Modified Basal Elements in Dicroidium Fronds (Corystospermales)
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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/233935696 Modified basal elements in Dicroidium fronds (Corystospermales) ARTICLE in REVIEW OF PALAEOBOTANY AND PALYNOLOGY · DECEMBER 2012 Impact Factor: 1.94 · DOI: 10.1016/j.revpalbo.2011.10.012 READS 83 4 AUTHORS, INCLUDING: Ignacio H Escapa Edith Taylor Museo Paleontológico Egidio Feruglio University of Kansas 51 PUBLICATIONS 485 CITATIONS 160 PUBLICATIONS 2,697 CITATIONS SEE PROFILE SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Ignacio H Escapa letting you access and read them immediately. Retrieved on: 05 February 2016 Review of Palaeobotany and Palynology 170 (2012) 15–26 Contents lists available at SciVerse ScienceDirect Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo Research papers Modified basal elements in Dicroidium fronds (Corystospermales) Benjamin Bomfleur a,⁎, Ignacio H. Escapa b, Edith L. Taylor a, Thomas N. Taylor a a Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Institute, University of Kansas, Lawrence, KS 66045-7600, USA b CONICET, Museo Paleontológico Egidio Feruglio, Av. Fontana 140, Trelew, Chubut 9100, Argentina article info abstract Article history: We describe a distinct type of heteromorphic basal frond element in four species of the seed-fern foliage Received 12 August 2011 Dicroidium, based principally on a survey of plant-fossil collections from the Triassic of Antarctica. The mod- Received in revised form 28 October 2011 ified foliar elements are conspicuously enlarged, and arise at wide angles and obliquely to the frond plane. Accepted 31 October 2011 Those of D. elongatum and D. crassinerve arise more or less directly from the petiole base; they are overall Available online 6 November 2011 wedge- to fan-shaped with variably dissected margins and fan-shaped venation. Those of D. odontopteroides and D. dubium are (sub)circular, reniform, or obcordate, also with fan-shaped venation, or divided into nar- Keywords: cyclopterid row tongue-shaped segments with alethopteroid venation. The basal elements of some species commonly aphlebia occur isolated. By analogy with comparable structures among other plant groups, we conclude that the stipule basal elements represent a distinct, early leaf-ontogenetic architectural unit in the frond bauplan of Dicroi- taxonomy dium, which provides an important additional character for a more accurate delimitation and systematic clas- palaeoecology sification. Similar structures also occur on the corystosperm reproductive organs Umkomasia and Pteruchus.It seed ferns appears that the basal elements were particularly common among high-latitude Dicroidium trees that flour- ished in a strongly seasonal light regime. We hypothesize that the modified elements may represent the ini- tial foliar outgrowths during spring flush, ensuring a rapid re-initiation of sap flow and metabolic activity after the extended period of winter dormancy. © 2011 Published by Elsevier B.V. 1. Introduction et al., 2007; Morel et al., 2010), there is increasing evidence that these foliage types should more appropriately be included in a single, Detailed documentation of leaf architecture contributes to a better more naturally defined genus, Dicroidium Gothan emend. Townrow understanding of the biology and ecology of seed ferns, and can also (e.g., Townrow, 1957; Bonetti, 1966; Archangelsky, 1968; Anderson provide significant characters for systematic classification (e.g., and Anderson, 1983; Holmes and Anderson, 2005; Bomfleur and Laveine, 1967, 1997; Laveine et al., 1993; Krings et al., 2001, 2006). Kerp, 2010). Compared to Palaeozoic foliage taxa, however, few studies have The concept of Dicroidium as a biological entity has been greatly addressed details of leaf architecture among Mesozoic seed ferns advanced, especially in recent decades. In their work on the Triassic (e.g., Rees and Cleal, 1993). A prominent type of Mesozoic seed-fern Molteno Formation of South Africa, Anderson and Anderson (1983) foliage consists of the characteristically bifurcate fronds of the Corys- have provided a comprehensively illustrated documentation of the tospermales from the Triassic of Gondwana. The foliage shows wide natural variability of Dicroidium fronds sensu lato. Of particular im- morphological variability and plasticity, which renders taxonomic de- portance are several specimens that Anderson and Anderson (1983) limitation somewhat problematic (e.g., Townrow, 1957; term hybrid fronds—ones that bear different types of pinnules that Archangelsky, 1968; Retallack, 1977; Baldoni, 1980; Petriella, 1981; would be assigned by some authors to separate genera when found Anderson