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Complexity Trends in the Evolutionary History of Dasycladalean Algae Michael Stearns

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Complexity Trends in the Evolutionary History of Dasycladalean Algae Michael Stearns Eastern Michigan University DigitalCommons@EMU Senior Honors Theses Honors College 2006 Complexity Trends in the Evolutionary History of Dasycladalean Algae Michael Stearns Follow this and additional works at: http://commons.emich.edu/honors Part of the Geology Commons Recommended Citation Stearns, Michael, "Complexity Trends in the Evolutionary History of Dasycladalean Algae" (2006). Senior Honors Theses. 29. http://commons.emich.edu/honors/29 This Open Access Senior Honors Thesis is brought to you for free and open access by the Honors College at DigitalCommons@EMU. It has been accepted for inclusion in Senior Honors Theses by an authorized administrator of DigitalCommons@EMU. For more information, please contact lib- [email protected]. Complexity Trends in the Evolutionary History of Dasycladalean Algae Abstract Dasycladalean algae are a group of marine algae that have an excellent fossil record extending back more than 500 million years. In our study, we investigated morphological complexity over the evolutionary history of this group. To determine complexity for each of more than 400 species, we assigned numerical values for six morphological features; the sum for each species forms a quantitative expression of complexity. We found that although maximum complexity increased over time, forms with minimum complexity persisted during most of the evolutionary history of the group. Degree Type Open Access Senior Honors Thesis Department Geography and Geology First Advisor Dr. Steven T. LoDuca Keywords Algae, Fossil, Dasycladales, Fossil Subject Categories Geology This open access senior honors thesis is available at DigitalCommons@EMU: http://commons.emich.edu/honors/29 Complexity Trends in the Evolutionary History of Dasycladalean Algae by Michael Stearns Advisor: Dr. Steven T. LoDuca Honors Thesis ABSTRACT Dasycladalean algae are a group of marine algae that have an excellent fossil record extending back more than 500 million years. In our study, we investigated morphological complexity over the evolutionary history of this group. To determine complexity for each of more than 400 species, we assigned numerical values for six morphological features; the sum for each species forms a quantitative expression of complexity. We found that although maximum complexity increased over time, forms with minimum complexity persisted during most of the evolutionary history of the group. 2 TABLE OF CONTENTS INTRODUCTION………………………………………………………….1-2 MORPHOLOGY AND REPRODUCTIVE MODES.…………………...3-7 THE DASYCLAD FOSSIL RECORD……………………………………7-8 METHODS…………………………………………………………………8-10 RESULTS AND CONCLUSIONS……………………………………….10-12 REFERENCES CITED…………………………………………………..13 APPENDIX I: DASYCLAD COMPLEXITY DATABASE…...…………14-24 3 INTRODUCTION Dasycladalean algae, or dasyclads, are an Order within the Division Chloraphyta (“green algae”) in the Kingdom Protista (single-celled eukaryotes) (Van Den Hoek et. al., 1995). The order is based on the radial symmetry and siphonous body plan of the cell (Fig. 1,2). Dasyclads are epifaunal organisms that must live in the photic zone of warm marine water. Dasycladalean Algae ("Dasyclads") Kingdom Protista Division Chlorophyta “green algae” Class Ulvophyceae flagellar apparatus: cruciate X-2-X-2 roots, with scales mitosis: closed, persistent spindle Order Dasycladales body plan: siphonous thallus architecture: radial symmetry Figure 1: Dasyclad classification (from Van Den Hoek et. al., 1995) Dasyclad algae are an important group for many fields of geology. In certain areas where dasyclads were widespread, there are entire packages of carbonate rocks that consist of their skeletons. Dasyclads are used as index fossils for stratigraphic correlation and as environmental facies indicators by 4 sedimentary geologists. Paleontologists use dasyclads to understand evolutionary trends, and biologists use them for genetic research (Berger Kaever 1992). a b Figure 2: Dasyclad thalli showing the radial symmetry characteristic of the Order Dasycladales (from Berger and Kaever, 1992) 5 Figure 3: Fossil dasyclad casements. The central cavity of the large casement contained the main axis; the perforations in the walls contained the laterals (from Berger and Kaever, 1992) MORPHOLOGY AND REPRODUCTIVE MODES The dasyclad thallus is composed of a central upright main axis and extensions of the main axis called laterals. Many shapes of dasyclad laterals are known, including phliophore, vesicular, acrophore, trichophore, and pirifer. (Fig. 4) The important thing to note about the different lateral shapes is that only acrophore can be cylindrical. 