Cephalopod Hooks, Both Recent and Fossil

7 Cephalopod Hooks, Both Recent and Fossil THEO S. ENGESER

M. R. CLARKE1

Geologishes-Palaeontologisches Institute und Museum Universität of Hamburg 2000 Hamburg 13, Federal Republic of Germany

Marine Biological Association of the United Kingdom The Laboratory, Citadel Hill Plymouth PL1 2PB, England

I. Introduction

Several families of recent and fossil coleoid ( = endocochleate) cephalopods have hooks on their arms and/or tentacles. Fossil evidence suggests that pre-Tertiary arm hooks are restricted to the extinct belemnoid cephalopods which are known from the early Devonian to the Cretaceous-Tertiary boundary. The belemnoid cephalopods had a varying number (20- 100) of paired hooks on each of 10 arms which were nearly equal in length (Fig. 1). Fossil hooks are found in association with shell remains of the belemnoid cephalopods, separately preserved in marine sediments in quite large numbers, or in the fossilized gastric contents of such ancient predators as fish, ichythosaurs, pliosaurs, and other cephalopods. Fossilized arm hooks, either loose or associated with belemnoid shells, have been very inadequately studied. What literature does exist is usually very old and loose hooks are labeled as scolecodonts and described under Annelida. Although hooks are of frequent occurrence in the Mesozoic, particularly the Jurassic, there are only two modern studies on isolated hooks (Kulicki and Szaniawski, 1972; Wind et al, 1977). In recent coleoids, hooks are found in all or some of the members of the teuthoid families Onychoteuthidae, Enoploteuthidae, Octopoteuthidae, Gonatidae, and Cranchiidae. Although they have often beenfiguredin taxonomic descriptions, their detailed morphology has not been compared 'Present address: Ridge Court, Court Road, Newton Ferrers, Plymouth PL8 1DD, England. 133 THE MOLLUSCA, VOL. 12 Paleontology and Neontology of Cephalopods

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across the groups or with the fossil coleoid hooks nor has it been related to muscle function. In this chapter we shall combine a short review of what is already known of fossil and recent hooks with the results of new examinations and shall examine the evolutionary implications of this research. II. Fossil Hooks Arm hooks of early coleoids were probably chitinous and, in life, were hollow with a central cavity of different sizes and shapes. After fossilization, the cavity has either remained hollow, become filled with calcite, calcium phosphate, or sediment, or has collapsed. The hooks are usually very friable and frequently fractured or broken, and their surface is usually dull black. They may occur in all marine sediment types. After treatment of sediment samples with hydrogen peroxide, sodium pyrophosphate, or acetic (or monochloracetic) acid, they can be picked from dried or wet residue. Cephalopod arm hooks have been reported from the Late Carboniferous (295 m.y.) to the Late Cretaceous (65 m.y.), but they will probably also be found in earlier Paleozoic sediments since coleoid cephalopods have recently been discovered in the Early Devonian "Hunsriick Shale" (390 m.y.) (West Germany) (Bändel et al, 1983). Their absence from Tertiary sediments may be due to the extinction of the hook-bearing coleoids at the CretaceousTertiary boundary and perhaps the different chemical composition of the arm hooks of Tertiary and Modern teuthids caused a low fossilization potential (see below). Because all recent cephalopods are marine and it is thought likely that no extinct members of the group penetrated fresh water, fossil cephalopod hooks are a useful microfossil indicator of marine conditions. The first and only existing descriptive diagram of fossil cephalopod arm hooks was published by Kulicki and Szaniawski ( 1972), and is based on arm hooks from the Jurassic of Poland. Recent discoveries of isolated hooks and complete cephalopods with hooks require a more sophisticated descriptive terminology for some morphological feature previously undescribed (Engeser, 1987). The three principal morphological characteristics of an arm hook, the base, the shaft, and the uncinus (Kulicki and Szaniawski, 1972), its orientation, and the standard measurements are shown in Fig. 1. Variations of this basic form include internal and external spurs, laterally and internally/externally extended hook bases, strongly or slightly internal curved uncini, or even slightly external curved uncini. The surface may bear small ornamentations (ridges, furrows, grooves, knobs). Particular features of the fossil coleoid cephalopod arm hook which we shall compare (Fig. 1 ) with the hooks of recent coleoids are the orbicular scar, the spur, and the base. The