and Anderson, 1983; Boucher et al., 1993; Holmes and detached (see, e.g., Anderson and Anderson, 1983: pl. 74, Fig. 7). Anderson, 2005). As a consequence, no uniform concept for the clas- The anatomy of Dicroidium fronds has also been described based on sification of these fronds has emerged since they were first identified structurally preserved material from the Middle Triassic of the central almost a hundred years ago (Gothan, 1912). Whereas some authors Transantarctic Mountains (Pigg, 1990). The common occurrence of a continue to distinguish among a varying number of separate genera, particular type of secretory cavity has made it possible to affiliate including Dicroidium Gothan, Xylopteris Frenguelli, Johnstonia these permineralized fronds with anatomically preserved pollen or- Walkom, and Zuberia Frenguelli (e.g., Frenguelli, 1943; Retallack, gans, ovulate organs, and stems with attached leaf bases from the 1977; Gnaedinger and Herbst, 1998; Spalletti et al., 2005; Artabe same site (Meyer-Berthaud et al., 1992, 1993; Yao et al., 1995; Klavins et al., 2002). In addition, it has been demonstrated that Dicroidium fronds have a distinctive epidermal anatomy and cuticle morphology, ‘ ’ ⁎ Corresponding author. Tel.: +1 785 864 4255; fax: +1 785 864 5860. including amphistomatic fronds with pseudosyndetocheilic stomata E-mail address: bomfl[email protected] (B. Bomfleur). (sensu Retallack, 1977), which is especially helpful in genus and species 0034-6667/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.revpalbo.2011.10.012 16 B. Bomfleur et al. / Review of Palaeobotany and Palynology 170 (2012) 15–26 identification (Gothan, 1912; Townrow, 1957; Archangelsky, 1968; 3. Results Retallack, 1977; Bomfleur and Kerp, 2010). Such anatomical details, however, can only be studied when the specimens are sufficiently 3.1. Modified basal elements in Dicroidium elongatum (Carruthers) well preserved. Traditionally, it has been thought that Dicroidium was Archangelsky (Plate I; Fig. 1A–C) restricted to the Triassic of Gondwana (Gothan, 1912); however, the re- cent descriptions of well-preserved Dicroidium fronds from the Permian Localities. (1) Schroeder Hill (85°23′S; 175°12′W) in the Cumulus palaeotropics (Kerp et al., 2006; Abu Hamad et al., 2008)demonstrate Hills, central Transantarctic Mountains, East Antarctica; (2) Mount that a diagnostic concept of Dicroidium as a biological entity should Wisting (86°27′S; 165°26′W), Queen Maud Mountains, central Trans- not include the stratigraphic and palaeogeographic occurrences of the antarctic Mountains, East Antarctica; (3) undetermined locality in the fossils. As a result, it becomes increasingly difficult to maintain a suit- Brisbane area, Queensland, Australia. able taxonomic delimitation of this taxon on one hand and a natural Stratigraphy and age. (1 and 2) Falla Formation (Victoria Group, classification approach on the other. Beacon Supergroup), Late Triassic; (3) Tingalpa Formation (Ipswich Heteromorphic basal pinnules have been reported to occur spo- Coal Measures), early Late Triassic. radically in Dicroidium fronds from South Africa (Townrow, 1962; Material. (1) Twenty-eight individual plant fossils on fifteen spec- Anderson and Anderson, 1983) and Australia (Holmes, 1982), but imens: T1672, 1676, 1678, 1679, 1681, 1690, 1697, 1699, 1706, 1711, this feature has not been studied in detail. Here we describe a distinct 1726, 1731, 1734–1736; (2) one specimen: T1886; (3) one specimen: type of foliar dimorphism among several Dicroidium species, based T6382. mainly on a survey of compression assemblages from several Triassic Description. Fronds are generally 15–20 cm long, bifurcated in the sites in the Transantarctic Mountains. This report provides details lower third at an acute angle, pinnate, and characterized by only a that can contribute to a more robust systematic classification of few distantly spaced, long needle-like and single-veined pinnules, Dicroidium based on biological criteria, and offers a further step in each arising at an acute angle (Plate I, 1, 2). better defining and understanding the physiology and growth strate- Heteromorphic basal elements occur in the form of one opposite gy of the Southern Hemisphere Corystospermales. pair of 1–2 cm long foliar appendages that arise either directly or at a distance of up to ~5 mm from the swollen petiole base (Plate I, 1–5, 10; Fig. 1A–C); they are inserted at angles between 25° and 70° 2. Material and methods and in most cases appear to be inclined or slightly curved basiscopi- cally and obliquely to the frond plane (Plate I,1–5, 10; Fig. 1A–C). This study is based on a survey of collections of Dicroidium assem- The basal elements have a variable morphology ranging from ob- blages