Novtatesamerican MUSEUM PUBLISHED by the AMERICAN MUSEUM of NATURAL HISTORY CENTRAL PARK WEST at 79TH STREET, NEW YORK, N.Y
NovtatesAMERICAN MUSEUM PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2878, pp. 1-39, figs. 1-19 June 5, 1987
Cranial Anatomy of the Lower Jurassic Shark Hybodus reticulatus (Chondrichthyes: Elasmobranchii), with Comments on Hybodontid Systematics
JOHN G. MAISEY1
ABSTRACT Hybodus reticulatus Agassiz, from the LowerJu- species ofHybodus. Some differences are noted in rassic of England, is considered to be the type the detailed arrangement ofthe lateral otic process, species ofHybodus. It is founded on skeletal frag- cephalic spines, and ethmopalatine articulation in ments and associated teeth. The cranial anatomy H. reticulatus and H. basanus. These differences is described from several incomplete specimens, are considered to be of systematic importance; H. including some of the type material. Two speci- basanus and H. fraasi are placed in a new genus, mens from the Upper Lias ofGermany are referred Egertonodus, on the basis of several differences to H. cf. reticulatus. Comparisons are made with from H. reticulatus. The genus Hybodus remains the Lower Cretaceous Hybodus basanus, whose an assemblage of nominal species, but increased cranial morphology is more completely known. anatomical data will gradually alleviate this situ- The neurocranium and jaws are similar in these ation. A list of genera retained within the Hybo- species, suggesting that many of the anatomical dontidae is given. Hybodontid tooth histology is peculiarities noted in the ethmoid and otico-oc- reviewed, and forms the basis of a phylogenetic cipital regions of H. basanus characterize a larger hypothesis which is open to falsification by other group of hybodontid sharks, including the type anatomical data.
INTRODUCTION Hybodus is a species-rich genus of Meso- upon isolated teeth and fin-spines. A few zoic elasmobranchs. Most of these nominal species are known from more complete skel- taxa are dubious, however, being founded etal material, such as H. reticulatus, H. de-
I Associate Curator, Department of Vertebrate Paleontology, American Museum of Natural History.
Copyright C American Museum of Natural History 1987 ISSN 0003-0082 / Price $4.00 2 AMERICAN MUSEUM NOVITATES NO. 2878 labechei, and H. hauffianus from the Lower Hybodus reticulatus is one ofseveral species Jurassic (Agassiz, 1837; Charlesworth, 1839; occurring in the Lower Lias of Lyme Regis Day, 1864; Woodward, 1889a, 1889b; Fraas, in Dorset, England. There are also some early 1889, 1896; Brown, 1900; Jaekel, 1906; Ko- references to H. reticulatus from the Upper ken, 1907), H.fraasi from the Upper Jurassic Lias of Germany; fin spines and a fragment (Koken, 1907; Maisey, 1986), and H. ba- of mandible with teeth from Ohmden were sanus from the Lower Cretaceous (Egerton, described by Quenstedt (1858, p. 222, pl. 27, 1845; Woodward, 1889a, 1916, 1919; Mai- fig. 1; 1885, p. 274, pl. 21, fig. 1). Woodward sey, 1982, 1983). (1889a, p. 268) also noted "an undetermined There are a number ofproblems surround- species allied to Hybodus reticulatus" from ing the nomenclature and type species ofHy- the Upper Lias of Boll, Wiirtemberg, repre- bodus. The name first appears in Alberti sented by BM(NH) P5880. According to Fraas (1834), where H. plicatilis is quoted without (1896), all these German specimens are re- description from a then-unpublished Agassiz ferable to another species, H. hauffianus. The manuscript. The species name is thus un- latter has since become known from magnif- available as an indicator for the genus in that icent complete specimens (Brown, 1900; Ko- work. Hybodus was first defined (along with ken, 1907; Hauff and Hauff, 1981), and is 20 nominal species, including H. plicatilis) widely envisioned as a "typical" shark ofthe by Agassiz (1837), but a type species was not Mesozoic (e.g., Romer, 1945; Gregory, 1951; designated. Some authors have subsequently Schaeffer, 1967). Fraas (1896) distinguished regarded H. plicatilis as the type species (e.g., H. hauffianus from H. reticulatus on strati- Glikman, 1967), but Woodward's (1916, p. graphic grounds and on differences in the teeth 4) designation ofH. reticulatus takes priority. and fin spines. Hybodus reticulatus is consequently retained A recently acquired specimen at the Staat- here as the type species of Hybodus. liches Museum fur Naturkunde (Stuttgart; Of the 20 original Hybodus species de- catalog no. 52460) has more gracile and acu- scribed by Agassiz (1837), 17 are founded minate teeth than H. hauffianus from the same partly or entirely upon teeth or partial den- stratigraphic horizons (see latter part of this titions. The remaining three species were paper). The teeth of this specimen are rem- based on fin spines; none ofthese species are iniscent of H. reticulatus in having vertical considered taxonomically reliable (Wood- lingual and labial striations confined to the ward, 1889a, p. 250; Maisey, 1978). lower two-thirds of the crown (cf. H. hauffi- Koken (1907, p. 4) commented that H. re- anus teeth which are more extensively striat- ticulatus was widely regarded as the type ed and have relatively shorter cusps; Fraas, species of Hybodus, and was extremely crit- 1896). Other Hybodontidae with similar teeth ical of Jaekel's (1889) proposal to relegate to H. reticulatus include H. plicatilis (Mu- Hybodus to a form genus of fin spine, partic- schelkalk of Germany and France) and H. ularly since Jaekel's (op. cit.) new genus Or- basanus (Wealden of Southern England). thybodus supposedly included Hybodus retic- Fortuitously, H. reticulatus is represented ulatus (see comments on taxonomy in the by a number of incomplete skeletons and latter part of this work). Thus the grounds skeletal fragments associated with teeth, all for regarding H. reticulatus as the type species from Lyme Regis (the type locality). Only of Hybodus are well established. material having teeth identical with Agassiz's From a pragmatic viewpoint, Woodward's original specimens has been included in the (1916) proposal is highly justified: (1) the present hypodigm. species has page priority over others founded upon teeth; (2) it is one of the three original species known both from teeth and fin spines; ACKNOWLEDGMENTS (3) it is represented by skeletal material (in- I thank Drs. Patterson and Forey for per- cluding some of the original specimens), un- mitting preparation and study of H. reticu- like the majority of other Hybodus spp.; and latus specimens in the British Museum (Nat- (4) it is the first Hybodus species to be reliably ural History), and for allowing me to borrow figured (in de la Beche, 1822). material for closer examination. My thanks 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 3 are also extended to Dr. Powell (Oxford), Dr. crm mesial crest M. D. Crane (Bristol City Museum and Art crp posterior crest Gallery), and Ms. Alison Longbottom (Brit- csp cephalic spine ish Museum) for their efforts in tracking down ebr epibranchial ect pr ectethmoid process actual and purported Agassiz type specimens. end f endolymphatic fossa Dr. R. Wild kindly allowed me to study the ethp pr ethmopalatine process Holzmaden sharks in the Stuttgart collection feha foramen for efferent hyoidean artery including the new specimen resembling H. fica foramen for internal carotid artery reticulatus, and Dr. P. Wellnhofer generously fm foramen magnum allowed me access to the holotype ofH.fraasi fora foramen for orbital artery in Munich. Preparation of BM(NH) and glda groove for lateral dorsal aorta Stuttgart specimens was undertaken at the hbr hypobranchial American Museum by Walter Sorensen and hym VII hyomandibular branch of facial nerve Ed Pederson. Illustrations were prepared by hyp 1 hypotic lamina Ellen Garvens, Peter Goldberg, and Frank hy r hyoid rays jc jugular canal Ippolito. The manuscript was typed by Ale- lab labial cartilage jandra Lora and edited by Brenda Jones. This 11 lateral lobe work is an integral part of a project concern- lm mesial lobe ing Mesozoic Hybodontidae, and is funded lot pr lateral otic process by the National Science Foundation (Award lp posterior lobe no. BSR 8308419). I thank Drs. G. Johnson Mc Meckel's cartilage (University of South Dakota) and B. Schaef- mil lateral marginal indentation fer (AMNH) for critically reviewing the MS, mim mesial marginal identation and Drs. L. Herman and P. Meylan (AMNH) not c notochordal canal for discussing the nomenclatural situation. not f notochordal foramen oc ar occipital arch oc cot occipital cotylus ABBREVIATIONS olfc olfactory canal Institutional ot cap otic capsule pbr pharyngobranchial AMNH American Museum ofNatural History po pr postorbital process BCM Bristol City Museum, England pq palatoquadrate BM(NH) British Museum (Natural History) pr cf precerebral fontanelle MCZ Museum of Comparative Zoology, prf com prefacial commissure Harvard q con quadrate concavity OUM Oxford University Museum, England q fl quadrate flange SMNS Staatliches Museum fuir Naturkunde in rb rostral bar Stuttgart sc scapulocoracoid sof spino-occipital foramen Anatomical sp c spinal canal acl accessory lateral cusp sub s suborbital shelf acm accessory mesial cusp sup cr supraorbital crest add f adductor fossa t teeth afsp anterior fin spine vgf vagus-glossopharyngeal fossa art cot articular cotylus II optic nerve art k articular knob V trigeminal nerve art pr articular process VII facial nerve ba barb IX glossopharyngeal nerve bh basihyal X vagus nerve bp basal plate c crown Orientation (teeth) cbr ceratobranchial ch ceratohyal lab labial cik caudal internasal keel lat lateral crd dorsal crest ling lingual crl lateral crest occl occlusal 4 AMERICAN MUSEUM NOVITATES NO. 2878
CLASS CHONDRICHTHYES ommended that, as in the present work, these indeterminate remains be referred to Hybo- SUBCLASS ELASMOBRANCHII dus until some generically diagnostic char- PLESION HYBODONTIFORMES acters (either from more complete material or from the teeth themselves) are identified. FAMILY HYBODONTIDAE OWEN, 1846 EMENDED DIAGNOSIS: Elasmobranchs with Genus Hybodus Agassiz two dorsal fin spines having alternated pos- terior denticles near the midline, and smooth DIAGNoSIS: Hybodontidae with an eth- unenameled ribbing which may be continu- mopalatine articulation extending into the ous proximally or broken up into rows of midorbital region and with a subocular fold wrinkled and radially striated tubercles; fin- in the posterior part of the orbit to accom- spine trunk osteodentine has distinct concen- modate the dorsal margin ofpalatoquadrate; tric layering; paired cephalic spines (one or basicranium with deep aortic grooves ex- two pairs) present in males; teeth with an- posed for their entire length; multicuspid aulacorhize basal platform and tumid to mul- acuminate teeth with osteodont crowns. ticuspid crown. TYPE SPECIES: Hybodus reticulatus Agassiz. SYSTEMATC NoTE: The term "Plesion" here follows the use outlined by Patterson and Ro- Hybodus reticulatus Agassiz sen (1977). Hybodontiformes may be further 1822, "Fossil jaw with a triple row of teeth," H. sequenced into nested taxa, using a suite of T. de la Beche, p. 44, pl. 5, fig. 3. anatomical characters, but to do so at this 1837-43, Hybodus reticulatus Agassiz, vol. 3, pp. point would be premature since several taxa 50, 180, pl. 9, figs. 1-9 (some may pertain to are awaiting revision (Maisey, in prep.). Some Acrodus); pl. 24, fig. 26; pl. 22a, figs. 22, 23. ofthe anatomical differences noted below be- 1889a, Hybodus reticulatus: Woodward, p. 266, tween species of Hybodus are slight, even pl. 10, figs. 16-18. and one could argue nauseam over 1907, Hybodus reticulatus: Koken, p. 3. trivial, ad 1916, Hybodus reticulatus: Woodward, p. 4 (des- the relative merits of "lumping" and "split- ignated type species). ting." Modern elasmobranch teeth are usu- 1966, Hybodus reticulatus: Patterson, p. 287. ally distinctive at generic level, but there are cases of similar teeth occurring in apparently LECTOTYPE: Bristol City Museum C4727, different genera (e.g., various carcharhinids, right mandible with teeth and labial carti- particularly those with "Physodon" teeth; lages; see figures 1, 2; also Agassiz (1837, pl. Applegate, 1978; Compagno, 1979; Cappet- 24, fig. 26). ta, 1980). Rarer cases ofdifferent tooth mor- LOCALITY AND HORIZON: Lyme Regis, phology supposedly within one genus (e.g., Dorset, England. Lower Lias (Sinemurian "Mustelus") are perhaps an artifact of sys- Stage), Lower Jurassic. tematic inattention. DIAGNOSIS: Teeth narrow, with a relatively From this point on within the present pa- high crown, the median eminence and all the per, taxa with teeth similar to those of Hy- lateral cones slender and sharply pointed; su- bodus reticulatus, but whose anatomy oth- perficial coronal wrinkles fine and numerous, erwise differs only slightly or else is unknown, often not extending to the apex; the principal continue to be referred to Hybodus. Follow- cusp of anterior and anterolateral teeth be- ing a discussion ofthe cranial morphology of tween two-thirds and three-quarters the H. reticulatus, two Hybodus species (H. ba- breadth of the tooth, usually recurved both sanus, H. fraasi) are removed to a new genus; posteriorly and lingually; supraotic cephalic until then they are also referred to Hybodus spines with-low accessory cusps. in the text. Eventually it is hoped that a REFERRED MATERIAL: broader systematic scheme will be generated J3088, Oxford University Museum, scattered teeth in which other species will be either placed and cartilage fragments (Philpot coll.). more firmly within Hybodus or else removed J3920, broken mandible with teeth, Philpot coll. to other genera. For those unfortunates work- J3099, fin spine, Philpot coll. ing primarily with isolated teeth, it is rec- J3 100, fin spine, Philpot coll. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 5
A 3 cms
B
Fig. 1. Hybodus reticulatus, BCM C4727, lectotype, figured Agassiz, 1837, pl. 24, fig. 26; right mandible with associated teeth (shown enlarged in C and D) and labial cartilages; A, lateral view; B, mesial view. C and D enlarged to same scale.
