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Evolutionary History of the Metazoa

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Evolutionary History of the Metazoa If we make the time animals have Evolutionary History existed to be 1 day, humans have of the Metazoa been around for 61 seconds. Pushing back the origin of animals Mysteries of ancient animal life Oldest traces of animals: ~ 645 mya: 24- isopropylcholestane (2009) Goals: Consider the central questions Oldest animals sponges, ~650 mya about the origin of Metazoa. (Science Daily, 2010) Review modern evidence about time of origin, patterns of evolution, important processes. “ ” Outline for History of Life Meteorites bombard the planet 4.6-3.8 bya Making it inhabitable I. Cambrian Explosion Earth formed 4.5 bya 4.6 bya 1st evidence of life 4 bya II. What we knew 15 yrs ago Fossil b-g algae 3.6 bya III. More Recent Discoveries 3 bya 2.5 bya A. New Fossil Discoveries First cells 2.0 bya First multicelled algae 1.5 bya B. Molecular Dating with nucleus IV. Some explanations for deep divergence 1.0 bya Cambrian Period V. Why so many body plans in Cambrian? Precambrian, 4.6 bya 543-510 mya To 543 mya 600 mya 543 500 mya 400 Source Newsweek Biology’s “Big Bang”: The Cambrian Fauna (543-510 mya) In Burgess Shale alone, 15 of the 33 modern phyla are recognized n = 40,000 Astonishing array of And many Multicellular life. All More! Modern phyla present. Also 19 unique body forms of Unknown affinity Cambrian Explosion Cambrian deposits known since 1909 from >34 locations including the U.S.A., Russia, China, Canada Morris 1989 1 Summary: ~ 15 years ago ’ • Enigmatic Ediacaran assemblage (565 mya), many of Darwin s Dilemma which apparently never survived to the Cambrian • A 20 my gap in the fossil record before the Cambrian Why did the Cambrian Fauna present a challenge to the Darwinian view of Evolution? • Cambrian explosion of body plans Skeletized How did Darwin and his contemporary evolutionists fauna respond to this criticism? All Modern phyla present and many other types that did not survive. Why does Sukumaran argue that in evaluating the Cambrian explosion it is important to recognize that the difference between the concepts of diversity and disparity ? Artist’s rendition of Vendian assemblage Different Views on the Exact Nature as primitive multicelled animals (565 mya) of the Vendian assemblage: 1946: Martin Glaessner, an Australian paleontologist called them simple soft bodied animals related to jellyfish, corals, segmented worms 1983: German biologist Adolf Seilacher believed they were giant single celled organisms; their own kingdom of life; a failed experiment 1995: Gregory Retalack, Univ. of Oregon, argues they are ancient lichens (structural expts). 2008: Xiao and Laflamme….biota of protists, fungi, algae, and metazoa, but there is growing evidence that among the 100 or so disparate species there were also a few bilateria. Outline for “History of Life” A. Dating Techniques 1995: Groetzinger et. al . (Physical Chemistry) I. Cambrian Explosion II. What we knew 15 yrs ago Precise lead and zircon dating of Vendian-Cambrian boundaries III. More Recent Discoveries “bridges” the 20 my gap. A. New Fossil Discoveries Some Vendian species survive to the Cambrian; transitional rocks B. Molecular Dating between Vendian and Cambrian explosion bear traces of Ediacara IV. Some explanations for deep divergence metazoan life gaining momentum. Fauna V. Why so many body plans in Cambrian? 2 A. New Fossil Discoveries: Diploblasts Stromatoveris from early Cambrian Modern Comb Jelly Is strikingly similar to Pre-Cambrian Fronds A. New Fossil Discoveries Phosphatized remains 1997-98 Li et. al. & Xiao et al. Report finding multicellular sponges, and embryos of various phyla in 570 + 20 mya formation. Prof. Shuhai Xiao, Virginia Tech Univ. Review Trends in Ecology and Evolution Vol.24 No.1 The phylogenetic uncertainty of Ediacara organisms not Emphasis is restricted to classical Ediacara fossils that only limits their role in testing hypotheses about the tempo are soft bodied, macroscopic and morphologically diverse. of early animal evolution but also compromises our ability to interpret their ecology using modern analogs. Fortu- The Ediacara biota in space and time nately, ecological inferences can be independently made on Ediacara fossils are mostly restricted between 575 and the basis of trace fossils, functional morphology and taph- 541 Ma (Figure 1; Box 1). Discoid fossils from the onomy. In the past decade, investigation of trace fossils >635 Ma Twitya Formation in northwestern Canada associated with Ediacara body fossils has shed important [12] are similar to some simple forms in the Ediacara biota, light on the autecology of several Ediacara taxa, whereas but the absence of co-occurring complex forms and their recent advances in the paleoecology of Ediacara organisms significantly older age suggest that the Twitya discs are [9] are of close relevance to the evolutionary radiation in possibly simple forerunners rather than parts (e.g. hold- the Ediacaran–Cambrian transition. fasts) of complex Ediacara fossils. A few Cambrian fossils The