6 Figure 4: Lateral shapes, e-g phliophore, h vesicular, l acrophore, m trichophore, n pirifer (from Berger and Kaever, 1992) Another aspect of thallus morphology is lateral disposition, which refers to the arrangement of the laterals on the main axis. Disposition has two main variations. The first variation, called euspondyl, refers to the arrangement of laterals in whorls around the main axis. The second variation, aspondyl, refers to a random arrangement of laterals on the main axis. (Fig. 5) A structure related to the laterals is called a cortex. This structure is formed of distally inflated and fused laterals that form an outer “shell”. Some dasyclad species have a cortex; others do not (Fig. 6). Branching of the laterals is another aspect of thallus morphology. Laterals may be undivided or have several orders of branching (Fig. 7). 7 a b Figure 5: Lateral disposition: a. aspondyl b. two variations of euspondyl; note the arrangement of laterals into whorls around the main axis (from Berger and Kaever, 1992) Figure 6: The structure of the cortex. a. External view of the cortex. Note the fusing of the lateral terminations into a solid outer “shell” b. Cutaway view showing the main axis, laterals, and cortex (from Berger and Kaever, 1992) Figure 7: Lateral branching a. undivided lateral, b-d. Laterals showing several different orders of branching (from Berger and Kaever, 1992) 8 The presence of a heterocladous thallus is another variation in the morphology of the dasyclad thallus. A heterocladous thallus bears alternating whorls of fertile and non-fertile laterals along the length of the main axis (Fig. 8). Sterile whorls Fertile whorls Figure 8: Heterocladous thallus showing the alternation of fertile and sterile whorls (from Berger and Kaever, 1992) There are three different kinds of reproductive modes that dasyclads exhibit based on the location of cyst production (Berger and Kaever, 1992) (Fig. 9). The vast majority of dasyclads produce reproductive structures called cysts or gametangia, which contain and protect the gametes. Endospore cyst production refers to cysts being produced in the main axis. Cladospore cyst production refers to the cysts being produced in the laterals. Choristospore cyst production refers to the cysts being produced in specialized structures called 9 gametophores. Gametophores are outgrowths of the laterals, and may extend from any part of the lateral (Fig. 10). a b c Figure 9: Three modes of cyst production corresponding to different sites of cyst production in the thallus a. Endospore (in the main axis) b. Cladospore (in the lateral) c. Choristospore (in gametophores) c a b 10 Figure 10: Three views of cysts and gametophores a. cysts (small spheres) within gametophores (large clear spheres) b. several gametophores extending from the sides of laterals c. view of thallus with many gametophores (from Berger and Kaever, 1992) THE DASYCLAD FOSSIL RECORD Dasycladalean algae have a fossil record that extends from the Cambrian Period, starting 544 million years ago, to the present (Berger and Kaever, 1992). Most dasycladalean algae produce a calcareous casement made of aragonite (CaCO3). This casement covers the main axis and all or some of the laterals (Fig. 11). The majority of the dasyclad fossil record consists of these casements. In addition, dasyclads that don’t produce this casement have occasionally been preserved as fossils in anoxic lagoon settings (LoDuca, 1997, 2003). 11 a b c Figure 11: Carbonate casements a. living dasyclad’s casement that covers the entire thallus b. fossil casement showing where the cysts, main axis, and laterals would have been contained c. diagrammatic views of fossil dasyclad casements (from Berger and Kaever, 1992) METHODS To investigate patterns of complexity in dasyclads over time, we constructed a large database. In the database, dasyclads were treated at the species level and at the temporal resolution of geologic Series. The database spans from the beginning of the Ordovician, 490 million years ago, to the present. In total, it incorporates over 400 species, including the 35 living species found in today’s seas. The complete database is provided in Appendix 1. Figure 12 shows some sample entries from the database. 12 Quantifying relative complexity for dasyclads, as for other organisms, is challenging. To quantify complexity for the various morphological character states and for the different reproductive modes in dasyclads, we assigned numerical values based on inferred relative complexity, such that a higher value was assigned to the character state or reproductive mode regarded as more complex (see Table 1). Overall complexity for a given species is the sum of all the individual complexity values for that species We used three guidelines to determine the relative complexity for each character state and reproductive mode: the order of appearance in the fossil record, geometric complexity, and information from developmental analysis
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