J3109, fin spine, Philpot coll. C4725, Bristol City Museum, fin spine; syntype, J3318, fin spine. figured Agassiz (1837, pl. 9, fig. 5). 6 AMERICAN MUSEUM NOVITATES NO. 2878
BM(NH) 40335, crushed head with teeth and sha- is now lost. Furthermore, some institutions green (some teeth figured Woodward, 1889a, pl. hold specimens that have been regarded as 10, figs. 16-18), purchased from J. Tennant, forming part ofAgassiz's type series but which 1867. bear little or no resemblance to his figured BM(NH) P3160, crushed head with scattered den- are tition and both fin spines (Enniskillen coll.). examples. Among these J3088 (Oxford BM(NH) P3162, scattered teeth and cartilage frag- University Museum, Philpot coll.) and ments (Enniskillen coll.). BM(NH) P2166. J3088 bears an Agassiz la- BM(NH) P3163, crushed head with palatoquad- bel, numbered 43c X; it had been identified rate and teeth (Enniskillen coll.). as a doubtful syntype, thought to be the spec- BM(NH) P2198, fragment of cartilage with sha- imen figured by Agassiz (1837, pl. 24, fig. 26). green and teeth (Egerton coll.). In fact, the specimen figured there is in the BM(NH) P2198a, parts of mandibular arch witb Bristol City Museum (C4727). J3088 cannot teeth and cephalic spine (Egerton coll.). be recognized as one ofthe specimens figured BM(NH) P2198b, two groups of teeth (Egerton by Agassiz (1837), nor as the one shown by coll.). de la Beche or Buckland It is BM(NH) P2203a, broken teeth and cephalic spines (1822) (1837). (Egerton coll.). in a fragmentary state, however, and could BM(NH) P2203b, broken teeth and cephalic spines conceivably have once been the original piece. (Egerton coll.). BM(NH) P2166 is one fin spine ofmany orig- BM(NH) P2203d, occiput, part of left palato- inally cataloged under that number (now as- quadrate, tooth and two cephalic spines (Eger- signed other numbers). It had been consid- ton coll.). ered one of the spines figured by Agassiz BM(NH) P2208, crushed head with teeth, three (1837, pl. 9, fig. 5), but this spine is also in incomplete cephalic spines and fin spine. Bristol (C4725). There is no evidence that BM(NH) P3168, crushed head and anterior trunk the two Bristol specimens, nor the fin spine region ofjuvenile (Enniskillen coll.). BM(NH) P4355, pertain to the same indi- BM(NH) P3156, crushed head with traces ofteeth, vidual. The other original specimens are now two cephalic spines and a fin spine (Enniskillen coll.). considered lost. In view ofthe nondiagnostic BM(NH) P5876, fragmentary head with teeth and nature of the fin spines C4725 and P4355, fin spines (Enniskillen coll.). the mandible and associated teeth, C4727 is BM(NH) P4355, fin spine, apparently a syntype here designated the lectotype ofHybodus re- figured Agassiz (1837, pl. 9, fig. 2). ticulatus. MCZ 780, fragments oflargejaws with a few teeth. In Agassiz (1837, pl. 24), this specimen is MCZ 816, crushed head with part of palatoquad- figured in inner as well as outer aspects. Cur- rate, shagreen and teeth (Marder coll.). iously, the inner view was incompletely fig- ured and seems smaller than the outer view NOTES ON THE MATERIAL because the posterior part of Meckel's carti- The first illustration ofa jaw fragment with lage, with its mandibular joint, is not shown Hybodus teeth was published by de la Beche (this can be seen in the photograph of the (1822, p. 44, pl. 5, fig. 3). The figure of this complete specimen, fig. 1). specimen, by then identified by Agassiz (1837) According to Woodward (1889a, p. 266), as Hybodus reticulatus, was reproduced in the a fin spine referred to H. formosus by Agassiz "Bridgewater Treatises" (Buckland, 1837, pl. (1837, pl. 9, figs. 11, 12) pertains to H. retic- 27d, figs. C1 and 2). According to Woodward ulatus, although Woodward (op. cit., p. 267) (1 889a, p. 266) the original specimen ("As- went on to admit that, "dorsal fin-spines of sociated teeth, cartilage, and dorsal spines") this species are not readily distinguishable is in the "Oxford Museum" (i.e., Oxford Uni- from those ofAcrodus anningiae," which he versity Museum). regarded as a valid taxon, and he gave no The earliest reliable account of this and reason for reassigning Agassiz's specimen. other specimens was published by Agassiz The following specimens, referred to H. re- (1837). However he did not designate any ticulatus, have been prepared using acetic acid particular specimen as a holotype. His orig- and mechanical techniques: BM(NH) P2208, inal material has become scattered and much P2203d, MCZ 780, and MCZ 816. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 7
Of the material listed earlier, BM(NH) two cephalic spines) shows the posterior ends P3168 was identified as H. raricostatus by of the aortic grooves on its ventral (external) Woodward (1889a, p. 258), and another surface, and part of the calcified notochordal (P3160) was identified as H. delabechei (ibid., canal internally. p. 260). I consider the tooth morphology of MCZ 780 comprises a a great many frag- these specimens to be more like that of H. ments of a large head. The best preserved reticulatus, however. P3168 is a small indi- parts are from the jaws of the left side. A vidual, and any slight differences in its teeth substantial part of the left palatoquadrate is may be accounted for as ontogenetic varia- intact, as is the posterior part of the corre- tion like that noted in H. basanus (Maisey, sponding mandible. 1983, p. 52). Since other specimens of H. BM(NH) P3160 displays the neurocrani- delabechei and H. raricostatus equal those of um in ventral view, although part of the left H. reticulatus in size, these species are still side is obscured by other pieces of cartilage considered distinct on the basis oftooth mor- (fig. 6). This specimen has provided most of phology. the information on the basicranium pre- One of the most informative specimens of sented below. Hybodus reticulatus is BM(NH) P2208 (figs. From the material available it seems that 2, 3). A complete but damaged braincase is the cranial anatomy of H. reticulatus is very preserved, with three cephalic spines in situ similar to that of H. basanus (fig. 7; cf. Mai- on the dorsal surface of the otico-occipital sey, 1982, 1983). Preservation of H. reticu- region. A large part of the cranium is visible latus does not approach that of H. basanus dorsally. In addition the inner wall ofthe left in quality, however, and consequently a orbit is displaced laterally so that some fea- number of aspects remain doubtful or un- tures are discernible within it. The braincase known in the type species. ofP2208 is not well preserved, however, and it would be almost impossible to interpret DESCRIPTION much of the finer morphology without ref- THE NEUROCRANIUM erence to H. basanus (see Maisey, 1982, 1983). All but the most anterior extremity ofthe left Since almost all the specimens of H. retic- palatoquadrate is well preserved. The pos- ulatus are crushed, the general proportions of terior halfofthe left mandible (Meckel's car- the braincase can only be estimated. The in- tilage) is also present. Thejaws ofthe left side tact occiput in BM(NH) P2203D (fig. 5) is are preserved in their original articular re- proportioned as in H. basanus (e.g., BM(NH) lationship, along with the ceratohyal and P60110; Maisey, 1983, figs. 4, 5) and the hyomandibula. Behind the head are some overall shape and proportions of the brain- pieces ofthe branchial skeleton, parts ofwhich case in dorsal and ventral views (figs. 2, 3, 6) appear to be undisturbed. The coracoid moi- are also close to those of H. basanus. There- ety of one scapulocoracoid is also present be- fore the neurocranium of H. reticulatus and low the branchial skeleton. H. basanus are thought to have been similarly Part of the basicranium is exposed in proportioned. The braincase is widest be- BM(NH) P3168, a small and presumably ju- tween the postorbital processes. The orbits venile specimen seen in ventral aspect. The lie approximately midway along the brain- right mandible is missing, but the right pal- case, and are underlain by a suborbital shelf atoquadrate is exposed in ventral/mesial view, (figs. 6, 7). The postorbital process has a slight and its mandibularjoint is visible (fig. 4). The anterior inclination distally, giving the orbit ceratohyals and basihyal lie centrally over the an oval outline (P2208; MCZ 816). Nasal braincase. Parts of the occiput and branchial capsules are not preserved, although part of arches are also discernible. The ethmopala- the olfactory canal is present in P2208, lateral tine region is damaged and incomplete. to the precerebral fontanelle (fig. 3). The eth- A complete and uncrushed occipital coty- moid region is prismatically calcified, even lus is preserved in BM(NH) P2203D (fig. 5). in the presumedjuvenile specimen P3168 (fig. This fragment (associated with a tooth and 4). This suggests early calcification ofthe eth- 8 AMERICAN MUSEUM NOVITATES NO. 2878
Fig. 2. Hybodus reticulatus, BM(NH) P2208, complete but damaged head in dorsolateral view. moid region, as in H. basanus and Permian ticulatus, although the olfactory canal seems xenacanths (Schaeffer, 1981; Maisey, 1983). better calcified than in H. basanus. The ethmoid region constitutes about a quar- The ventral surface of the internasal sep- ter of the braincase length, as in H. basanus tum is folded into a median keel in P3160 and Tristychius (Dick, 1978). This is more (fig. 6), and traces ofthis keel are also visible than in certain Paleozoic sharks (e.g., Cobe- along the broken anterior edge ofP2208. The lodus, Tamiobatis, Xenacanthus; Romer, floor ofthe precerebral fontanelle is fairly flat 1964; Zangerl and Case, 1976; Schaeffer, and does not contribute to the keel. The ar- 1981), but less than in most living sharks. rangement therefore closely resembles that in The olfactory canal is large in P2208, and H. basanus, although in H. reticulatus the has a mesial wall (cf. H. basanus). There is keel is a little wider relative to the rest of the an additional canal within the lateral wall of ethmoid region (fig. 7). Comparison with H. the intemasal lamina, mesial to the olfactory basanus suggests that this keel did not form canal. A corresponding canal in "H." basanus part of any rostral structure. A comparable was interpreted as possibly housing an an- keel is also found in Xenacanthus and Tami- teromedially directed branch of the lateral obatis (Schaeffer, 1981). ophthalmic nerve (Maisey, 1983, p. 13). The In H. basanus a bulbous ethmopalatine floor of the precerebral fontanelle contains a process arises from each side ofthe internasal foramen leading into a canal (P2208), as in keel, ventral to the precerebral fontanelle and H. basanus. I have previously suggested that olfactory canals (Maisey, 1983, figs. 2, 3, 8, this canal housed the anterior cerebral vein, 9, 12, 13, 14, 16). This process provides an and that it lay close to the floor of the olfac- important articulation with the palatoquad- tory canal (Maisey, 1983, p. 17). There is rate, whose dorsal margin lies within a round- nothing inconsistent with this view in H. re- ed groove between the ethmopalatine process 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 9
4 cms
Fig. 3. Annotated key to principal features displayed by BM(NH) 2208.