Ediacaran–Cambrian transition marks a rapid are interpreted as Ediacara survivors or as phyletic des- change in taxonomic diversity, morphological disparity cendants [13,14], but with rare exceptions [14] the most and ecosystem complexity of early animals. Does the early iconic members of the Ediacara biota – the rangeomorphs Downloaded from rstb.royalsocietypublishing.org on 24 August 2009 evolution of the Ediacara biota share similar patterns with and erniettomorphs (Box 2), for example – are unknown in the Cambrian radiation of animals? Only recently have the Cambrian. It has been proposed that the demise of the Earliest fossil record of animals G. E. Budd 1427 paleontologists begun to approach this question using Ediacara biota might be due to the closure of a unique quantitativeClosing methods and a growing databasethe of Ediacara 20taphonomic my window mediatedGap by microbial activities fossils [10,11]. [4,15], and that the Ediacara biota continued to strive after Ordovician (part) The Ediacara fauna bridges the evolution of Promising results from emerging research of the phylo- the Cambrian radiation but were simply not preserved. 490 genic affinities, ecological diversity and evolutionary pat- However, the scarcity of Ediacara fossils in exceptionally Upper terns of theXiao Ediacara biota promptand this review.Laflamme Thus, we preserved Cambrian 2008: biota such as the Burgess Shale [16] 500 Cambrian will begin with a brief description of the spatial–temporal points to a more likely scenario of extinction or at least Middle multicellular life leading to the Cambrian fauna distribution and bodyplan diversity of the Ediacara biota, ecological restriction [2]. 510 Burgess Shale Cambrian Review Trends in Ecology and Evolution Vol.24 No.1 followed by a“On review of recent the advances ineve the phylogenetic, of animalThe restricted temporal radiation..” distribution is in contrast with preservation paleoecological and evolutionary analyses of this biota. a wide spatial distribution of the Ediacara biota. Ediacara 520 8 Lower 530 Cambrian 4 1 6 540 7 3 550 560 5 E 570 Figure 2. The Ediacaran acanthomorphic acritarch Tanarium pluriprotensum from the Tanana Formation, in the Giles 1 580 D drillcore, Officer Basin, Australia; 100!. At least some Gaskiers glaciation Precambrian acanthomorphic acritarchs may be the eggs of 590 animals. Courtesy of S. Willman. Ediacaran interval formative only in the upper part of the Doushantuo Fm, it ranges 600 2 C down to very close to the base, and thus to 630 Myr ago 610 or so. The claim would be that the oldest animal fossils of the Doushantuo Fm, dating back to just after the 620 Nantuo glaciation (i.e. the Chinese glacial deposits normally correlated with the Marinoan) are of this age, 630 a time that predates the first Ediacaran fossils by some B Marinoan glaciation 60 Myr, as well as the more conservative molecular 640 clock estimates for the divergence of the bilaterians. Despite the obvious uncertainties, the most reason- 650 able interpretation of the data thus is that embryo- forming animals of some sort had evolved by just after 660 A Marinoan time, and that sponges and presumed other animals had started to emerge by 580 Myr ago at the Figure 1. Provisional time scale for events around the latest, and that the Ediacaran biotas are likely to be a Precambrian–Cambrian boundary. 1, range of large, acantho- little younger than the Doushantuo embryos. The morphic‘Ediacaran’acritarchs (a genus thatcontains metazoan- upshot of the new data is that considerably more like embryos is found from close to the bottom of their range just above the Marinoan glaciation rocks); 2, possible range of the convincing evidence exists in the fossil record for an Figure 1. Temporal distribution (bars) and stratigraphic occurrences (black dots) of representative Ediacara genera, plotted against timescale of Ediacaran Period and fossil origin of the animals considerably before the Cambrian localities or stratigraphic units. The three Ediacara assemblages (Box 1) are indicated by different shades of gray. The Marinoan and Gaskiers glaciations, as well as the age Doushantuo embryos and cnidarian-like fossils according to range of the Doushantuo biota, are also marked. Modified from Ref. [58] with permission from the AAAS. Barfod et al. (2002);3,possiblerangeofthesameaccordingto than it did 10 years ago (Budd & Jensen 2000), with an Condon et al. (2005) (which is correct is uncertain, but the inferred documented fossil origin of the entire clade Figure 2. Possible phylogenetic placement of bilateral Ediacara fossils (vendomorphs, parvancorinomorphs, Yorgia, Kimberella and Dickinsonia), tri-, tetra-, penta- and 32 former is favoured here); 4, the ‘Ediacaran’ biota; 5, trace being datable to just after 635 Myr ago—a significant octoradial forms, and rangeomorphs in the metazoan tree. The diverse array of morphological constructions exemplified by the Ediacara biota suggests a greater fossils; 6, Cloudina and Namacalathus;7,classicalsmallshell result (see figure 2 for summary). phylogenetic diversity than typically assumed.
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