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Cephalopod Hooks, Both Recent and Fossil

Fig. 1. A schematic diagram of the main part of a typical fossil hook.

orbicular scar represents the margin up to which the hook was probably covered with soft tissue. The spur, whose function is unknown, may be situated on the internal, external, or even lateral side of the shaft, closer to the base or to the uncinus or almost exactly equidistant between them. The base was, in life, enclosed in muscles and attached the hook to the arm. Because they have similar shapes from the smallest to the largest sizes found on arms, these hooks were clearly not derived from a sucker ring during ontogeny as are the hooks of living coleoids. A. Fossil Cephalopods w i t h Hooks

To date, fossil arm hooks have only been found isolated or associated with belemnoid coleoid remains. There is no evidence that any other pre-Tertiary cephalopod group possessed arm hooks. For example, despite thousands of finds of fossil "teuthids" with preserved soft part remains, none of them

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showed arm hooks or even impressions of hooks (as might be expected if they had been of different chemical composition and not preserved). First let us consider the different groups of the belemnoid coleoids. The classification of the belemnoid coleoids is currently under discussion, but most authors agree in recognizing four groups, the aulacocerids, the phragmoteuthids, the belemnoteuthids, and the belemnitids (=belemnites s. str.). Whether these groups should be regarded as orders or suborders remains to be decided. Although hooks have not yet been found in association with aulacocerids, the last three groups share the typical belemnoid arm hook (Fig. 1 ) with some variations, arranged in 20-50 pairs on each of the 10 subequal arms; this suggests a monophyletic origin of the belemnoid coleoids (apart from other features they have in common). In addition to the belemnoid coleoids, there are other coleoids with phragmocones, small aragonitic rostra, and proostraca, but they probably have no arm hooks (e.g., the Groenlandibelidae of the Late Cretaceous) and this line probably led to the Tertiary and Modern spirulids, sepiids, and teuthids. 1. The

Phragmoteuthids

The phragmoteuthids are characterized by very thin, aragonitic rostra and three-part, ventrally open body chambers or proostraca. They have been reported from the Late Permian (240 m.y.) to the Early Jurassic (200 m.y.). Five phragmoteuthid species have so far been described with preserved soft parts and hooks. Other species were based on shells or arm crowns alone. The five described species are: Phragmoteuthis bisinuata (Bronn 1859), Middle Triassic, Alps. "Phragmoteuthis" montefiorei (Buckman 1880), Early Jurassic of southern England. "Phragmoteuthis" conocauda (Quenstedt 1849), Early Jurassic of south Germany. Sueviteuthis zellensis Reitner & Engeser 1982, Early Jurassic of south Germany. Sueviteuthis schlierbachensis Reitner & Engeser 1982, Early Jurassic of south Germany. Phragmoteuthis bisinuata and Sueviteuthis zellensis have more slender hooks with a slightly curved uncinus compared with the "normal" belemnoid arm hook (Fig. 1). The hooks of Sueviteuthis schlierbachensis are very long and slender, and slightly externally curved (Fig. If). The hooks of "Phragmoteuthis" montefiorei and "Phragmoteuthis" conocauda (Fig. 2d) are quite different and show a well-developed, strongly curved uncinus and laterally extended bases forming large, rounded "knobs." The two species

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Cephalopod Hooks, Both Recent and Fossil Λ

e

f

g

h

Fig. 2. Eight basic variations of the typical belemnoid arm hook, (a) Belemnoteuthis antiqua Pearce 1847, Middle Jurassic, England; (b) schematic belemnitid arm hook (from Reitner and Urlichs, 1983; Riegraf and Hanff, 1983); (c) parataxon "Onychites" ornatus Quenstedt 1857, Middle Jurassic, Germany; (d) "Phragmoteuthis" montefiorei (Buckman 1880), Early Jurassic, southern England; (e) Chondroteuthis wunnenbergi Bode 1933, Early Jurassic, Germany and England; (f) Sueviteuthis schlierbachensis Reitner & Engeser 1982, Early Jurassic, south Germany; (g) parataxon Comuncus Kulicki & Szamawski 1973, Middle Juras­ sic, Poland; (h) parataxon "Onychites" rhynchoides Putzer 1939, Early Jurassic, south Germany.