and palatobasal ridge. In H. basanus the pal- reticulatus and H. basanus, although the ar- atoquadrate has a short dorsal flattened area ticular relationship between the braincase and which articulates with the ventral surface of palatoquadrate is essentially similar. In both the ethmopalatine process. species the dorsal margin of the palatoquad- This articular surface is also present in H. rate lies within the orbit and overlies the sub- reticulatus, but evidence for the cranial eth- orbital shelf. In H. basanus and H. fraasi the mopalatine process is poor. In P2208, how- suborbital shelf underlies the palatoquadrate ever, the badly crushed left ethmopalatine only anteriorly (Maisey, 1983, 1986), where- process seems to be present, immediately as in H. reticulatus this overlap seems to have overlying a corresponding flattened articular extended farther toward the postorbital pro- surface ofthe palatoquadrate. In P3160 parts cess. The suborbital shelf ofP3160 has a dis- ofwhat may be the left ethmopalatine process tinct groove flanked by oblique ridges toward are exposed lateral to the internasal keel. In the rear ofthe orbit, interpreted as part of an this specimen that is also evidence for a short articulation with the palatoquadrate. Topo- median rostral bar. Its shape suggests that H. graphically this articulation lies much farther reticulatus had a somewhat longer median posteriorly than in H. basanus, and is more rostral bar than in H. basanus and H. fraasi. comparable with the ethmoidal articulation The suborbital shelf of P3160 tapers an- in "Cladodus" and Xenacanthus (Gross, 1937; teriorly until it merges with the intemasal Schaeffer, 198 1). In P3160 there is a foramen keel. In H. basanus this shelfis broader below located immediately posterior to the articu- the postnasal wall than farther posteriorly, lation. Schaeffer (1981) interpreted a simi- and then narrows abruptly beneath the eth- larly positioned foramen in Texas Xenacan- mopalatine process. Consequently the con- thus and "Cladodus" from Germany as figuration of the basicranium differs in H. housing the efferent pseudobranchial artery. 10 AMERICAN MUSEUM NOVITATES NO. 2878
Fig. 4. Hybodus reticulatus, BM(NH) P3168, jaws and visceral arches in ventral aspect; parts of the occiput and ethmopalatine region are also visible.
The foramen for this vessel in H. basanus anterior position of the ethmoidal articula- apparently lies dorsal to the suborbital shelf tion in H. basanus and H. fraasi is derived in the posterior part of the orbit (Maisey, relative to the condition in H. reticulatus, 1983, figs. 9, 13, 14). Hybodus reticulatus Xenacanthus, and "Cladodus." therefore seems to have its ethmoidal artic- The only specimen of H. reticulatus in ulation and efferent pseudobranchial fora- which the orbital wall is visible is P2208; men arranged as in some Paleozoic sharks even here the cartilage is broken and few fea- and not as in H. basanus. I suspect that the tures are discernible (figs. 2, 3). Two large 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS I1I
B
not f
S( E F
sof I
2 cms II Fig. 5. Hybodus reticulatus, BM(NH) P2203D, uncrushed occipital cotylus prepared in acid and associated with a tooth (fig. 10) and two cephalic spines (figs. 12, 13). Occiput shown in (A) dorsal, (B) ventral, (C) posterior, (D) anterior, (E) left lateral, and (F) right lateral views. holes, separated by a bridge of calcified car- ings are evident. If a trigeminofacialis recess tilage, may mark the positions of the optic was developed as in H. basanus and Xena- and trigeminofacialis foramina. Aside from canthus, it has been obliterated during fos- a few foramina for laterosensory innervation silization in P2208, and the postorbital pro- through the roofofthe orbits, no other open- cess is crushed down over the posterior part 12 AMERICAN MUSEUM NOVITATES NO. 2878
Paired foramina for the orbital arteries are present in P3160. Their position relative to the median internal carotid foramen is sim- ilar to that in H. basanus. Shallow grooves for the lateral dorsal aortae extend from the orbital foramina posteriorly and converge to- ward the occiput. These grooves become much deeper adjacent to the occipital cotylus (P2203d, P3168), and in transverse section the basicranium is interrupted by two deep invaginated clefts within the calcified carti- lage (figs. 5, 6). In modern elasmobranchs the lateral dor- sal aorta is entirely superficial to the basicra- nium, and in some taxa is reduced or even absent, e.g., Heterodontus, where the head receives most of its blood via the efferent hyoidean vessel (Maisey, 1982, p. 37). In var- ious Paleozoic elasmobranchs (e.g., "Clado- dus," Xenacanthus, Tamiobatis) the lateral aorta would have been enclosed within a pris- matically calcified canal for much ofits length (Stensio, 1937; Gross, 1937; Romer, 1964; Schaeffer, 1981). In Hybodus basanus there are extremely short bridges of prismatic car- tilage enclosing the aortic grooves posteriorly (Maisey, 1982, 1983), but these are lacking in H. reticulatus (P2203d and P3168). Deep aortic grooves also occur in H. hauffianus (e.g., Fig. 6. Hybodus reticulatus, BM(NH) P3160, Brown, 1900, pl. 16, fig. 2; Maisey, 1982, fig. disarticulated braincase (ventral view) and viscer- 5). The shape of the basicranium in the oc- al arches. Scale bar = 25 mm. cipital region of H. reticulatus suggests that strong infolding ofthe parachordals occurred early in ontogeny, perhaps prior to calcifi- ofthe orbit. Nevertheless this at least suggests cation. that the postorbital process of H. reticulatus Slightly anterior to the orbital foramen is was positioned over any trigeminofacialis re- a smaller one (P3160), comparable to the gion as in Xenacanthus, not behind it as in supposed palatine nerve foramen of "Clad- H. basanus (see Maisey, 1983, p. 22 and figs. odus," Xenacanthus, Tristychius, and Hy- 13, 14). H. fraasi resembles H. basanus in bodus basanus (Gross, 1937; Dick, 1978; this respect (Maisey, 1986). Schaeffer, 1981; Maisey, 1983). This foramen There is a single median foramen for the is of uncertain function in the fossils, how- internal carotids, lying in a shallow depres- ever, since separate orbital and palatine fo- sion which also contains a hypophyseal fo- ramina have been reported in only two Re- ramen (P3160). The carotid foramen lies cent elasmobranchs (Deania, Centroscymnus; approximately level with the efferent pseu- Holmgren, 1941) in which the lateral com- dobranchial foramina and the posterior limit missure is relatively small and does not en- ofthe ethmoidal articulation, as in Xenacan- close the hyomandibular nerve. In Squatina, thus (Schaeffer, 1981, figs. 5,6). In H. basanus where the lateral commissure is as well de- the efferent pseudobranchial lies at this level veloped as in Hybodus and many Paleozoic (but farther dorsally), but in Tamiobatis and sharks, there is no palatine foramen. Cladodus the internal carotid foramen is lo- The left postorbital process ofP2208 is seen cated posterior to that of the efferent pseu- in dorsolateral view, overlain partly by a ce- dobranchial. phalic spine (figs. 2, 3). The right postorbital 1 987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULA4TUS 13
A .
Fig. 7. Outline restoration of the cranium of H. reticulatus in (A) dorsal and (B) ventral views; positions of cephalic spines shown stippled. process of MCZ 816 is preserved but the en- overlies the palatoquadrate adductor fossa tire head is covered with shagreen denticles (P2208, MCZ 816). The hyomandibular head that obscure many details. In P3168 the ex- articulates with the otic region behind the treme tip of the right postorbital process is postorbital process, but as in H. basanus the exposed ventrally between the palatoquad- lateral otic process forms an eave which over- rate and ceratohyal. Part of the right post- hangs the articular region. In P2208 the lat- orbital process is preserved in P3160; the floor eral otic process is shaped so as to articulate of the jugular canal has broken away, re- with a large dermal cephalic spine. This pro- vealing the roof of this canal. Although bro- cess and its spine presumably lay directly over ken, the floor of the process was apparently the lateral head vein. A comparable arrange- pierced by a large foramen, comparable to ment is found in H. basanus (P1 1872). The the crescent-shaped opening in the postor- single pair of cephalic spines in this species bital process of H. basanus (where it may corresponds to the posterior pair in H. reticu- have housed the efferent hyoidean artery and/ latus and other taxa including H. hauffianus or spiracle and/or spiracular organ; Maisey, and H. delabechei (Woodward, 1 889b; Brown, 1983, p. 26). A similar opening has also been 1900; Maisey, 1982). The posterior cephalic located in the floor ofthe postorbital process spines are slightly larger than the anterior pair in H. hauffianus (Maisey, in prep.). (there is a much greater disparity of size in The postorbital process of H. reticulatus, H. hauffianus, although the posterior pair is like that ofH. basanus, is located on the lat- again the larger; Maisey, in prep.). In H. re- eral wall of the otic capsule, and is similarly ticulatus the anterior (supratemporal) ce- pierced by a wide jugular canal. Distally the phalic spine is situated over the posterior part process is expanded and flattened where it of the orbit. Its base is in contact with the 14 AMERICAN MUSEUM NOVITATES NO. 2878
A B
Fig. 8. Indeterminate hybodontid, BM(NH) P50869; (A) left side, showing hyomandibula and cer- atohyal plus mesial surface of palatoquadrate and Meckel's cartilage, and oblique view of the floor of the occipital region; (B) right side, showing postorbital process, jugular canal, palatoquadrate (label is on quadrate flange), Meckel's cartilage, end of ceratohyal plus calcified hyoid rays. BM(NH) photos. supraorbital shelf and the proximal part of prism layers collectively about 2.5 mm in the postorbital process in P2208, and there thickness. are indications ofa very low projection, from There are at least three ventral spino-oc- the skull roof, upon which the spine rests; cipital nerves in P2203d on the right side, this cannot be confirmed from other speci- with two preserved on the left, but within the mens. A more detailed description ofthe ce- calcified floor of the spinal canal there are phalic spines is given below. several small openings which may indicate The occiput of P2203d (fig. 5) is 38 mm an originally greater number of spino-occip- wide, about 28 mm from top to bottom, and ital nerves. Six are discernible in BM(NH) its center (containing the notochordal canal) P50869, an acid-prepared partial head of an is approximately 16.5 mm deep (measured indeterminate Liassic hybodont (fig. 8). from the lateral cotylar projections). The cot- ylus thus resembles a deep cup. Its inner sur- VISCERAL SKELETON face is rugose and pitted, resembling the ar- The left palatoquadrate is exposed in lat- ticular surface ofa fossil bone where it would eral view in P2208 (figs. 3, 4). Part of the originally have been overlain by a cartilagi- right palatoquadrate is preserved (covered by nous pad. The occiput is calcified (although shagreen denticles) in MCZ 816. In P3168 not prismatically), and is fairly solid, al- the mesial surface of the right palatoquad- though broken areas of P2203d reveal large rate is seen, but the left element is overlain internal vacuities and strutlike infoldings of by a mandibular cartilage (fig. 9). Various calcified tissue. The floor of the occipital re- parts of the palatoquadrate are preserved in gion immediately anterior to the cotylus is other specimens, e.g., BM(NH) 40335, P3160, prismatic, however, and consists of several P3163, P2198a, P2203d, and MCZ 790. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 15
- -.f ethp
I *
A
B ch Fig. 9. Restoration of cranium and jaws ofH. reticulatus in (A) lateral and (B) ventral views; labials and basihyal shown as dashed lines, position of cephalic spines shown stippled.
