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The Anatomy of Turinia Pagei (Powrie), and the Phylogenetic Status of the Thelodonti Philip C

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The Anatomy of Turinia Pagei (Powrie), and the Phylogenetic Status of the Thelodonti Philip C Transactions of the Royal Society of Edinburgh: Earth Sciences, 92, 15±37, 2001 The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti Philip C. J. Donoghue and M. Paul Smith ABSTRACT: Little is known regarding the internal anatomy of thelodonts, with most of the available information coming from a single specimen, the holotype of Turinia pagei (Powrie). Previous descriptions have led to many partial, or con¯icting interpretations. Herein, we describe fully the anatomy of T. pagei based on the holotype and additional material. T. pagei possessed a branchial system composed of eight pairs of gills, a buccal/nasohypophyseal region lined with minute denticles, comparable to buccopharyngeal denticles of sharks, and possessed a stomach which is preserved by sediment in®ll in the holotype specimen. Contrast between the petrological character of the gut in®ll and the sediment in which the animal is preserved suggests that the gut was in®lled in vivo and that T. pagei was probably a deposit feeder. Phylogenetic analysis resolves T. pagei and the Galeaspida as sister-taxa, comprising a sister-group to the Osteostraci plus jawed vertebrates. In contrast to the view that has prevailed hitherto, thelodonts with a dorsoventrally compressed cross-sectional pro®le comprise a monophyletic group, of which T. pagei is the least derived member. The furcacaudiforms are resolved as an unnatural group, one taxon being the sister taxon to the `conventional’ thelodont clade, and the other, the sister taxon to this clade plus galeaspids, osteostracans and jawed vertebrates. The analysis agrees with the earlier view that thelodonts lack distinct synapomorphies, but we argue that distinct synapomorphies are not a requisite of monophyly. KEY WORDS: `agnathan’, chordate, cladistics, homoplasy, palaeobiology, stem-gnathostome, thelodont, vertebrate. `The discovery by Powrie (1870) of the ®rst intact thelodont, 2000 for a de®nition of crown-group and stem-groups concepts ``Cephalopterus’’ pagei, from the Lower Devonian of Scotland, and how they are applied with respect to lower vertebrates). In did little to elucidate the relationships of the group. Instead, contrast, Turner (1991) and Turner & van der Brugghen (1993) over the years, this specimen seems to have confused the argued that all animals which possess thelodont-grade scales issue’ (Turner & van der Brugghen 1993, p. 127). are united by a synapomorphy of scale histology and structure, although they concurred with the view of thelodonts as primi- Thelodonts are extinct jawless vertebrates, which character- tive crown-group gnathostomes, or as the sister-group of istically possess a dermal skeleton composed of thousands of crown-gnathostomes. microscopic scales that were commonly dispersed after death Conventionally, thelodonts have been perceived as dorso- and the decay of supporting soft tissues. Thus, the fossil ventrally compressed, with unsupported pectoral ¯aps, dorsal record of these animals is dominated by jumbled collections and anal ®ns, and a hypocercal tail (Turner 1991). However, of scales that are recovered by acid dissolution of limestones whilst this bauplan may stand for most thelodonts, it does (e.g. Turner 1973). Rarely, however, rapid burial, lack of not encompass the recently discovered Furcacaudiformes current activity and absence of scavengers have combined to (Wilson & Caldwell 1993), which also possess thelodontiform preserve the skeleton of these animals in such a way that the scales. The Furcacaudiformes are characterised by a deep, articulation of the scales has been aVected only by decay of