therefore should be separated on the generic level from the typical Phragmoteuthis. 2. The Belemnoteuthids

The belemnoteuthids have one-part dorsal proostraca and small aragonitic rostra (stratigraphie range: Jurassic, ?Cretaceous). Five species of this group have been found almost complete with arm hooks. Belemnoteuthis antiqua Pearce 1847, Middle Jurassic, England. Belemnoteuthis mayri Engeser & Reitner 1981, Late Jurassic of south Germany. Acanthoteuthis speciosa Münster 1839, Late Jurassic, south Germany. Chondroteuthis wunnenbergi Bode 1933, Early Jurassic, Germany, England. "Belemnoteuthis" syriaca Roger 1944, Late Cretaceous, Lebanon.

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The first three species have typical belemnoid arm hooks (Fig. 2a) which are almost indistinguishable from each other. On the other hand, Chondroteuthis wunnenbergi is characterized by peculiar hooks with external spurs (Fig. 2e) which are situated on the proximal lower third of the arms. The hook form changes gradually to a more typical belemnoid hook on the distal two-thirds of the arms. The hooks of "Belemnotheuthis" syriaca have not been properly described but are short and weakly curved and otherwise are fairly typical belemnoid hooks. 3. The Belemnitids (= Belemnites s. str.)

Belemnitids are characterized by big, calcitic rostra and one-part proostraca. Some of the previously described belemnites with "soft parts" were proved to be forgeries (Donovan, 1977; Riegraf and Reitner, 1979). More recently, four specimens representing three species or varieties of belemnitids have been described with genuinely preserved soft parts and hooks (Reitner and Urlichs, 1983; Riegraf and Hauff, 1983). These are: Acrocoelites (Toarcibelus) raui (Werner 1912). Passaloteuthis paxillosa (Schlotheim 1820), var. A, Schwegler 1962). Passaloteuthis paxillosa (Schlotheim 1820), var. C, Schwegler, 1962 (specimen 1). Passaloteuthis paxillosa (Schlotheim 1820), var. C, Schwegler, 1962 (specimen 2). All four specimens come from the Early Jurassic (200 m.y.) Posidonia Shale of south Germany. Acrocoelites (Toarcibelus) raui is reported as having two types of arm hooks. The hooks of the proximal two-thirds of the arms are almost of the same type as the "typical" belemnoid hook. Toward the distal ends of the arms they change gradually to hooks with an internal spur (Reitner and Urlichs, 1983) (Fig. 2b). The Passaloteuthis paxillosa (Schlotheim), var. A of Schwegler, has only "typical" arm hooks, however, the distal parts of all 10 arms are missing, as are those of Passaloteuthis paxillosa (Schlotheim), var. C of Schwegler (specimen 2) (Riegraf and Hauff, 1983). On the other hand, Passaloteuthis paxillosa (Schlotheim), var. C, Schwegler (specimen 1), has internal spurs on all of the hooks along the arms. This specimen exhibits one very large hook called an "onychite" at the base of the arm crown with its peak looking forward (it is more likely that this hook was paired). It is more than 10 times the length of the normal-sized hooks. To which arm pair such hooks belong is not clear. Riegraf and Hauff (1983) regarded them as tentacular arm hooks (similar to Onychoteuthis, see below), but this is improbable considering their situation and orientation and the fact that belemnites had 10 subequal arms each equipped with 30-50 pairs of normal-sized hooks.