The palatoquadrate in general resembles flange, part of which is closely overlain by that ofH. basanus in lacking a cleaver-shaped the postorbital process (P2208; MCZ 816). otic process. As in that species, there is a deep The posterior margin of the palatoquadrate adductor fossa defined dorsally by a quadrate slopes abruptly toward the articular process, 16 AMERICAN MUSEUM NOVITATES NO. 2878 above which there is a slight concavity to the palatoquadrates, as in H. basanus. A ten- accommodate the hyomandibula. There is no tative reconstruction of the head with jaws suggestion ofa postorbital articulation ofthe in place is shown in figure 9. kind found in hexanchoids and many Paleo- The hyomandibula is visible behind the zoic sharks. Instead, there may have been an palatoquadrate of P2208, although its prox- articulation in the back of the orbit, analo- imal end is overlain by the lateral otic process gous to the osteichthyan basal articulation. and its cephalic spine. The distal end of the The posterior margin of the suborbital shelf hyomandibula is blunt, with a shallow con- in P3160 bears a pair of oblique ridges and cave articulation accommodating the round- grooves in the region where (in H. basanus) ed end of the ceratohyal. Both ceratohyals the palatoquadrate would leave the orbit and are visible in P3168, together with part of a pass beneath the suborbital shelf(fig. 6). This much-crushed basihyal. "articular" fold is located on the subocular The basihyal of H. basanus is character- cartilage (rather than on the interorbital wall istically a very bulbous, rounded element with as in osteichthyans, where it forms a basitra- large paired ventral concavities on either side becular articulation), suggesting that the of a narrow median septum (Maisey, 1983, "basal" articulation in H. reticulatus may be figs. 2, 6, 16, and p. 45). A faintly comparable a novel feature, not homologous with that of element in H. reticulatus was found after acid osteichthyans. preparation of MCZ 780, but is extremely The palatoquadrate ethmoidal articulation scrappy. It could be interpreted as part ofthe is more extensive than in H. basanus and H. basihyal but seems to lack bilateral symme- fraasi. There is an articular process on the try, and it may instead be the upper (hyo- palatoquadrate, bearing a series of mesial mandibular) end ofa ceratohyal; it seems too transverse ridges and grooves (P3160). From large to be part of a gill arch. this process the palatine region tapers toward The branchial skeleton is poorly preserved the symphysis, and its dorsal margin bears a in all the available material. In P3168 there long, narrow articular surface (P2208). In H. are traces of five arches (represented by ce- basanus a corresponding surface articulates ratobranchials) anterior to the scapulocora- with the ventral side of the ethmopalatine coids (fig. 4). Additionally, what may be part process (Maisey, 1982, 1983). The main dif- of a basibranchial copula lies across the ba- ferences between the palatoquadrates in these sicranium, behind the right ceratohyal. There species are found in the greater extent of the are five branchial arches in H. basanus ethmoidal articulation and the posteriorly in- (Woodward, 1889a, p. 274; 1916, p. 8; Mai- dented subocular cartilage of H. reticulatus. sey, 1982, p. 15; 1983, p. 47), but nothing is Meckel's cartilage is deepest below the cen- known ofits basibranchial skeleton. In P2208, ter of the palatoquadrate adductor fossa three or four epibranchials and parts of two (P2208). It lacks a groove on its lateral surface or three pharyngobranchials are discernible comparable to the one which contains the (figs. 2, 3), but they are too badly damaged anterior lower labial cartilage in H. basanus for comparative purposes. Pharyngobranchi- and H. fraasi. There are traces of labial car- als have been noted in H. hauffianus (Koken, tilages in P2208, but they are poorly pre- 1907) and H. cassangensis (Maisey, 1982). served. Parts of anterior and posterior upper Epibranchials are known in these species and labials, and traces of a posterior lower labial in H. basanus (Woodward, 1916; Maisey, seem to be preserved. Possibly the labials of 1983). H. reticulatus were less elaborate than in H. basanus, especially in view of the absence of DERMAL SKELETON a groove for the anterior lower labial. P2208 1. TEETH: Hybodus reticulatus is recog- suggests that the labials were prismatically nized primarily by its tooth morphology calcified, as in H. basanus. (Agassiz, 1837, p. 178; Woodward, 1889a, p. Little else can be said concerning the jaws. 266; 1916, p. 4; see diagnosis given earlier). It is unknown whether the palatoquadrates The present account relies heavily on a hy- met symphysially, but the internasal keel of podigm of several specimens (including one the ethmoid region may have lain between of the originally figured pieces). All of these 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 17
cm
lab
occi
I r.. I~~~~~~~~~' 1- .,,a-P 37
lat
A B ff c Fig. 10. Teeth from some H. reticulatus specimens; (A) MCZ 780, (B) MCZ 816, (C) BM(NH) P2203D. Scale bar = 10 mm. are identified on the basis ofassociated teeth, of single crystallite enameloid (SCE), con- some of which are illustrated here (figs. 1, 2, firming Reif's (1973) suggestion (founded on 10). None of the specimens is sufficiently in- observations ofother Hybodus spp. teeth) that tact to establish the range ofdental variation this type of enameloid is typical of Hybodus. and number of tooth rows as completely as There is no indication of parallel-fibered or in H. basanus (Maisey, 1983, figs. 19, 20). haphazardly fibered enameloid layers. The A broken tooth from BM(NH) P2203d was single-crystallite enameloid of H. reticulatus subjected to SEM (scanning electron micro- is very similar to that described in H. dela- scope) examination in order to determine the bechei by Reif( 978a, fig. 7b), with vertically enameloid ultrastructure in H. reticulatus (fig. oriented crystallites except at the very surface 11). The area shown in figure 1 lA is from a ofthe tooth. SCE seems most parsimoniously large lateral cusp next to the principal one. to be a primitive character among Hybodon- The ridge crossing the upper part of figure tidae. By contrast, the "triple-layered" enam- 1 lA is one of the longitudinal striae orna- eloid of modem elasmobranchs (Reif, 1973) menting the accessory cusp. Figure 1 lB shows is almost certainly a derived condition (Reif, the area outlined in figure lA at higher mag- 1978a; Duffin, 1981; Thies, 1982; Duffin and nification. The etched surface reveals a layer Ward, 1983; Maisey, 1984a, 1984b). Some 18 AMERICAN MUSEUM NOVITATES NO. 2878
Fig. 11. Tooth enameloid ultrastructure in H. reticulatus, from a broken tooth of BM(NH) P2203D, etched in 2 N HCl for 5 seconds, gold-coated, and photographed using a Cambridge Stereoscan 250 at 10 kV. See text for details. fossils previously thought to be Hybodonti- osteodont teeth, as in H. reticulatus. Jaekel's dae (e.g., Synechodus, Palaeospinax) also have (1898, 1906, 1911) notion that H. hauffianus "triple-layered enameloid" (Reif, 1973, should be referred to Polyacrodus (and that 1978a), which (together with various other Hybodus be relegated to the status of a fin- skeletal and dental characters) distinguishes spine form genus, which has been vigorously them as primitive neoselachians (Maisey, disputed, e.g., Koken, 1907; Stensio, 1921) 1975, 1977, 1984a, 1984b, 1985; Duffin and now seems to be without foundation. Tooth Ward, 1983). morphology is discussed further below (see The tooth crown of H. reticulatus is com- Comments, Section 3). posed largely of trabecular osteodentine 2. DERMAL DENTICLES: Dermal denticles (Agassiz, 1837, pl. Ml, fig. 5). Osteodont his- cover much ofthe head in MCZ 816. Simple tology is of fairly restricted distribution in unicuspid scales and multicuspid compound shark teeth; it has been identified in Hybodus scales are both present, the latter especially and Acrodus spp., but not in all other Hy- around the mouth. Over most ofthe head the bodontidae. In this regard, it is important to denticles are under 1 mm diameter, but larger note that the crowns of teeth generally re- denticles (1.5-2+ mm diameter) occur near ferred to Polyacrodus are orthodont (e.g., Pat- the snout, dorsally above the occiput, and in terson, 1966, pl. 5, fig. 1), whereas many Hy- a patch above the rear of the orbit (where bodontidae such as H. haufjianus possess there is an anterior cephalic spine in males). 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 19
According to Woodward (1889b), compound (height measured vertically from the center scales occur only behind the head in H. de- of the basal platform; fig. 12). The posterior labechei, but this is not true ofall specimens lobe of the basal platform is 10.5 mm wide (Reif, 1978b). In that species the denticles and extends posteriorly about 9 mm from the are slightly larger dorsally and anterior to the crown base. There is a "gum-line" extending orbits and there is also a mixture of com- approximately 2 mm from the crown base pound ("growing") and unicuspid ("non- laterally and posteriorly but not anteriorly. growing") scales (Reif, 1978b). In H. basanus The lateral lobe projects almost 11 mm, and only unicuspid scales occur on the head the mesial lobe about 12.5 mm from either (Maisey, 1982, 1983), and in H. fraasi such side of the posterior lobe. The mesial and scales are present over the entire body, no lateral lobes are slightly oblique, so that the compound scales having been found (Maisey, mesial marginal indentation is more acute 1986). than the lateral indentation. Both indenta- As Reif (1978b, p. 120) found in H. de- tions are deep, giving the basal platform a labechei, the compound "growing" scales of strongly triradiate appearance. The crown H. reticulatus have a low cusp count (gen- consists of a principal retrorse cusp with a erally only two or three), suggesting that few weak apical barb posteriorly. The barb is odontodes were involved in forming each joined to the cusp apex by a posterior crest, denticle and that even these "growing" den- and a nearly straight mesial crest extends ticles were periodically shed. In general, the obliquely across the curved crown from the simple unicuspid scales of H. reticulatus are barb toward the basal platform. Numerous tumid and blunt, whatever their size, and weak striae branch from this crest proxi- would have formed a rugose covering to most mally, and the crest disappears among several ofthe head. The multicuspid scales are most- stronger proximal crenulations about 5 mm ly acuminate, with sharply pointed cusps. from the base. A laterally displaced "dorsal" Similar (but unicuspid) denticles fringe the crest extends from near the apex toward the mouth in living Chlamydoselachus where they base, also disappearing among the basal cren- take on an almost toothlike appearance (Gar- ulations about 3 mm from the base. Unlike man, 1885, pl. VI, fig. 12). Examination of the mesial crest, however, the dorsal crest cleared and stained 175 mm and 205 mm does not give off any striae. Chlamydoselachus embryos (personal ob- The supraotic spine of P2203d is 38 mm serv.) shows that the first unicuspid tooth wide, 26 mm long, and approximately 28 mm generation is followed by replacement teeth high (fig. 13). The posterior lobe is slightly having two or three acuminate cusps which more than 11 mm across and projects pos- may have secondarily fused together, al- teriorly about 15 mm. Its "gum-line" is al- though it cannot be determined from the most 4 mm deep posteriorly. The lateral and available material whether pre- or postpapil- mesial lobes both project approximately 13.5 lary concrescence has actually occurred. mm. Lateral and marginal indentations are 3. CEPHALIC SPINES: Cephalic spines ofH. deep. The barbed crown has a weak mesial reticulatus are of two sizes. The terminology crest, a stronger posterior crest, and a dorsal used in this description is taken from Maisey crest that is prominent only near the apex, (1982). The anterior (supratemporal) pair is then becomes faint and indistinct proximally. slightly smaller than the posterior (supraotic) There are several proximal wrinkles ante- pair. In BM(NH) P2208 the left supratem- riorly, but these are weaker and not so rec- poral and both supraotic spine bases are pre- tilinear as on the suprapostorbital spine. served in situ, but their crowns are missing Both spines have small, rounded accessory (figs. 2, 3). In P2203d, two almost complete cusps flanking the main one. In both spines cephalic spines of different sizes (presumed the mesial accessory cusp is more prominent, to be the right supratemporal and supraotic and on the supratemporal spine the mesial spines) have been acid prepared from the ma- accessory cusp bears a few striae. The basal trix (figs. 12, 13). platforms are typically spongy and porous. The supratemporal spine ofP2203d is 33.5 They lack any ordered pattern of canals or mm wide, 21.5 mm long, and is 21 mm high foramina, but there are one or two larger 20 AMERICAN MUSEUM NOVITATES NO. 2878
C
mil bp
Im mim 2 cms
Fig. 12. Smaller cephalic spine of BM(NH) P2203D; comparison with BM(NH) P2208 suggests this is the right supratemporal spine. Views are of(A) mesial, (B) posterior, (C) lateral, (D) dorsal, (E) anterior, and (F) ventral aspects. openings (e.g., on the dorsal side of the pos- delabechei there are two pairs, arranged as in terior lobe of the supratemporal spine, fig. H. reticulatus (Woodward, 1889a, pl. 8, fig. 12). 1; Maisey, 1982, figs. 3B, 15C). There are two For comparison, the following measure- pairs in H. hauffianus (Brown, 1900). As far ments were obtained from P2208; left supra- as can be determined, the supraotic pair is temporal spine 28 mm wide, 16.5 mm long, arranged as in H. reticulatus and H. dela- maximum width of posterior lobe 12.5 mm, bechei, but the supratemporal spines lie clos- mesial lobe projects 11 mm, lateral lobe 7.5 er together (Maisey, 1982, fig. 15B). Two dif- mm; left supraotic spine 32 mm wide, 19.5 ferent-size pairs of cephalic spines have also mm long, maximum width of posterior lobe been found in Acrodus lateralis (Rieppel, 15 mm, mesial lobe projects 11.5 mm, lateral 1981). lobe 11.5 mm. From this it appears that The arrangement ofcephalic spines can be P2203d represents a much larger individual studied in only a handful of specimens, and than P2208. This is supported by tooth size; consequently the extent to which spine mor- although those of P2203d are from an un- phology varies, both between and within taxa, known position in the mouth, they are rather is largely unknown. The principal morpho- squat anterior teeth. One of these (fig. 10C) logical features of cephalic spines have been measures 13.5 mm across, whereas the largest described by Maisey (1982, p. 29, figs. 15, lateral tooth visible in P2208 is only 14.5 mm 16), who also identified several asymmetrical across, and its anterior teeth are generally less characteristics which may be useful in distin- than 9 mm across. guishing between left and right antimeres. In H. reticulatus the supraotic cephalic These characteristics include striation pat- spines occupy the same topographic position terns on the principal cusp and the asym- as the single pair in H. basanus (Maisey, 1982, metrical shape of the basal plate. Investiga- fig. 15), and it would seem that the supratem- tion of the cephalic spines in H. reticulatus poral pair is absent in that species. In H. partly corroborates these earlier suggestions 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 21
C
Ip
bp
Im
Fig. 13. Larger cephalic spine of BM(NH) P2203D; comparison with BM(NH) P2208 suggests this is the right supraotic spine. Views are as in figure 12, and are to same scale.
