fax: +1 715 836 5089. Sample SBT03-01 was collected from a freshly-worked road cut along E-mail address: [email protected] (W.A. Taylor). Route 131 southwest of LaFarge, Wisconsin (N 43° 33.185', W 90° 0034-6667/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2008.09.001 Author's personal copy W.A. Taylor, P.K. Strother / Review of Palaeobotany and Palynology 153 (2009) 296–309 297 Plate I. 1–6 Light micrographs of Agamachates casearius gen. et sp. nov. Scale bar in each figure is 10 µm. “a” and “b” designations represent different focal planes of the same specimen. 1. Two attached synoecosporal packets forming an apparent dyad. 2. A quartet of packets that gives the appearance at first of being a simple spore tetrad. 3. Four synoecosporal packets, each with apparent interior spores. 4. A quartet of packets in which the folded walls of the interior spores are more apparent than in Fig. 2. 5. A quartet of adpressed packets that appear as two dyads, but which appear to each contain one or more interior spores. 6. A larger, irregular cluster of packets. Author's personal copy 298 W.A. Taylor, P.K. Strother / Review of Palaeobotany and Palynology 153 (2009) 296–309 Author's personal copy W.A. Taylor, P.K. Strother / Review of Palaeobotany and Palynology 153 (2009) 296–309 299 Plate III. 11–14 The sectioned specimen and the holotype. Scale bar in each image is 10 µm. 11. LM of the sectioned specimen with 4 of the attached synoecosporal packets labeled A through D. 12. SEM of the sectioned specimen, arrows point to extra-sporal material attached to all packets. 13. This is a composite image of the sectioned specimen constructed by superimposing the images from Figs. 11 and 12 in Photoshop (see methods.) 14. Agamachates casearius holotype. Slide SBT03-01B(SEM), England Finder coordinates, H54/3, Natural History Museum accession number FM2097. 39.747'). In southern Wisconsin, the Lone Rock Formation is one of two chaspsis-Prosaukia biozones), corresponding to the middle portion of the glauconite-bearing Upper Cambrian units (Mahoney et al.,1997). The age Upper Cambrian. The sample, a light grey mudstone, is thoroughly of the Lone Rock Formation in this region ranges from the Elvinia to bioturbated at the mm to cm scale. Individual burrowed fabrics are filled Illanurus priscus trilobite biozones (from Runkel et al., 2007). Since our with fine-grained quartz sand containing large well-rounded glauconite collection is from the middle part of the formation, we consider the age grains, consistent with their likely origin as fecal pellets. The paleogeo- to be within the lower half of the Sunwaptian Stage (Conapsis to Pty- graphic position of the locality is close to the eastern margin of the Plate II. 7–10 SEM images of A. casearius. Scale bar in each image is 10 µm. 7. Cluster of synoecosporal packets (SP), (F) furrows within packets, (ESM) extra-sporal material, which, in some cases occurs as linear features that appear to represent attachment scars, (DIM) dimple in the synoecosporal wall indicating the presence of an underlying lumen within the spore. 8. Another cluster, (SP) synoecosporal packet. 9. A. casearius single packet with infolded contact area and extensive extra-sporal material. 10. A quartet of closely adpressed packets which appear without a common enclosing wall. Author's personal copy 300 W.A. Taylor, P.K. Strother / Review of Palaeobotany and Palynology 153 (2009) 296–309 sequence in Wisconsin, corresponding to a proximal, very near shore Table 1 setting on a very shallow, low angle slope. Packet designation 1st section Last section (last=230) Total maximum diam Approximately 10 g of crushed sample were dissolved in cold A013114µm concentrated HF followed by treatment with cold fuming HNO3 for Dyad 1 (bot left) 0 100 5 min and without further oxidation. Gravity separation was Dyad 2 (up left) 0 60 performed using ZnCl2 (s.g. 2.0). Isolated specimens were picked, Dyad 3 (up right) 0 76 dried on an SEM stub, coated and scanned on an AMRAY 1600 series Dyad 4 (up right) 16 131 B011014µm SEM. Single grains were then picked off the SEM stubs, embedded in Dyad 1 0 110 glycerin jelly and mounted on standard 1 in.×3 in.