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Also, because only one of the four specimens had this large hook, it seems more likely that they are a sexual modification, probably of the male belemnite, similar to modified suckers on the hectocotyli of some living coleoids. 4. Uncertain

Forms

Jeletzkya douglassae Johnson & Richardson 1968, the oldest belemnoid species with hooks, is recorded from the Pennsylvanian (295 m.y., Late Carboniferous) of the Mazon Creek Formation (Illinois, U.S.A.) and has strongly curved, "typical" arm hooks. The systematic position of this genus is uncertain. B. Isolated Hooks and A r m Crowns

Isolated hooks and arm crowns are recorded from various localities and stratigraphical levels. Since an exact attribution of these isolated hooks and arm crowns to particular orthotaxa (guards) is quite uncertain or even impossible, a parataxonomic system was proposed for them (Quenstedt, 1856 — 1857; Kulicki and Szaniawski, 1972). Although these hooks have been given a binominal name, they represent "morphonyms." Some hook types do not correspond with a particular specific orthotaxon and occur in more than one. Others are characteristic for particular guards, and a combination of the hook types and the features of the guard may even be more specific than the shell alone (see above for belemnitids). Whereas some hook types found isolated can now be attributed at least to a particular suborder of the belemnoids, two hook types are unknown in combination with an orthotaxon, the form genus Cornuncus Kulicki & Szaniawski 1972 (Fig. 2g) and the form species of "Onychites" rhynchoides Putzer 1939 (Fig. 2h). Because it is beyond the scope of the present work to describe all records of isolated hooks and arm crowns, a compilation is given in Fig. 3 in which all records of complete belemnoid specimens, isolated arm crowns, and isolated hooks are listed stratigraphically and attributed to eight basic variations of the typical belemnoid arm hook. C. Musculature of Hooks

Generally there is nothing left of the muscles attaching or surrounding the hooks to suggest how they were attached or used. However, the fact that the hooks can lie on the arm with their outer sides either toward the end of the arm or toward its base (Fig. 4) suggests that they were attached to the arm by a narrow stalk. In one fossil, Belemnoteuthìs antiqua Pearce 1847 (Mantell, 1852), muscles were wrapped around the base. However, no suckerlike structure as interpreted by Mantell (1852) seems to have been present and

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the form of the bases of most of the hooks suggests that they provided a "handle" for the muscles to hold (Fig. 2). Perhaps where the handle was long it gave some fulcrum for leverage and when it was laterally expanded it provided a means for the muscle to move the hook laterally (Fig. 2). III. Hooks of Recent Coleoids

Hooks of recent coleoids are chitinous and have been developed from the chitinous rings found in the suckers of all decapods. Stages of their formation can be followed during growth, in which certain distinct suckers in the young squids gradually become modified to form hooks (Kristiansen, 1977). Because of the differences in origin, the terminology relating to hooks from living forms is different from that relating to fossil forms (Fig. 5). The adult form always bears two structures which indicate the evolutionary stages and ontogenetic steps in the hook's formation; these are the external opening and the groove into which the muscle is inserted. As a sucker ring becomes modified (Fig. 5), itfirstloses all but the central tooth and becomes flattened

Fig. 4. England .

"Phragmoteuthis" montefiorei (Buckman, 1880), Early Jurassic of souther

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Fig. 5. Stages in the ontogenetic and evolutionary development of recent coleoid chitinous hooks from sucker rings, ( a - e ) External (oral) views of ring and hook to show the development. 1, External opening; 2, internal opening; 3, groove of insertion of muscle; 4, groove of fusion; 5, basal lobe; 6, crenulations; 7, notch, ( f - j ) Transverse sections at positions of dashed lines in a - e. (k - n) Sagittal sections of stages in a - e (assuming axes of hooks are straight).

laterally (Fig. 5b). The distal part of the ring bulges out and extends distally, carrying the attached tooth with it. The insertion groove of the muscle is carried distally with the tooth and, when the two sides of the anterior part of the external opening progressively fuse from behind the tooth toward the proximal end of the opening (Fig. 5c), the two sides of the groove come to lie close and parallel to one another. As the external opening becomes restricted, the sides of the ring near the proximal end become blown out to form large basal lobes (Fig. 5d), which may be flattened, crenulated, or have notches to take muscle insertions. The line of fusion of the edge of the

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external opening along the external face of the shaft of the hook sometimes disappears so that the shaft is round, elliptical, triangular, or almost bladelike in cross section (Fig. 5e). Hooks are found infivefamilies of recent teuthoids.