concerning spine asymmetry. Asymmetrical there is greater corresponding morphological features of the principal cusp (e.g., laterally variation in the skull roof of Hybodontidae offset dorsal crest; mesial crest meets poste- than is presently recognized. rior crest at barb apex; lateral crest very short) A further note may be added concerning are consistent among those taxa investigated the accessory (lateral and mesial) cusps of (H. reticulatus, H. delabechei, H. hauffianus, hybodontid cephalic spines. These are lack- H. basanus, Asteracanthus ornatissimus). ing in many Cretaceous examples, including There seems to be less consistency in the H. basanus (Woodward, 1916, pl. 1, figs. 2, asymmetry of the basal plate, however. As 2a; Maisey, 1983, fig. 24) and specimens re- Maisey (1982) has noted, some cephalic spines ferred to Lonchidion (Estes, 1964; Patterson, have an almost bilateral symmetry (e.g., 1966), as well as in Jurassic Asteracanthus Woodward, 1916, pl. 1, fig. 4; Patterson, 1966, (Woodward, 1888, pl. 12, figs. 7, 8; Maisey, fig. 26B; Cappetta and Case, 1975, pl. 1, figs. 1982, fig. 16A-F). Woodward (1889b, p. 260) 3-6). Comparison of cephalic spines of As- noted that in a specimen of H. delabechei, teracanthus ornatissimus (e.g., BM(NH) BM(NH) 39880, the posterior (supraotic) ce- P12522; Maisey, 1982, fig. 46A-F) and Hy- phalic spines lack accessory cusps, while the bodus reticulatus (figs. 12, 13) reveals that, anterior (supratemporal) ones possess them. according to the asymmetrical features ofthe In H. basanus, the single pair of cephalic crown, the basal plate's "lop-sidedness" is spines (which are topographically equivalent reversed. Since the basal plate is closely at- to the supraotic pair in other hybodonts) also tached to special areas on the skull roof, this lacks accessory cusps. Both of the associated plasticity in spine asymmetry suggests that H. reticulatus cephalic spines, preserved in 22 AMERICAN MUSEUM NOVITATES NO. 2878
-_- %. Fig. 14. Hybodus cf. reticulatus, BM(NH) P5880, Upper Lias, Boll. Wiurtemburg; disarticulated visceral arches, teeth, and ?cranium.
BM(NH) P2203d, have accessory cusps on ment, otherwise spines of gradational sizes either side of the principal one, although in would probably be found on individual males neither example are the accessory cusps more (analogous to the bizarre presence of several than low, tumid lobes. In BM(NH) P2208 caudal spines of differing sizes in some sting the bases ofboth supraotic spines and the left rays). However the periodicity (if any) is un- supratemporal spine are preserved in situ. likely ever to be determined. The possibility None of the crown is preserved on the su- is raised that some incomplete "female" hy- pratemporal spine, but there are traces ofac- bodontid specimens lacking cephalic spines cessory cusps adjacent to the base ofthe prin- are simply immature males, or the elasmo- cipal one. Therefore it is concluded that the branch equivalent of a stag in winter. In absence of accessory cusps is not a reliable smaller cephalic spines the principal cusp has means of distinguishing supraotic from su- an open pulp cavity, but in larger specimens pratemporal cephalic spines. However the this is filled in, usually by concentric ortho- possibility remains that the relative devel- dentine lamellae. This suggests that these opment of these accessory cusps is of taxo- spines were retained for an appreciable time nomic value among various Hybodontidae, before being lost. despite the paucity of data at present. 4. FiN SPINEs: The apex ofa dorsal fin spine There are no indications of incremental is preserved in P2208 (figs. 2, 3). It bears growth of H. reticulatus cephalic spines ex- several longitudinal ribs, and has retrorse ternally or from histological sections (e.g., denticle rows posteriorly (see Maisey, 1978, Agassiz, 1837, pl. M2, figs. 1-3). We may for terminology). There are no distinctive surmise that hybodontid cephalic spines were features to separate the spines of H. reticu- periodically shed and replaced by new, larger latus from those ofsome other Hybodontidae ones. This process would not have been con- such as Acrodus or Palaeobates. The speci- tinuous, unlike tooth or denticle replace- mens described by Agassiz (1837, p. 180, pl. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 23
Fig. 15. Hybodus cf. reticulatus, SMNS 51949, almost complete specimen lacking tail.
24, fig. 26, pI. 22a, figs. 22, 23) and Wood- sible to make as detailed a comparison as one ward (1889a, p. 267) are regarded here as would like (fig. 16). specifically indeterminate. The dorsal surface of the braincase is ex- posed in SMNS 51949, but only the posterior Hybodus cf. reticulatus portion (as far forward as the postorbital pro- cess) can be discerned. The arrangement of REFERRED MATERIAL: the occiput, otic capsules, lateral otic process, BM(NH) P5880, Upper Lias, Boll, Wiirtemburg, and postorbital process is as in H. reticulatus, large slab with disarticulated elements of the H. hauffianus, and H. basanus. The basicra- jaws, hyoid and visceral arches, teeth, fin spine, nium and ethmoidal region are not exposed and possibly parts of the braincase. in SMNS 51949, and only fragments of the SMNH 51949, Upper Lias, Schwarzjura EIIe, occiput are visible in BM(NH) P5880. Spec- Holzmaden, Wiirtemburg, coll. G. Fischer, 1980, imens ofH. hauffianus from Holzmaden pos- almost complete specimen, preserved length ap- sess a broader internasal keel than H. retic- prox. 2 m, lacking caudal fin. ulatus from England (which, in turn, has a A detailed revision of Upper Liassic Hy- wider internasal keel than H. basanus; see bodontidae from Wiirtemburg, Germany, is above), so it would be interesting to compare in preparation, and it would be premature to this feature in H. cf. reticulatus from Ger- discuss them in detail here. Two specimens many. The neurocranium of SMNH 51949 deserve mention at this point, however, as features broad supraorbital crests, but the left their tooth morphology resembles that ofH. postorbital process is missing and the right reticulatus rather more than H. hauffianus one is overlain by visceral arch elements (cer- from the same locality. One of these speci- atohyal, Meckel's cartilage), so the dorsal mens, BM(NH) P5880, from Boll (Wood- outline of the braincase does not entirely re- ward, 1889a, p. 268) is poorly preserved and flect its original shape (fig. 17). lacks a braincase (fig. 14). The other speci- The upper and lowerjaws are preserved in men, recently collected at Holzmaden (SMNS both specimens. Their overall morphology is 51949), is more complete than P5880, and similar to that found in H. reticulatus from has been extensively prepared (figs. 15, 17). England. The palatoquadrate of SMNH Its teeth are multicuspid, with gracile acu- 51949 has a broad but low "palatine" pro- minate cusps which are striated proximally cess, as in BM(NH) P2208. This process is (those ofH. hauffianus are generally less acu- more prominent in adult H. hauffianus (Ko- minate and more completely striated; Fraas, ken, 1907; Maisey, 1982), but in BM(NH) 1896). Teeth in the BM(NH) example are P3168, a small (juvenile?) specimen of H. poorly exposed and it is not therefore pos- reticulatus, the process is also well developed. 24 AMERICAN MUSEUM NOVITATES NO. 2878
The hyomandibula and ceratohyal ofSMNH e. lateral otic process positioned immedi- 51949 are shaped as in H. reticulatus ately behind and in part dorsal to post- (BM(NH) P2208-P3168) and H. basanus orbital process, (Maisey, 1982, 1983). The ceratohyal bears f. large, well calcified labial complex, a deep fossa at its posterior end, as in H. g. palatoquadrate with elongate and deep basanus and Xenacanthus (Maisey, 1983, p. adductor fossa overlain by quadrate 45), perhaps for an adductor hyomandibularis flange, no postorbital articulation, but muscle (no such muscle occurs in Recent with an elongate ethmoidal articulation chondrichthyans, according to Edgeworth, extending from the rostral bar to the or- 1935). bitonasal lamina, In the absence of conflicting evidence, h. hyomandibula located in part dorsal to therefore, the Holzmaden fauna seems to in- the palatoquadrate, clude H. reticulatus or at least a very closely i. shagreen and oropharyngeal denticles related species, and H. reticulatus may con- with several "neck" canals, but lacking a sequently range from the Lower to the Upper strong constriction between base and Lias. crown, j. fin-spine morphology (see Maisey, 1978), SOME COMMENTS ON k. teeth lacking specialized nutritive foram- HYBODONTID PHYLOGENY ina ("anaulacorhize" condition; Casier, AND TAXONOMY 1947c, p. 6), 1. "Sphenonchus" cephalic spines present 1. COMPARISON OF (in males). H. RETICULATUS AND H. BASANUS From the preceding description it is pos- Hybodus reticulatus differs from H. basanus sible to determine that the cranial anatomy in having: ofH. reticulatus closely resembles that ofH. m. longer rostral bar, basanus, and that together these taxa differ n. larger and stronger ethmoidal articula- from Recent elasmobranchs in several re- tion, spects (see also Maisey, 1982, 1983). Some o. broader intemasal keel, of these differences probably represent apo- p. olfactory capsule with a calcified mesial morphic characters (or character states) of wall, Recent elasmobranchs, including the ar- q. efferent pseudobranchial foramen locat- rangement of subcranial arteries; lack of an ed farther ventrally, ethmoidal keel; generally uncalcified lateral r. postorbital process overlying trigemino- commissure and unenclosedjugular canal (cf. facialis region, Squatina, however); the relative positions of s. anterior paired cephalic spines present, occiput and otic capsules; the presence ofcal- t. deeper grooves for lateral aorta, with no cified centra (including an occipital half-cen- posterior bridge of calcified cartilage, trum in Recent sharks); and closure of the u. higher number of spino-occipital nerves, adult hypophyseal foramen (see Maisey, 1982, v. folds in subocular cartilage to accom- p. 36; 1984a). Other features have been con- modate palatoquadrate in rear of orbit, sidered apomorphic for Hybodus and a few w. no groove on Meckel's cartilage for an other genera (e.g., Acrodus, Lissodus, Aster- anterior labial, acanthus, Palaeobates; Maisey, 1982, p. 40). x. compound ("growing") dermal denticles. Among those characters, the following are Some of these differences (e.g., characters known to be shared by H. reticulatus and H. p, u) may be trivial (such variations being basanus: common among Recent elasmobranchs), and a. large, downtumed postorbital processes, others (e.g., character q) might be artifacts b. inflated and long jugular canal, due to preservational differences. Many more c. massive ethmopalatine process ventral to seem to be primitive attributes of H. reticu- olfactory canal, latus (suggested by comparison with other d. lateral commissure broadly plastered on hybodontids, Xenacanthus, Tamiobatis, lateral surface of otic capsule, Ctenacanthus, Cladoselache, Hopleacanthus, 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 25
a1
4 p., . Fig. 16. Teeth from (A) SMNS 51949, (B, C) BM(NH) P5880. Scale bar = 10 mm. "Cladodus," Cobelodus, Tristychius, and On- 5. suprapostorbital cephalic spines absent ychoselache), including m-o, r, t, w, x. Hy- 6. shallow grooves for dorsal aorta bodus basanus is therefore distinguishable 7. groove in Meckel's cartilage for anterior from H. reticulatus in the following presum- lower labial cartilage ably apomorphic features: 8. absence of "growing" dermal denticles 1. short rostral bar The holotype (and still the only example) 2. shortened ethmoidal articulation of Hybodus fraasi in Munich agrees with H. 3. narrow intemasal keel basanus in characters 1, 2, 4, 7, and 8 (re- 4. trigeminofacialis fossa not overlain by maining features unknown in H. fraasi; Mai- postorbital process sey, 1986). These similarities suggest that an 26 AMERICAN MUSEUM NOVITATES NO. 2878
Fig. 17. Head ofSMNS 51949 showing principal elements ofcranium (in dorsal aspect), disarticulated jaws and hyoid arch, ventral elements of the branchial arches, scapulocoracoids, and fin spine.
