Herpetology – the Study of Amphibians and Reptiles • Both Are Very Diverse
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Herpetology – the study of amphibians and reptiles • Both are very diverse groups and are as different from one another as are birds and mammals • Why study them collectively? – Traditionally, they’ve been studied together because of complimentary life histories and the same or similar study techniques can be used in the field and laboratory – Also, it may have something to do with historical lack of interest in studying these groups because of: • Relative small size and secretive nature (among vertebrates) • Lack of commercial value • Un-endearing qualities (i.e., they’re creepy and slimy to some folks, even some biologists) – There is no indication that herpetology may become two separate fields of study 1 1 Vertebrate Evolution • Phylum Chordata characteristics – may be with organism its entire life or only during a certain developmental stage 1. Dorsal, hollow nerve cord 2. Flexible supportive rod (notochord) running along dorsum just ventral to nerve cord 3. Pharyngeal slits or pouches 4. A tail during some point of development • Phylum Chordata has 3 subphyla 1. Urochordata – tunicates; adults are sessile marine animals with gill slits • Larvae are free-swimming and possess notochord and nerve cord in muscular tail • Tail is reabsorbed when larvae transforms into an adult 2 2 2. Cephalochordata –lancelets; small marine animals that live in sand in shallow water • Retains gill slits, notochord, and nerve cord thru life 3. Vertebrata – chordates with a “backbone”; persistent notochord, or vertebral column of bone or cartilage • All possess a cranium • All embryos pass thru a stage when pharyngeal pouches are present • Recently, Vertebrata has been changed to Craniata with 3 superclasses: Myxini (hagfish), Petromyzontida (lampreys) and Gnathastomata (jawed vertebrates) Lamprey ammocoete 3 3 Jawless Fish Lamprey Hagfish Vertebrate Evolution 1. Tunicate larvae 2. Lancelet 3. Larval lamprey (ammocoete) and jawless fishes 4. Jaw development from anterior pharyngeal arches – capture and ingestion of more food sources 5. Paired fin evolution A. Eventually leads to tetrapod limbs B. Fin spine theory – spiny sharks (acanthodians) had up to 7 pairs of spines along trunk and these may have led to front and rear paired fins 4 4 • Emergence onto land – Extinct lobe-finned fishes called rhipidistians seem to be the most likely tetrapod ancestor • Similar to modern lungfish, had gills and probably lungs to breathe air • Teeth and limb bones closely resemble early amphibian bones 5 5 – Modern “walking” fish include walking catfish, mudskippers, and lungfish Lungfish – found in South Mudskipper America, Africa, and Australia Walking catfish – from southeast Asia but now found in Florida – Earliest known amphibians were labyrinthodonts • Had traits of lobe-finned fish and later tetrapods • Most modern salamanders still cannot fully support themselves with their limbs and have unshelled eggs like fish 6 6 • Evolutionary timeline: – Jawless fish – Cambrian Period, 530 MYA – Jawed fish – explosion of fish diversity in Silurian Period (425 MYA) – Terrestrial amphibians – Devonian Period (400 MYA) • Adaptations of some lobe-finned fish that allowed emergence onto land: – Limbs with digits – Lungs – A primitive neck • Fish-like ancestors probably evolved these traits in shallow swamps with stagnant water – Competition and an abundance of unexploited resources may have drove vertebrates onto land – Another theory is that early amphibians lived and fed in water but deposited eggs in moist places on land for better survival of eggs and larvae Coelacanth – modern lobe-finned fish 7 7 • The amniotic egg: reptiles, birds, mammals (amniotes) – Carboniferous Period (320 MYA) – To this day, amphibian eggs are still very similar to those of fish and must be placed in moist areas to develop; no protective shell – Seems to have developed to increase protection of terrestrial eggs from microbes – First, a fibrous shell evolved then, as added protection, a calcerous (calcium) layer was added – All modern-day reptiles deposit calcium crystals in a fibrous matrix 8 8 – Today, most reptile eggs must absorb moisture from the environment to complete development – It is not clear whether extraembyronic membranes evolved within primitive eggs or female’s oviducts – Earliest amniotes were a group of labyrinthodonts called anthracosaurs (below) • Amniotes – extraembryonic membranes – Do not need water to reproduce, no larval stage – Chorion, amnion, and allantois provide metabolic support for developing embryo – Yolk sac provides food 9 9 • Adaptive modifications required for terrestrial life: 1. Embryonic development not dependent on standing water 2. Direct development of embryo without a