A. Onychoteuthidae

In Onychoteuthidae, hooks (12-38 in number) are present only on the tentacles and those of the dorsal row are more elongated than those of the ventral row of the club. The form is generally similar in all species (Fig. 6a - i), with the large basal lobes usually having crenulated edges and one or two notches on the proximal sides. The single notch is characteristic of Onychoteuthis (Fig. 6a), Ancistroteuthis, and the nearly related Moroteuthis knipovitchi (Fig. 6b-e). This asymmetry appears to be apomorphic, being derived from the double-notched form possessed by Kondakovia andEnoploteuthis. Whether it has a functional significance is not known, but if it is an insertion for a muscle it may cause a twisting of the hook when it contracts.

B. Enoploteuthidae

In Enoploteuthidae, the arms as well as the tentacles bear hooks. The tentacular hooks are very similar to those in the Onychoteuthidae in Enoploteuthis (Fig. 6j,k), but in Abralia and Abraliopsis (Fig. 6r,s) several developments have taken place. The groove of insertion does not extend very far up the shaft so that the hood enveloping the hook is free from the hook except at its base and around the edge of the external opening, the shaft has flattened and become like a knife edge on the external side (i.e., inside the curve of the hook), the basal lobes have become flatter and thicker on the edges, the external opening has almost closed, and the internal opening has become smaller and come to lie more toward the proximal side. Arm hooks show developments similar to those of the tentacle hooks but the shafts never become so long, the groove of insertion extends up the shaft to beyond or as far as the main change in direction of the hook, and the basal lobes do not have distinct notches. The external opening is often "drawn up" on a narrow ridge which forms part of the external cutting edge of the hook. Ancistrocheirus lesueuri tentacle hooks (Fig. 6m-q) differ from most of the species described in the family by having no notches in the basal lobes. C. Conatidae

The large tentacular hooks oiGonatus species (Fig. 6cc-ee) have a basic structure similar to that of the Onychoteuthidae except that the base is not

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Fig. 6. Hooks of recent coleoids. (a) Onychoteuthis; ( b - e ) Moroteuthis knipovitchi; ( f - i) Kondakovia; (j -1) Enoploteuthis; (m - o ) tentacle and (p - q) arm hooks oi Ancistrocheirus; (r, s) Abraliopsis; (t) tentacle and (u) arm hooks of Pyroteuthis; (v) Octopoteuth'is; (w, x) hooks of Taningia; (y) Taonius; (z) arm and (aa) tentacle hooks of Mesonychoteuthis; (bb) Galiteuthis; (cc, dd) tentacle hooks (distal and central) and (ee) arm hook of Gonatus. (b - i, m - q, w , and x after Clarke (1980); j - l , r - v , and c c - e e after Young (1972).)

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expanded to form lateral basal lobes, the shaft and uncinus together curve around to form almost a semicircle with no marked inflexion, and there are no notches in the base. There are one to six hooks on the tentacle. Arm hooks of Gonatus and Gonatopsis have expanded basal lobes with no notches. According to Kristiansen (1977), the point of the hook does not develop from the medial tooth as described by Naef ( 1923) and as would appear (but has not been definitely established) to be the case for squids in other families. In Gonatus he has described the point to grow from the sucker ring inside the tooth, which is then lost. D. Octopoteuthidae

In Octopoteuthidae, the hooks are characteristic of the family and are only present on the arms. Tentacles are lost in early life and bear only suckers. The hooks are characteristic in having a groove of insertion which lies well back from the point of the uncinus, a single pair of ancillary teeth on the external opening, no lateral basal lobes, and a very broad internal opening (Fig. 6v-bb). E. Cranchiidae

In Cranchiidae, hooks are only present on the tentacles oî Galiteuthis, on both arms and tentacles of Mesonychoteuthis hamiltoni Robson 1925, and on the tentacles of Taonius. Hooks of Mesonychoteuthis are not very advanced in development since they have short shafts and long external and internal openings and each retains a pair of ancillary teeth even in the tentacular hooks (Fig. 6z,aa). In Galiteuthis (Fig. 6bb) the hook is more advanced since it has a long shaft and basal lobes which are particularly developed on the medial sides of the hooks (i.e., toward the sagittal plane of the tentacle club). Possibly this provides leverage to pull the hooks of the two rows toward one another. In Galiteuthis glacialis, the base has winglike expansions. In all species the hooks lack crenulations or notches. The "double hook" of Taonius is little modified from the simple sucker ring since the internal and external openings are very broad and the "hooks" are really two large teeth on a slight upward extension of the distal side of the ring. IV. Evolution of the Hook