advanced group, represented by H. basanus generalized Hybodontidae, but Hybodus it- and H. fraasi, can be distinguished from more selfremains poorly diagnosed. The genus may 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 27 be derived in lacking any calcified bridge over Orthybodus Jaekel, 1898 the dorsal aortic grooves, and possibly in pos- Parahybodus Jaekel, 1898 session of subocular folds in the rear of the Type species: Hybodus reticulatus Ag., orbit, but it must be admitted that at present 1837 these characters do not provide a substantial Egertonodus, g. nov. or particularly useful diagnosis. As defined Type species: Hybodus basanus Egerton, above, the family Hybodontidae includes 1845 many nominal species of Hybodus, some of Acrodus Agassiz, 1838 which cannot be distinguished above the Syn. Sphenonchus Ag., 1843 (in part) species level from H. reticulatus, plus a few Thectodus von Meyer and Plein- derived species (e.g., H. basanus, H. fraasi) inger, 1844 distinguishable on the basis of several non- ?Hylaeobatis Woodward, 1916 dental characters. To make the classification Type species: Acrodus nobilis Ag., 1838 reflect this phylogeny, I propose the erection Palaeobates Meyer, 1849 of a new genus to separate H. basanus and Syn. Strophodus Ag., 1838 (in part) H. fraasi from H. reticulatus. Type species: Palaeobates angustissimus (Ag.), 1838 Lissodus Brough, 1935 Egertonodus, new genus Syn. Lonchidion Estes, 1964 Type species: Hybodus africanus Broom, 1909 DIAGNOSIS: Hybodontidae with ethmopal- Polyacrodus Jaekel, 1889 atine articulation extending only onto the an- Type species: Hybodus polycyphus Ag., terior part of orbitonasal lamina; trigemi- 1838 nofacialis fossa not overlain by postorbital Asteracanthus Agassiz, 1837 process; shallow grooves for lateral dorsal Syn. Strophodus Ag., 1838 (in part) aortae; single (posterior) pair of cephalic Sphenonchus Ag., 1843 (in part) spines; only nongrowing dermal denticles on Curtodus Savage, 1867 body and in oropharynx; groove on Meckel's Type species: Asteracanthus ornatissi- cartilage for anterior lower labial cartilage; mus Ag., 1837 multicuspid osteodont teeth. Bdellodus Quenstedt, 1882 ETYMOLOGY: Named after Sir Philip Eger- Type species: Bdellodus bollensis Quen- ton, who first described the type species. stedt, 1882 TYPE SPECIES: Egertonodus basanus (Eger- ton). [Hybodus basanus Egerton, 1845, p. 197, The earliest named species of Acrodus is pl. iv.] Note: see Maisey (1983) for list of A. gaillardoti, quoted from an unpublished citations and for details of the cranial anat- Agassiz manuscript by Gaillardot (1835, p. omy in this species. 49). This is regarded as the type species (by OTHER REFERRED SPECIES: Egertonodus monotypy) by some authors (e.g., Glikman, fraasi(Brown). [Hybodusfraasi, Brown, 1900, 1967; Johnson, 1981). Since the species is not p. 151, pl. xv, fig. 1.] Note: The assignment described or defined, however, the name is of H. fraasi to Egertonodus must be consid- unavailable and it does not constitute an in- ered tentative in the absence of data con- dication for the genus (International Code of cerning the subcranial aortic arrangement, Zoological Nomenclature, 1985, art. 12). cephalic spines, and oropharyngeal denticles. Agassiz (1838) described the genus and var- ious species (including A. gaillardoti), but 2. HYBODONTID SYSTEMATICS failed to designate a type species. Woodward Family Hybodontidae Owen (1916, p. 14) proposed that A. nobilis Agassiz Hybodus Agassiz, 1837 be considered the type species; that decision Syn. Leiacanthus Ag., 1837 is supported here. ?Meristodon Ag., 1843 Rather fewer taxa are retained within the Sphenonchus Ag., 1843 (in part) Hybodontidae here than in the majority of ?Selachidea Quenstedt, 1852 previous works. In Woodward (1889a), for 28 AMERICAN MUSEUM NOVITATES NO. 2878 example, the equivalent higher taxon (Ces- should be admitted. In the above list, all but traciontidae) included Orodus, Campodus, two of the included taxa (Polyacrodus, Bdel- Diclitodus, Sphenacanthus, Tristychius, lodus) are now known from more than iso- Wodnika, Palaeobates, Hybodus, Acrodus, lated teeth. In most cases, parts of the en- Asteracanthus, Strophodus, Bdellodus, Pa- doskeleton are available, as well as dermal laeospinax, Synechodus, and Cestracion elements, and in future papers some of this (=Heterodontus), an assemblage that admi- material will be discussed (meanwhile, see rably illustrates the Victorian propensity for Rieppel, 1981; Maisey, 1982, 1983, 1985, erecting nongroups. Owen (1860) had earlier 1986). Nevertheless, it is the teeth which have classified selachian fishes into four great fam- customarily attracted the attention of pale- ilies ofequal rank: Raiidae (skates and rays), ontologists, and hybodontid tooth morphol- Squalidae (Recent sharks except Heterodon- ogy has been central to various hypotheses tus), Cestraciontidae (Heterodontus), and of elasmobranch evolution for well over a Hybodontes (many extinct taxa). century (e.g., Owen, 1940-45; White, 1937; This scheme was modified by Owen (1866) Casier, 1947a-c; Radinsky, 1961; Glikman, by combining the Cestracionts and Hybo- 1964, 1967). In almost every case, however, donts into the suborder Cestraphori (fossil the "Cestracion Ikon" of Agassiz (1833-44) and Recent sharks characterized by dorsal fin and Owen (1860, 1866) has had a decided spines). Since that time, the notion that Het- influence. erodontus is some kind of "Living Fossil" TOOTH HISTOLOGY: Hybodontid tooth his- hybodontid has remained popular (e.g., tology and ultrastructure has been critically Goodrich, 1909, 1930; White, 1937; Smith, examined by Stensi6 (1921), Seilacher (1943), 1942; Radinsky, 1961; Young, 1962; Patter- Radinsky (1961), Patterson (1966), Reif son, 1966; Andrews et al., 1967; Schaeffer, (1973, 1978a), Johnson (1981), and Rieppel 1967; Blot, 1969), despite an increasingly (1981), although the number of species sur- overwhelming body ofopposing data (Brown, veyed remains a small fraction of the total 1900; Zittel, 1911; Brough, 1935; Holmgren, described. In such circumstances there is al- 1941; Compagno, 1973; Reif, 1973, 1978a, ways a danger that the paucity of data may 1978b; Maisey, 1975, 1977, 1978, 1982, create artificial and misleading distinctions 1983, 1984a, 1984b, 1985). in the mind of the investigator. A classic ex- It is by now well established that the Hy- ample of this is Glikman's (1964, 1967) ma- bodontidae are an extinct group of sharks jor division of sharks into two infraclasses, whose cranial morphology and dermal skel- Orthodonta and Osteodonta, and its subse- eton was highly specialized. The group is quent rejection by Patterson (1966) and probably ancient, and some Paleozoic taxa Compagno (1973). At face value, Glikman's may be related to it (e.g., Tristychius, Ony- suggestions amount to no more than a gross choselache; Dick, 1978; Dick and Maisey, oversimplification of reality, but with the 1980). However, many other taxa have er- incorporation of supplementary data con- roneously been assigned or allied to the Hy- cerning enameloid ultrastructure (Reif, bodontidae by means of homoplasies, spu- 1973, 1978a), Glikman's concept has taken rious and ambiguous characters (see Maisey, on renewed significance (Maisey, 1982). Pat- 1982, p. 42 for a review), and even by no terson's (1966) objection to Glikman's sub- characters at all [e.g., Orodontidae (Wood- division of orthodont and osteodont Hybo- ward, 1889a, p. 229)]. dontidae is somewhat blunted, since it is no longer "necessary to believe that Lissodus and Lonchidion are only related to the well known 3. TOOTH HISTOLOGY, Jurassic and Cretaceous species of Hybodus TOOTH ARCHITECTURE, AND insofar as the Orthodonta and Osteodonta HYBODONTID PHYLOGENY shared a hypothetical common ancestry in For the term "Hybodontidae" to possess the Lower Devonian" (ibid., p. 332), al- any kind of taxonomic credibility, it would though Duffin (1985) espoused the view that seem reasonable that only those taxa sharing the Lissodus/Lonchidion lineage has indeed some apomorphic peculiarities with Hybodus been distinct since at least the Lower Car- 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 29
boniferous. Reif's (op. cit.) discoveries fairly ization ofthe orthodentine, largely attributed conclusively demonstrate what Radinsky by Stensi6 (1921, p. 38) to ontogenetic dif- (1961) and Patterson (1966) could only sur- ferences, although Johnson (1981) suggested mise, namely that tooth dentine histology has that some variation is related to dentitional undergone parallel transformations in the position. In a Polyacrodus minimus tooth Hybodontidae and Recent elasmobranchs. sectioned by Patterson (1966, pl. 5, fig. 1) Under these circumstances it seems worth- there are traces of canals (apparently not while to review available data for hybodontid "contained" by denteonal trabeculae) passing teeth and, if possible, to formulate a phylo- into the orthodentine (perhaps corresponding genetic hypothesis that is open to falsification to "tubular" orthodentine; see Zangerl in or modification by the discovery of new an- Peyer, 1946). According to Johnson's (1981) atomical data. criteria, this arrangement would be typical of The terminology for various dentinous tis- an anterior tooth in Polyacrodus. Such vas- sues is a complex issue that will not be ap- cular canals are absent from the lateral teeth praised here (see Orvig, 1951, 1967; Radin- of Palaeobates angustissimus (Patterson, sky, 1961; Glikman, 1967; Rieppel, 1981; 1966, pl. 5, fig. 2; Rieppel, 1981, fig. 14B) Zangerl, 1981). The observed histological and "Lonchidion" spp. (Estes, 1964, fig. 2d; variation among hybodontid teeth can be Patterson, 1966, pl. 5, fig. 3), and also from summarized as follows (mostly after Peyer, supposed lateral teeth ofPolyacrodus (e.g., P. 1946, 1968; Radinsky, 1961; Patterson, 1966; wichitaensis; Johnson, 1981, p. 14). Rieppel, 1981). At the present time, a limited number of Teeth of Hybodus and Acrodus typically competing scenarios are feasible: consist of a thin outer enameloid layer plus 1. Tooth histology has changed many an underlying layer of pallial dentine (sensu times, perhaps for functional reasons, and is Rieppel, 1981; ="Manteldentin" ofWeiden- therefore an unreliable taxonomic and phy- reich, 1926), below which is a central core of logenetic tool (e.g., Radinsky, 1961; Patter- osteodentine with relatively little osteonal son, 1966; Compagno, 1973). tissue. 