Plant Evolution • Major events in the evolution of land plants – The Devonian Period was a time of rapid evolution for the land plants Subphylum Vertebrata • Subphylum Vertebrata has several divisions you need to be familiar with: – Superclass Agnatha – Jawless Fish; Lampry Eel; Ostracoderm (fossil) – Superclass Gnathostomata • Class Placodermi – First Jawed Fish (Fossils) • Class Chondricthyes – Cartilagenous Fish; Sharks; Rays • Class Osteicthyes – Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea Horse – Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth Geologic Ranges of Major Fish Groups Devonian Jawless Fish Devonian Seafloor Ray-Finned and Lobe-Finned Fish • Arrangement of fin bones for (a) a ray-finned fish (b) a lobe-finned fish – muscles extend into the fin allowing greater flexibility Crawling out of the Water • A mass extinction occurs when a large fraction of all living species becomes rapidly extinct. • The fossil record shows that at least five major mass extinctions have occurred in the past 500 million years. • Impacts of asteroids on Earth are suspected as a primary cause of mass extinction. • Three of the five major mass extinctions occurred during the Paleozoic era: – At the end of the Ordovician period, – during the late Devonian period, and – at the end of the Permian period. • This extinction occurred at end of the period, about 440-450 mya. • It is thought to be caused by a global cooling, which caused the continent Gondwana to glaciate. Geologists have found glacial deposits in the Saharan desert, which provided the evidence for this theory. • This event is theorized to be caused by an episode of global cooling similar to that of the Ordovician extinction. • This time, the glacial deposits have been found in northern Brazil. • However, meteorite impacts have also been thought a possible cause of this mass extinction, although the evidence remains inconclusive. • This extinction had little effect on land animals, mostly affecting (again) the marine life, in particular, the reef-builders. Devonian Timeline PLANT ANCESTRY • Plants represent a monophyletic group – Evolved from a common ancestor – Who was this common ancestor? • Multiple lines of evidence indicate that plants evolved from a group of green algae termed Charophytes – What is this evidence? MAJOR PLANT GROUPS • Four major periods of plant evolution – New structures evolved, adaptive radiations followed • Origin of plants from aquatic ancestors • Diversification of vascular plants • Origin of seeds • Emergence of flowering plants PLANT EVOLUTION • Plant ancestry can be viewed in terms of a nested set of monophyletic groups BRYOPHYTES – first in Ordovician • The gametophyte is the dominant generation in the life cycles of bryophytes VASCULAR PLANTS Adaptations of vascular plants • Differentiated bodies – Subterranean root systems (water & minerals) – Aerial stems and leaves (photosynthesis) • Vascular tissue – Xylem (water & minerals) – Phloem (organic nutrients) • Lignin – Cell wall component providing mechanical support Psilopsida & Rhyniopsida • Earliest land plants • Does not possess leaves or true roots • Stems photosynthesize • Simple, dicotomous branching • Apical reproductive structures, spores in sporangia • Rhynia is earliest fossil • Reconstruction of Early Devonian landscape. SEEDLESS VASCULAR PLANTS • Dominated forest landscapes of Devonian and Carboniferous period Three living divisions – Lycophyta – Horsetails (Shenophyta) – Ferns (Pterophyta) LYCOPHYTES • Division Lycophata • Evolved in Devonian period – Prevalent in Carboniferous period – Woody tree lineage • Became extinct near end of Carboniferous period – Herbaceous lineage • Represented today by ~1,000 species LYCOPHYTES Sphenophyta • Division horsetail • Ancient lineage of seedless vascular plants – Dates back to Devonian Prevalent during Carboniferous • Modern survivors include ~15 species in the genus Equisetum – Most common in Northern hemisphere – Generally found in damp locations, streambanks FERNS • Division Pterophyta • Ancient ancestry – Origins in Devonian period – Prevalent in Carboniferous period • Currently most prevalent seedless vascular plant – >12,000 species exist today – Most diverse in tropics SEED PLANTS Key adaptations of seed plants • Reduction of the gametophyte – Minute gametophytes retained within and protected by the sporophyte • Advent of the seed – Seeds replaced spores as a means of dispersing offspring • Evolution of pollen – Eliminated the liquid H[2]O fertilization requirement Plant Evolution • Major events in the evolution of land plants – The Devonian Period was a time of rapid evolution for the land plants Evolution of Seeds • The evolution of the seed during the Late Devonian – liberated land plants from their dependence on moist conditions – and allowed them to spread over all parts of the land • In the seed method of reproduction – the spores are not released to the environment – but are retained on the spore-bearing plant – where they grow into the male and female forms Progymnosperms • Seedless vascular plants – but likely progenitors of seed plants • Unlike other seedless vascular plants, progymnosperms had secondary vascular tissue (both xylem and phloem) and its structure is very like that of modern conifers • Some had a eustele and were heterosporous Evolution of Conifer Seed Plants • The appearance of heterospory – was followed several million years later – by the emergence of progymnosperms • Middle and Late Devonian plants • with fernlike reproductive habit • and a gymnosperm anatomy – which gave rise in the Late Devonian – to such other gymnosperm groups as • the seed ferns • and conifer-type seed plants Gymnosperms • Paraphyletic grouping of unrelated woody seed plants that are just not Angiosperms • Flowerless, seed-bearing plants • Possess cones, pollen carried by wind • Seeds dispersed by gravity or some animals • Important groups: – Pteridosperms - “seed ferns” – Cordaitales - strap-like leaves with parallel venation – Cycads – Ginkgos – Conifers (pines & firs) - next lab Evolution of Seeds • The evolution of the seed during the Late Devonian – liberated land plants from their dependence on moist conditions – and allowed them to spread over all parts of the land • In the seed method of reproduction – the spores are not released to the environment – but are retained on the spore-bearing plant – where they grow into the male and female forms Pteridosperms • Seed ferns • Have similar phenotypic characters as the true ferns, but with seeds & cones instead of spores Fossils • Oldest seeds from Devonian (365 mya) • Elkinsia • Archaeosperma Seedless Vascular Plant • Generalized life history of a seedless vascular plant • The mature sporophyte plant produces spores SEEDLESS VASCULAR PLANTS Vertebrate Phylogeny