When the fossil hooks (Figs. 2 and 7) are compared with those of livingcephalopods, it is at once clear that their shape and structure have little in common. The fossil hooks have no suggestion of a basal ring structure, there

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Fig. 7. Scanning electron microscope photographs of a selection of isolated belemnoid hooks from the Middle Jurassic of southern England.

is never an external opening, the internal (basal) opening can be far longer than that of the recent hooks, there is no evidence of a groove of insertion along the external side of the shaft and the insertion ( = orbicular scar) is not commonly a very distinct feature on the uncinus, the most usual form of the base (Fig. 1) is quite different from the bases of recent hooks, and there is often a single spur (Figs. 2d and 7) with no homolog in recent hooks. There is no doubt that the hook has evolved more than once and the extinct precursors of the recent hooks are not known to us. Regarding the evolution of the hook in recent forms, Fig. 8 shows a possible scheme of development from a primitive sessile sucker with no chitinous ring which led to little further differentiation in Octopoda and Vampyromorpha (M). From this stem, a branch developed stalks to the suckers and chitin on the sucker rim (I). This led to the simple smooth sucker rings seen in some Loliginidae, Sepiolidae, and Sepiidae (L). Rings of some loliginids, however, developed teeth (K), as did many oegopsids (J). Because close Loligo species may have either teeth or smooth rings, it seems likely that the teeth were formed secondarily and not at the same time as in the oegopsids. From the toothed ring of the oegopsids two lines probably arose. In the first line, two teeth became enlarged (3) to form a double hook as in Taonius, and in the second line one tooth enlarged (4) and two small teeth

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Cephalopod Hooks, Both Recent and Fossil

Belemnite hooks disappear

\

\

\ A. Taonius

B. A bra Hops is Ab raiia

C. Moroteuthis Onychoteuthis

D. Kondakovia Enoploteuthis E. Pyroteuthinae Ancistrocheirinae F. Galiteuthis

G. Gonatus

H. Mesonychoteuthis

1. Octopoteuthidae

J. All other Oegopsida K. some Loliginidae L. some Loliginidae Sepiolidae Sepiidae M. Octopoda Vampyromorpha Fig. 8.

Possible evolutionary pathways in the d e v e l o p m e n t of hooks and suckers in

recent Coleoidea based o n structural features 1 - 1 1 described in the text.

were retained as in the Octopoteuthidae, or all the other teeth were lost (5) as in Mesonychoteuthis. In some forms the external opening of the sucker began to extend and close (6) to form hooks like Gonatus or the bases also expanded (7) as in the Onychoteuthidae, Enoploteuthidae, and Galiteuthis. While Galiteuthis has a simple unnotched base, two notches developed in one offshoot (8) and the

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lobes became crenulated (9) in another offshoot (E). The branch with two notches (8) led to Enoploteuthis and Kondakovia (D). From this line Abraliopsis and Abralia (B) developed ( 10) an almost closed external opening and a small internal opening that lie more toward the proximal side; the shaft forms a sharp edge on the inside of the curved hook and the fleshy hood does not extend over the hook. A second offshoot of the line D arises by loss of one of the notches in the base ( 11 ) as in Onychoteuthis and Moroteuthis knipovitchi (C). This possible course of evolution of hooks in Fig. 8 shows the Onychoteuthidae and Enoploteuthidae to be very close to one another and suggests Abraliopsis and Abralia are more advanced than the Pyroteuthinae and Ancistrocheirinae and possibly Enoploteuthis. Within the Onychoteuthidae, Kondakovia would appear to be more primitive than Onychoteuthis and Moroteuthis knipovitchi. The close proximity of Galiteuthis and Mesonychoteuthis is no surprise, but based on hooks alone, Gonatus also seems very close to these genera. Taonius is a surprising distance from the other taonines, Galiteuthis and Mesonychoteuthis, in this scheme. V. Discussion