2. Tooth histology has changed relatively Teeth of Asteracanthus have a thin outer few times, and is phylogenetically informa- layer ofpallial dentine, although this is some- tive; in which case the primitive pattern for times lost, probably by in vivo abrasion. Be- Hybodontidae could be: (a) osteodont (e.g., low this is a layer of vascularized osteonal Hybodus, Acrodus), (b) orthodont (e.g., Poly- dentine in which the vascular canals are ar- acrodus, "Lonchidion," Palaeobates), or (c) ranged in vertical columns, with thick areas columnar (e.g., Asteracanthus). The example of interosteonal tissue, all overlying a thick cited by Compagno (1973) of histological layer ofcircumpulpar dentine (Rieppel, 1981, variation between two closely related car- fig. 13B, C). The lateral, noncuspid "molar- charhinoid taxa (Hemipristis and Dirrhizo- iform" teeth of Heterodontus are somewhat don) seems to be exceptional, and the ex- similar, but there only the upper region of ample given by Patterson (1966) in pristids the osteodentine has a columnar arrange- is hardly admissible, since it concerns spe- ment, whereas the deeper-lying part is irreg- cialized rostral "teeth" (modified dermal ularly arranged (Radinsky, 1961, p. 75). Lis- denticles) rather than dentitional teeth. sodus africanus teeth are generally not suitable The paleontological evidence for the his- for histological study (Brough, 1935; Duffin, tological instability ofshark teeth is also weak, 1985). In "Lonchidion," Palaeobates, and stemming mainly from studies of isolated Polyacrodus the teeth generally consist ofout- teeth from incertae sedis taxa (e.g., Ptycho- er enameloid and pallial dentine, overlying a dus, Hylaeobatis, "Orodus"). Where some- thick orthodentine layer which is usually dis- thing is known of the anatomy other than tinguished from pallial dentine by the num- teeth, we find that dental histology is re- ber and size ofdentinal tubules (Jaekel, 1889; markably uniform in spite of great variation Stensio, 1921; Estes, 1964; Patterson, 1966; in tooth morphology (and presumed func- Rieppel, 1981; Duffin, 1985). Variation is seen tion) among closely related taxa (e.g., Hy- in both the thickness and degree ofvascular- bodus, Acrodus; Radinsky, 1961). On the 30 AMERICAN MUSEUM NOVITATES NO. 2878 whole, neontological and paleontological data tation on the columnar osteodentine of both support the contention that shark tooth Heterodontus lateral teeth, in which the ear- histology is conservative, and that observed liest-formed dentine is "compact ortho- variations are potentially informative phy- dentine" (Marquard, 1946; ? = pallial den- logenetically. tine of Rieppel, 1981). While it might be Having reached this conclusion, it is still argued that a reorganization from ortho- a quantum leap to the assumption that var- dentine to osteodentine is no more than the ious dental histologies each arose only once ontogenetic equivalent ofswitching from pal- in hybodontid evolution. Nevertheless, this lial dentine to orthodentine, the conse- is the simplest falsifiable hypothesis, and as quences of this switch, as far as odontoblast such it provides an immediate basis for fur- reorganization is concerned, are surely more ther phylogenetic studies. However, there re- profound where osteonal tissue is subse- mains the controversial problem of which quently formed. Besides the odontoblast re- condition is primitive. According to Rieppel orientation itself, the involvement of fibro- (1981, p. 351) "osteodentine has been re- blastic mesodermal cells (in forming the placed by orthodentine" (i.e., phylogenet- collagenous denteonal framework) is also ically) in orthodont hybodontids. It is clear greatly enhanced. Thus the formation of os- from Jaekel's (1889, p. 329) remarks that he, teodentine involves considerably greater sec- too, regarded osteodentine as phylogenet- ondary modification to skeletal genome ically primitive. The columnar osteodentine expression than does orthodentine deposi- ofAsteracanthus has similarly been regarded tion. In consequence, I concur with Peyer's as a specialized "derivative" ofosteodentine, (1968) opinion that osteodentine formation from ancestral sharks possessing osteodont in elasmobranch teeth is secondary, and con- teeth (Peyer, 1946, 1968; Radinsky, 1961). clude that hybodontids (and other elasmo- Thus the view that osteodentine is primi- branchs) with orthodont teeth are probably tively present in hybodontid teeth has proven primitive in this respect (see also Maisey, attractive to many investigators, thereby 1982, p. 28). strengthening the popular idea that Hybodus Outgroup comparison among Recent elas- and Acrodus are form genera having no taxo- mobranchs (particularly among those taxa nomic reality. that are considered cladistically primitive) is This long-cherished view is somewhat un- relatively uninformative. Chlamydoselachus dermined by ontogenetic data. According to is orthodont (Pfeil, 1983, p. 19), but hexan- Peyer (1968, p. 61): choid teeth and Heterodontus lateral teeth are in elasmobranchs a very regular coat of ortho- osteodont (Agassiz, 1844; Marquard, 1946; dentine is always formed first, surrounding a Radinsky, 1961; Peyer, 1968). According to wide, uniform pulp cavity, even when the mode Compagno (1973), Heterodontus anterior of formation of dentine changes in the further teeth are orthodont. Among fossils, "Clad- development of the tooth .... In Raja, Mus- odus" and xenacanthid teeth sectioned by telus, and other elasmobranchs whose teeth con- Agassiz (1844, pls. M1, M2) are orthodont, sist of orthodentine, the genesis ofdentine pro- and most Paleozoic seem to gresses during the entire period ofits formation chondrichthyans in a rather uniform way .... Where trabecular possess either orthodont or tubular "brady- dentine occurs, there is a sudden, non-transi- odont" tooth histology, rather than irregular tional change in the mode of formation ... osteodentine as in Hybodus, Egertonodus, and brought about by the odontoblasts which, in- Acrodus. Thus the stratigraphic record, in- stead of retreating in closed formation toward complete though it is phylogenetically, at least the center of the pulp cavity, suddenly show a hints that the hybodontid osteodont condi- seemingly irregular alignment throughout the tion is derived (i.e., that Hybodus, Egerto- pulp cavity. nodus, and Acrodus together represent a It is clear from this developmental viewpoint monophyletic group, albeit of form species). that Peyer (1968) regarded the osteodont Further outgroup comparison with osteich- condition to be derived, since it involves sec- thyans corroborates Peyer's (1968) conten- ondary reorganization of odontoblast migra- tion that the osteodont condition is derived. tion patterns; he placed a similar interpre- The teeth ofvarious Recent and fossil Actin- 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 31
opterygii and Sarcopterygii largely consist of scribed Pennsylvanian Hybodontiformes orthodentine (Peyer, op. cit., pp. 90-118). from Kansas; Maisey, in prep.). "Labyrinthodont" folding ofthe dentine and TOOTH ARCHITECTURE. Historically, there enamel (e.g., Rhizodus, Holoptychius) can be is uncertainty and confusion surrounding so complex as to produce the appearance of "hybodont" tooth architecture and external osteodentine in transverse section (Peyer, op. morphology. While many "hybodont" taxa cit., fig. 61). Among cladistically primitive have been described solely on the basis of Actinopterygii (e.g., Polypterus, Lepisosteus, teeth, the latter are difficult to define collec- Amia) the teeth also consist mainly of or- tively in terms ofunique attributes, since cer- thodentine (sometimes modified, e.g., into the tain features are common to teeth of many folded "plicidentine" of Lepisosteus). extinct and apparently unrelated nonhybo- Provisionally accepting that osteodont Hy- dontids such as Xenacanthidae and "clado- bodontidae form a monophyletic group, it is donts" (e.g., Ctenacanthus, Cobelodus), or to much less clear whether to regard columnar neoselachians such as Heterodontus and the osteodentine (e.g., ofAsteracanthus) as an in- extinct taxa Palaeospinax and Synechodus dependently acquired character (i.e., from an (Maisey, 1982). orthodont ancestor), or as a specialization of In attempting to "define" a tooth from a the osteodont condition. Which viewpoint is hybodontid shark, in many respects it is eas- adopted will clearly have a radical effect on ier (though systematically questionable) to one's view of the systematic position of As- compare what they lack when compared with teracanthus. Again, perhaps the most con- other elasmobranch teeth. For example, the vincing evidence comes from the ontogeny following dentitional features are generally of Heterodontus lateral teeth, in which the not found in the teeth of taxa listed above: first-formed osteodentine (immediately be- 1. Expanded semicircular lingual torus low the outer layer of pallial dentine) is ar- with median or paired articular surfaces ranged in columnar fashion, while subse- (present in "cladodonts," Xenacanthidae; ex- quent osteodentine is deposited more panded torus but no articular surfaces in randomly (Marquard, 1946). Corresponding Palaeospinax, Synechodus, Orthacodus, modifications to dentine formation have been Chlamydoselachus). inferred in Acrodus (Radinsky, 1961). In an 2. Enameled surface of adjacent cusps is Asteracanthus cf. reticulatus tooth sectioned disjunct (enameled cusps separated by un- by Rieppel (1981, fig. 14B, C), the outermost enameled areas in Xenacanthidae, many denteons also form a columnar pattern, but "cladodonts," some Synechodus spp., an irregular arrangement occurs deeper with- Chlamydoselachus, and some lamniforms). in the tooth. It is tempting to suggest that the 3. Nonlinear progressive size reduction irregular osteodentine pattern (formed rela- from the median cusp to the lateral cusps tively late in ontogeny, after pallial or co- ("Cladodus," Phoebodus, or Xenacanthidae, lumnar dentine is deposited) is an extreme Chlamydoselachus; progressive linear reduc- specialization of certain Hybodontidae (par- tion of cusp height only apparent in multi- ticularly those with high-crowned teeth). One cuspid hybodontid teeth; a similar progres- consequence of this hypothesis is that the sive arrangement also occurs in some fossil multicuspid, high-crowned teeth observed in neoselachians, e.g., Palaeospinax, Synecho- Hybodus and Egertonodus represent a cla- dus). distically derived pattern that has been ac- 4. Bradyodont crown architecture, with quired independently from other Recent and denteonal pores at tooth surface (n.b. heavily fossil taxa (e.g., "Cladodus," Xenacanthus, abraded Asteracanthus teeth show "pseudo- Ctenacanthus, Cobelodus, Palaeospinax, Sy- bradyodont" pattern where columnar den- nechodus, various galeomorphs, Chlamydo- tine has become secondarily exposed). selachus), and that the Hybodontidae are 5. Fusion of successional teeth (occurs in plesiomorphically characterized by tumid, "Pleurodus" teeth of Helodus, iniopteryg- low-crowned teeth (a supposition which is ians, and edestid symphyseal teeth). loosely corroborated by the stratigraphic re- 6. Divided, downturned roots (lateral pro- cord and by the dentition in some unde- jections from basal plate in various neose- 32 AMERICAN MUSEUM NOVITATES NO. 2878 lachians, particularly Lamniformes; labio- (e.g., ofhemiaulacorhize, holaulacorhize, and lingual projections in Xenacanthidae). perhaps polyaulacorhize neoselachian teeth, 7. Specialized root foramina ("foramen sensu Casier, 1947c, p. 9) are located some individualise" ofCasier, 1947a, p. 9; see dis- distance below the root-crown junction. This cussion below). suggests that the enlarged foramina are mor- In some cases, hybodontid teeth are char- phologically and perhaps functionally dis- acterized only by absence of certain features tinct from the row of horizontal pores. (e.g., of 1,4, 6, 7). Furthermore, in most cases Support for this view comes from a few these features have a more widespread (even acid-prepared teeth of Hybodus reticulatus if disjunct) distribution among Recent and (e.g., MCZ 780; fig. 18). On the lingual surface fossil chondrichthyans, suggesting homopla- there is a horizontal series of pores, below sy. The one possible exception to this is the which are a few larger canals (fig. 18A, C), last character which, despite being an ab- penetrating straight through the root to sence, is sufficiently unusual to have captured emerge in the middle of the basal face (fig. the attention of several investigators. 