hump-backed and laterally-compressed body, approximately the internal organs and eventual collapse of the carcass. Such symmetrical tail, and lack the anal ®ns of dorso-ventrally com- events are extremely rare but of great importance because it pressed thelodonts (Wilson & Caldwell 1993, 1998; Caldwell & is only under these circumstances that the full variation of Wilson 1995). The existence of two distinct body forms of scale morphology can be fully appreciated (MaÈ rss & Ritchie thelodontiform scale-bearing jawless vertebrates reinforces 1998). Furthermore, it is precisely because such preservational Janvier’s (1981, 1996b) contention that, as conventionally per- events are so rare that we know so little about the anatomy and ceived, the Thelodonti are a paraphyletic group. However, the palaeobiology of this highly enigmatic group of vertebrates. lack of thelodont synapomorphies might result as much from Although thelodonts are commonly considered to be jawless absence of evidence as evidence of their absence. Considering vertebrates, it has been suggested that these animals are more the importance of thelodonts in scenarios currently being devel- closely related to sharks than to any other group of extinct or oped to explain the origin of jaws and teeth (e.g. Mallatt 1996; extant vertebrates (Traquair 1899a; Turner 1991). In contrast, Smith & Coates 1998, 2001) the need to understand thelodont Janvier (1981, 1986, 1996a, b) has argued that thelodonts anatomy more fully has never been of greater importance. express only symplesiomorphic characters (shared-primitive characters) and no autapomorphic features. This has led to the interpretation of thelodonts as a paraphyletic ensemble, 1. Turinia pagei (Powrie): a history of research of which some members may be more closely related to crown-group gnathostomes than others (e.g. Forey & Janvier The ®rst, and most complete articulated specimen of Turinia 1993, 1994; Wilson & Caldwell 1998; see Donoghue et al. pagei was discovered in the Lower Old Red Sandstone of 16 P. C. J. DONOGHUE AND M. P. SMITH e h t l x l e a n a i d d e e e e s y d h u n c c c c r r l u c t l a a c n n n n g o n n a e e e e h s e n a h i h a i t o l e d c d d d p c r h h n i i i i a a t s d i h a c o o b p r v v v v r e o e c s o r c t n p e e e e o a l m r g e n c u p s n a y e e t p o d o u e r o o o a o e s i e t h h n e b p i r 8 s n ? n ¯ n H t v n b o s o d k s s c n l e i a l a u f h a s m r a s t : n o y t t c i e l o r d t d e a t r t y f n a h e t n n u s h s m r i e k c o o n a a r c a c a b s o o h r l ) l r r t d e t c a t ) s c o l s a i h a 8 t e a a u n c t o l t o h 8 s - a i n p v b e 9 l m c p a ? r l s T s a d g b d b a a y e o 9 f n ( e i a o a o l u r i r n a n n n u s t r h 1 h r t N & ( t b g d s o b t e b p b a 7 ? s ? ? ± ® ± a h l x s c a s n l i p u s u y a h s a r u o c c s t l i ¯ a ’ g g d e n u r n a t h e r a a d i t t a d t o a r e p t e , d o h a h r h ) s n r l i i n e d s r d o c l p b p e 2 r a a e v m n a o o c n a n e o o s 9 l h r p o c s t d p s s a e u e c m 9 r e m u m u e r e r n n n r p l s y o 1 h s p T ( t ± a b o a a b a 8 ? o p a s ` r ® ± o s l l x a ’ a n l i e e s y r a h v r u c c s u t l i a a g t e n u r & n a t t h h r a a i t t a l t o l a r e n p e ) o a h r h l ) l s r a i u n e d e r 8 o c p b a e 1 r r k b a c e s e m n a s o c 9 t s i n e o s s t 9 l h e n r p r c i t d p p 9 x s n a e r e c m 9 u e u u o e u e r r u a 1 e n n r p s y p 1 h r ( T ( t v d a ¯ b o a a b a 8 ? t m p q s ? ¯ ± r e n r u T t l e x s n a t r & i n p o l a r o l h y a ) s r a a i e d n 8 a r c e 2 r r ¯ a y n o o c t a / n e 8 l i h r a r p c s t t n h a e c 9 g o e k u u r n e n r u e p s 1 h s T ( t v a b r ? b a 7 ? g ® ± t i v o n l N a ) a ; c s 4 e ) a e l s 6 n g 3 r s r a h 9 t o a n 3 i u e È c s 1 e o t 1 h h r l / b i i .
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