So far, no sucker rings have been recorded from fossils. This is remarkable in view of the good preservation of many fossils, the chitinous nature of sucker rings (Hunt and Nixon, 1981), and the many examples with hooks of, we suppose, a chitinous composition. It would seem that sucker rings had not been developed when these hooked forms were preserved in the Jurassic or they had a very low fossilization potential (as did beaks). Fleshy suckers have been described (Fischer and Riou, 1982). All the recent hooked teuthids live in the deep sea, while all those living on the continental shelves, with the exception oiMoroteuthis, which sometimes migrates into shallow water, have suckers with chitinous rings often bearing teeth. This is in marked contrast to the fossil coleoids with hooks, which were from shallow shelf deposits. Fossil hooks are preserved much better than fossil coleoid beaks (mandibles), which are usually preserved as little more than a black stain. This suggests that the fossil hooks differ chemically from beaks. In the stomachs of fossil ichthyosaurs, pliosaurs, and fish there are sometimes large numbers of belemnoid hooks (Pollard, 1968) but no beaks have been found. This is remarkable since, in stomachs of living cetaceans, fish, and birds, the beaks are only affected very slowly by digestive juices while the hooks, and sucker rings are rapidly distorted and dissolved (Clarke, 1980). The large numbers of fossil hooks in fossil predators' stomachs suggest that they had a chemical

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composition different from that of recent hooks or they would not have accumulated in the stomachs before being digested. However, this is not certain since resistance to digestion is not the same as resistance to fossilization or diagenesis. In gastric contents of ichthyosaurs, the substance between the hooks is often black and has previously been accounted for as coming from the ink of the coleoids, but it is more probably from flattened or deformed beaks which very rarely leave good remains even in specimens which are well preserved in other respects. Analysis of the element composition with an x-ray electron microscope showed that the fossil hooks now consist largely of pyrite and calcite. Titanium was present on the outside and in the cavity of the hooks, although it was not identified in the bed material or in the hook material itself. A recent hook, from Ancistrocheirus, contained significant amounts of lead or copper and zinc in the tip (unpublished analyses by authors).

VI. Summary and Perspectives

Fossil evidence strongly suggests that all belemnoid coleoids had 10 subequal arms each bearing about 20-100 hooks in two rows. These fossils are known from the Upper Carboniferous (290 m.y.) to the end of the Cretaceous (65 m.y.). The hooks vary in size along the arms but show distinctive shapes for each species of belemnoid. There is never any indication of ontogenetic development from a suckerlike structure and there is no doubt that they have evolved on a quite different evolutionary line than the hooks present in some living cephalopods, all of which developed from chitinous sucker rings during growth of the animal. In this chapter we reviewed the types of hooks on the arms of all those belemnoid species in which they occur, that is, in the groups Phragmoteuthida, Belemnoteuthida, and Belemnitida ( = belemnites s. str.), as well as those in the five living coleoid families in which they occur on arms and tentacles, the Onychoteuthidae, Enoploteuthidae, Gonatidae, Octopoteuthidae, and Cranchiidae. A possible scheme for the evolution of hooks in recent coleoids shows the close relationship of the Enoploteuthidae and Onychoteuthidae and suggests affinity between taonines and Gonatus. Fossil hooks frequently occur unattached to other fossil remains and the variation of such hooks which have beenfiguredin the literature is described and discussed. Because the evidence available suggests that no cephalopods have colonized fresh water, these hooks are good indicators of marine deposits and a more detailed study of their relationships to the known belemnoid species may well make them useful for stratigraphy.

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Acknowledgments We would like to express our warm thanks to the Director and staff of the Department of Palaeontology of the British Museum (Natural History) and in particular to Dr. M.K. Howarth and Mr. D. Phillips for their help and permission to examine specimens. One of the authors (T.S.E.) received a grant from the NATO Scientific Committee awarded by the German Academic Exchange Service and received support from the Marine Biological Association of the U.K. for which he would like to thank Professor E.J. Denton. We should also like to thank Mrs. Sylvia Marriott for typing the various drafts.

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