free-living larval stage 3. Skeletal and muscular support to withstand increased force of gravity 4. Feeding – shifts in morphology (shape of jaws) and behavior 5. Modification of integument to withstand friction, abrasion, and evaporation rates • Amphibians have more cell layers and a keratinized outer layer of skin that fish lack which helps reduce friction and punctures by foreign objects but does not reduce evaporation rates • Amniotes evolved even more cell layers and more keratin to outer layers to reduce evaporation • Thicker skin also aided in support of internal organs 10 10 • Early reptiles – Carboniferous Period 300 MYA – During this time, plants were becoming abundant and diverse on land providing a food source – Until then, vertebrates were primarily carnivorous, as most extant primitive fishes (catfish, lungfish, gar, bowfin) and amphibians are today – The only terrestrial animal prey available was fast-moving invertebrates (spiders, centipedes, and mites) – An explosion of reptile species began (adaptive radiation) when they became herbivorous – Earliest reptiles were cotylosaurs, represented by Hylonomus which still had many amphibian-like traits such as its skull, limbs, and girdle 11 11 Therapsida – the earliest mammals Permian Period 250 mya 12 12 13 14 15 16 8/26/2014 Systematics and Taxonomy of Herpetofauna • Systematics - the scientific study of the kinds and diversity of organisms and any and all relationships among them • The study of biodiversity and its historical (evolutionary) and contemporary patterns and processes, which involves the comparative study of living and fossil species • In the past, anatomical structures were used to infer relationships but there were problems discerning traits that were homologous (similar in appearance because of common ancestry) or a result of convergent evolution (analogous) • Convergent evolution has occurred often and is the result of animals with no recent common ancestor that have adapted to similar habitats and lifestyles in different regions of the planet Aardvark - Africa Giant anteater – South America 3 Species evolved to eat Sloth bear – ants and termites on India different continents 17 1 8/26/2014 • One great example is comparing Australian marsupial mammals with North American placental mammals • Now, DNA is used from living forms to derive relationships so we can more easily look at evolutionary descent • Taxonomy - the practice and science of classification – Taxonomies are composed of taxonomic units known as taxa (singular taxon) and are hierarchical in structure – As you drop from one level to the next, the taxa become more and more exclusive Examples of convergent evolution of placental mammals and Australian marsupials 18 2 8/26/2014 • Early tetrapod evolution – Early relationships among evolving vertebrates are confusing and several scenarios are presented – We believe all tetrapods descended from one group of bony fish (Osteichthyes) that were members of the fleshy-finned group (Sarcopterygii: coelacanth, lungfish) rather than ray-finned fish (Actinopterygii: sunfish, snapper, gar) – By mid-Permian (transition from Paleozoic to Mesozoic Era), amphibians and reptiles had diversified and radiated onto land • Amniotes were becoming dominant and consisted of more species and individuals than anamniotes 19 3 8/26/2014 20 4 8/26/2014 • The climate where tetrapods were evolving was hot and wet, and dense plant communities were found in valleys but upland plants were beginning to radiate • The herpetofauna living today represent remnant lineages of very diverse groups that shared some traits of amphibians and/or reptiles • Living and extinct (fossilized) species are grouped into higher taxa by temporal fenestrae (openings in the skull) 21 5 8/26/2014 • In the Triassic Period (early Mesozoic): – Dominant terrestrial vertebrates were reptiles and synapsids – Lissamphibians (modern amphibians) first appear in the fossil record although they probably date back to Permian – Some marine amphibians existed (a rarity) – The first lissamphibian in the fossil record is Triadobatrachus massinoti, a frog from the lower Triassic found in Madagascar • Its pelvic girdle and skull are very similar to modern frogs but it had more body vertebrae (14) that were unfused and 6 tail vertebrae 22 6 8/26/2014 Higher taxonomy of modern amphibian • Class Amphibia – Order Urodela (extant species)/Caudata – Salamanders • Lizard-like in appearance and most are small (largest is the giant Chinese salamander at 6 ft. and 140 lbs.) 23 7 8/26/2014 • Salamanders – Earliest fossils are from the middle Jurassic found in England – Fossils have been found in the northern hemisphere around the world and South America is the only southern continent with a significant distribution of species from one family – It appears they have always been