18B, D). In one example there is a median Casier (1947a, p. 9) was first to diagnose canal and a pair of lateral canals (each situ- hybodontid tooth morphology by this means. ated level with the penultimate lateral cusp; His "type hybodontoide" was distinguished fig. 18A, B). The canals have a thin floor of by (among other criteria): "L'absence de fo- dentine, although in another tooth from the ramen individualise (il n'y a qu'une rangee same specimen these canals are incompletely horizontale de pores, en nombre indefini, a floored, as in developing neoselachian teeth. chacune des faces externe et interne) et le Topographically these larger canals and their caractere tres poreux de la face basilaire, cor- foramina are similar to the enlarged median respondent a celui, tres amorphe, de la struc- and paired foramina of certain fossil neose- ture interne, sans canaux definis, mais a grand lachians (e.g., Palaeospinax, Synechodus) and nombre de sinus communiquant plus ou to corresponding enlarged canals in the roots moins entre eux, pour constituter une cavite of "Cladodus," xenacanthid, and Helodus centrale, et, selon toute apparence, a une ir- teeth. rigation dentaire par un systeme vasculaire Hybodus reticulatus is thus an embarrass- diffus." ment to Casier's (1947a) "hybodontoid" tooth In a review ofBritish Wealden shark teeth, pattern, since the type species of Hybodus Patterson (1966) noted that several taxa pos- displays a character that "hybodonts" are sessed Casier's (1947a) "hybodontid root." supposed to lack. However we may still be Some teeth had a horizontal series of pores justified in regarding Casier's (1947c) anau- just below the root/crown junction (e.g., Hy- lacorhize "stage" as a valid special condition bodus parvidens; Patterson, 1966, fig. 7; H. of the Hybodontidae, by emphasizing the brevicostatus; ibid., fig. 13). In subsequent presence ofthe horizontal pore system below works, these pores have come to be regarded the crown/root junction (which is apparently as specialized foramina (e.g., Johnson, 1981, unique), and by deemphasizing the lack of p. 4, fig. 1; Duffin, 1985, p. 108, fig. 1). How- specialized foramina ("foramen individua- ever, it is quite clear from Casier's (1947a) lise"), which are primitively present in Hy- original description, and from his subsequent bodus reticulatus. comments in that work (see also Casier, Restated in this way, the anaulacorhize root 1947b, and particularly his 1947c, p. 9 def- architecture ofthe Hybodontidae is a further inition ofthe "anaulacorhize" stage: "Pas de potential synapomorphy of the group, in- foramens definis; en principe rien que des stead of a primitive archetype from which pores") that "pores" and "foramen indivi- various neoselachian patterns might be de- dualise" are subtly different. Thus the "spe- rived. Such patterns are more readily "ex- cialized foramina" identified by Johnson plained" as phylogenetic "derivatives" ofthe (1981) and Duffin (1985) do not correspond vascularization pattern and root morphology to the "foramen individualise" of Casier found in Synechodus, Palaeospinax, and (1947a-c), but to the "rangee horizontale de "Cladodus" teeth. These extinct taxa plesio- pores ... a chacune des faces interne et ex- morphically retain specialized root canals but terne" of his "hybodontoid" or anaulaco- lack the horizontal pore system of the Hy- rhize teeth. In general, the enlarged foramina bodontidae, and do not therefore possess an- Fig. 18. SEM photographs of two Hybodus reticulatus teeth from MCZ 780, showing configuration of root foramina within lingual (A, C) and basal/labial (B, D) surfaces. Tooth in A and B has median and paired enlarged canals; tooth in C and D has less ordered arrangement. Immediately below the root/ crown junction is a horizontal series of numerous small pores which are separated from the enlarged canals by many large and primitively irregular openings, giving the root a spongy appearance. Teeth gold-coated and examined on Cambridge Stereoscan-250 at 3 kV. Scale bars = 2 mm. 34 AMERICAN MUSEUM NOVITATES NO. 2878 aulacorhize teeth (as defined by Casier, 1 947c, sage the fulfillment of a century-old proph- and modified above). ecy, namely "that future research in regard From the above remarks it seems that tooth to structures other than teeth will lead to the histology by itself does not permit the erec- subdivision ofthe multitudinous forms hith- tion ofany particularly convincing phylogeny erto grouped under one generic name" (i.e., of the Hybodontidae. When histological and Hybodus; Woodward, 1886, p. 223). It is sug- morphological data are combined, however, gested that H. basanus and H. fraasi are suf- some kind oftransformational pattern seems ficiently distinct from H. reticulatus to merit vaguely recognizable, with a primitive con- their placement within a new genus (Eger- dition of molariform anaulacorhize teeth tonodus). As additional species become bet- consisting of pallial dentine overlying non- ter known, it should be possible to elaborate vascular orthodentine (e.g., "Lonchidion," the phylogeny and taxonomy of the Hybo- Polyacrodus, Palaeobates), the acquisition of dontidae beyond the level attained here irregular osteodentine below columnar den- (hopefully before another century has tine (e.g., Asteracanthus), followed perhaps passed!), although the vast majority of nom- by secondary loss of columnar dentine (e.g., inal species will undoubtedly remain incertae "Acrodus," Hybodus; fig. 1 9A). A slightly sedis taxa. more parsimonious hypothesis can be for- I have suggested elsewhere (Maisey, 1986) mulated by assuming that the columnar os- that the Hybodontidae represent an extinct teodentine ofAcrodus and Asteracanthus is a group of ecological generalists with an ar- synapomorphy, since this does not involve a rested evolutionary pattern. However there character reversal for the Hybodus/Egerton- is evidence of some (presumably adaptation- odus group (fig. 1 9B). Hybodus and Egerton- al) changes in tooth histology among certain odus emerge from either scenario as derived hybodontid taxa. Furthermore, in at least one Hybodontidae united by high-crowned, mul- genus (Asteracanthus) it is certain that such ticuspid teeth; in the second scenario Acrodus histological modifications were accompanied and Asteracanthus also form a monophyletic by changes in palatoquadrate morphology group, characterized by columnar dentine which would presumably involve some de- within the tumid, low-crowned teeth. It re- parture from the suspensorial arrangement mains to be seen whether these hypotheses seen in H. basanus and H. reticulatus (Mai- of relationships are refuted by additional sey, 1982). Unlike some other investigators (nondentitional) data. (e.g., Radinsky, 1961; Patterson, 1966), I re- gard histological variation in hybodontid CONCLUSIONS teeth as a potentially useful preliminary basis At this time it would be premature to claim for phylogenetic analysis of this otherwise anything but modest progress in unraveling conservative group. The following conclu- the systematic relationships of the Hybo- sions have been drawn from the current in- dontidae. The genus Hybodus remains poorly vestigation: diagnosed, but this is no longer simply a con- 1. The primitive hybodontid tooth histol- sequence of a lack of data. Something is now ogy is orthodont (i.e., with a thin layer of known concerning the cranial anatomy in the enameloid and pallial dentine overlying a type species, H. reticulatus. Comparison with thicker layer of circumpulpar orthodentine), other material suggests, however, that vir- as in Polyacrodus, "Lonchidion," and Pa- tually every known aspect ofcranial anatomy laeobates. in H. reticulatus also characterizes a broader 2. At least some orthodont hybodontids group or groups (e.g., Hybodontidae, Hybo- possessed Hybodus-like cranial anatomy and dontiformes). Moreover, the status of most triradiate cephalic spines (e.g., Palaeobates; Hybodus species, founded upon isolated teeth Rieppel, 1981; Maisey, in prep.). Where or fin spines, remains unresolved. known, the detailed cranial and cephalic spine Nevertheless, in a few respects the cranial morphology of Paleozoic Hybodontiformes, anatomy ofH. reticulatus seems to differ from here excluded from the Hybodontidae (Mai- that of some other taxa (notably H. basanus sey, in prep.), is different from that of Hybo- and H. fraasi). Optimistically, this may pre- dus. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 35
6C: o> $\5 QO \>C) ~p(~g
A
CO o(\ 'p< p:
3 B
1
Fig. 19. Two plausible hypotheses of hybodontid relationships based on tooth histology. (A) is supported by Recent ontogenetic data for elasmobranch teeth, but involves a reversal (columnar osteo- dentine, acquired at node 1 and lost at node 3); (B) is more parsimonious but requires an assumption about columnar osteodentine being apomorphic. Characterization ofnodes is as follows: 1, osteodentine present (primitively columnar in A, but primitively irregular in B); 2A, irregular osteodentine present; 2B, columnar osteodentine present; 3, high-crowned multicuspid teeth. In each case, molariform ortho- dont teeth are considered primitive, multicuspid osteodont teeth derived.
3. Ontogenetic data indicate that ortho- mation ofenameloid and pallial dentine. The dentine, irregular osteodentine, and colum- formation of osteodentine involves consid- nar osteodentine are deposited after the for- erably greater reorganization of scleroblasts 36 AMERICAN MUSEUM NOVITATES NO. 2878 and fibroblasts than orthodentine deposition, Andrews, S. M., B. G. Gardiner, R. S. Miles, and however, and is consequently viewed as a C. Patterson derived condition of skeletal genome expres- 1967. Pisces. In W. B. Harland et al., Thefossil sion. Columnar osteodentine (if present) record. Geological Society of London, os- pp. 637-683. characteristically forms prior to irregular Applegate, S. P. teodentine, suggesting that the latter is a more 1978. Phyletic studies: Part 1, Tiger Sharks. extreme modification of skeletal genome Mexico, Universidad Nacional Auto- expression than columnar osteodentine. noma, Instituto de Geologia, Revista. 4. Some low-crowned, tumid hybodontid Mexico City, vol. 2, no. 1, pp. 55-65. teeth may be orthodont (e.g., Palaeobates); Blot, J. others consist largely of columnar osteoden- 1969. Holocephales et elasmobranches. Sys- tine (e.g., Asteracanthus), or of irregular os- tematique. In J. Piveteau (ed.), Traite teodentine with only a thin columnar layer de paleontologie, 4:702-776. (e.g., Acrodus). Osteodont hybodontid teeth Brough, J. 1935. On the structure and relationships ofthe may be low- or high-crowned and multicus- Hybodont sharks. Mem. Manch. