See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/311861377 Agnatha, Chondrichthyes and Osteichthyes Chapter · November 2016 CITATIONS READS 0 1,531 2 authors: Antonis Petrou Charitos Zapitis AP Marine Env.Consultancy Ltd & Enalia Physis Environmental Research Centre University of Derby 29 PUBLICATIONS 131 CITATIONS 1 PUBLICATION 0 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Lionfish in the Mediterranean View project PCY1 - Investigating the effects of recreational diving on the macroalgal communities of the 'Zenobia' shipwreck (Cyprus) View project All content following this page was uploaded by Charitos Zapitis on 24 December 2016. The user has requested enhancement of the downloaded file. Chapter 26 - Agnatha CHAPTER 26: AGNATHA, CHONDRICHTHYES AND OSTEICHTHYES FISHES Antonis Petrou and Charitos Zapitis 1. INTRODUCTION The classification of fishes is not straightforward since they do not form a natural scientific grouping like the other vertebrate classes, i.e. the amphibians, reptiles, birds and mammals. Indeed, fishes can be considered by exclusion to be vertebrates that are not tetrapods (see Chapter 25: Introduction to Vertebrates). They are aquatic, gill- bearing, ectothermic ('cold-blooded') animals with a distinguishable head and, when present, digit-less limbs. Traditionally, fish have been arranged into three groups: ➵ Agnatha 1, the jawless fish (Myxini [hagfishes] and Hyperoartia [lampreys]); ➵ Chondrichthyes, the cartilaginous fish (sharks, skates and rays); ➵ Osteichthyes, the bony fish (Actinopterygii [ray-finned fishes] and Sarcopterygii [lobe-finned fishes]). This classification is adequate for general purposes, although Agnatha is paraphyletic and includes several groups of extinct jawless fishes. 2. THE EVOLUTION OF FISHES The first proto-fish appeared in the mid-Cambrian period around 530 MYA. The best known was Pikaia gracilens (named after Pike Peak in Canada), a lancelet-like crea - ture around 5 cm long, with a slender worm-like body and large prominent head. Although technically not vertebrates, they had a primitive notochord, so were 'chor - dates', had the basic vertebrate body plan, and were at the root of the vertebrate line - age. During the Ordovician period, 490 - 430 MYA, the first indisputable fish appeared. 2.1. Agnatha Unlike modern fishes, they were jawless and toothless, and were traditionally named agnathans (Greek: 'no jaws'). The most primitive lacked fins and many were covered with thick bone-like plates, which provided some protection against arthro - pod predators such as the giant sea scorpion. The agnathans flourished and by the end of the Silurian period (445 - 420 MYA) reached the peak of their evolution. During the Devonian (420 - 360 MYA), competition from and predation by the sea scorpions (some of which grew up to 2 m in length) and the newly emerging jawed fish, resulted in a decline in agnathans from which they never recovered. There are just two orders of extant jawless fish as shown in Fig 1: Myxiniformes (hagfish) with around 76 species and Petromyzontiformes (lampreys) with Epinephelus marginatus around 38 species. These two lineages had already diverged before the (Dusky Grouper). 1The extant agnathans (the hagfish and lampreys), have been placed together in the grouping Antonis Petrou Cyclostomata (Greek: 'round-mouthed') (see Fig 1), the taxonomical status of which is contentious. 621 : ’ s r s , l s s s The Wildlife of Cyprus - a u d d r n o m o i o r f m a ‘ d i b o p , o f m b a a s i p a r h d e h t a l evolution of the hinged jaw and paired appendages (Shimeld & Donoghue, 2012). A . e i r p m ) p e t s t r h g e T h e d p o s m s g i i T f e n o a f fossil find in South Africa from the Devonian period has shown that lampreys have g e r m l n T a f u l s a e - i n o s c a h a c e e changed little in the last 360 MY (Gess et al ., 2006). i s r r p p m s e t r r r 5 o m e f e i o A t n h d e The dramatic evolutionary changes that occurred in the early fishes are described t o r u m i r s o n o S o o d & i p d i p in Chapter 25 and it is sufficient just to list these changes here, i.e. paired fins, jaws, a n e e a o L c ) D i h r h s f p s t ) e i A i f ( s c s g f ) e bony skeleton and lungs. At the start of the Devonian 420 MYA, the first lineages e n e p s u o m s l i r i e h 1 i n o f a s i i g h l o i t w a f diverged into the Chondrichthyes (the cartilaginous fish) and the Osteichthyes (bony n s r y t n i o e r s f i d u d p o e A i t v g ( t e a r fish). The Placodermi (placoderms) (Greek: 'plate-skinned'), heavily armoured bottom- r n D e p n e C u o n l s v i ( e c i f ) s c r s o dwelling fishes, which first appear in the fossil record from 435 MYA, were initially - e h e t a p e n s m d r a S 2 c b o f e a - i l i i o e successful but went extinct by the end of the Devonian period 365 MYA. h l t f o h ( i c n ( , l s t a a ) ' c e i e a n h c p l k i e l e - a . p p o s h s r C c e m c i r h i c s e a i o e f l s p ' p l ( l s 2.2. Chondrichthyes e a s m 0 f A e 0 o o 0 , : m r % 0 C 0 o 3 1 f 4 As suggested by their name, the cartilaginous fish or Chondrichthyes (Greek: i d . c a n r g c u e i s o P t r , r a n F e chondr = 'cartilage'; ichthys = 'fish'), lack true bones and their skeleton is composed e h s d t r i e r o w p t e s i s e r r t e e e of cartilage. By around 400 MYA, they had split into two subclasses: Holocephali g ) r e d ' t r a l l o s e e p s 0 h o e n 4 i T . ('complete heads'), commonly known as chimaeras or ghost sharks, and c e o a e m ) l c p n i s o b f s e 1 e w c T ' o m ( r Elasmobranchii (Greek: elasmos = 'metal plate/ductile'; Latin: branchus = 'gills'), b ( o f i i m commonly known as sharks, skates and rays. Holocephali are so called because they A s ) s e s a i t e i i e t c have a single gill opening on each side giving the head an undivided look (cf. the a s e a g h r e p ) s s s i y m r i b 7 r r m a ) e f e o ' g l e f ( t e Elasmobranchii, which have several gill openings on each side). Although Holocephali i o t e e t s o d t t r s s n e o p h o e l s o i o n o v p w o were successful and abundant in the Mesozoic Era (245 - 65.5 MYA), just one of the ) ' h e b n n s ( L i t i d e H s f h t e a i s e - i c f c e n e y seven known orders is still extant, the Chimaeriformes (chimaeras/ghost sharks) with l p w s A d s a e G d a 7 r a j 2 m ( p r . ( a o & f c just over 50 known species. i s r n e o s e n g e r p u The earliest undisputed fossil from a shark, a placoid scale, dates back 420 MY. The i t c s ( A ) s s e e i h Carboniferous Period (360 - 286 MYA), saw such proliferation of shark species that it is c s i i e f s p e d s e e e t 2 e m r s 1 r ) g ' o known as the ‘Golden Age of Sharks’. Around 305 MYA, the flatter-bodied rays and & f o i a s e r r r l i e i h s t n t d c e i p i r c y b o l skates (superorder Batoidea) split off from the sharks (superorder Selachimorpha). ( n e o a p b P o s c ' 0 h ( 0 ) 6 s During the Permian - Triassic transition 252 MYA there was a major extinction (the . C e a t c a k d s n a d r a s n 'Great Dying'), in which 99% of all marine species perished. However, many sharks e r a d o e i s d t o y r t a o s s r a ( e 4 i B e c came through the event and enjoyed a further period of adaptive radiation starting 8 e c p s n 0 i 0 a i 200 MYA during the Jurassic and Cretaceous.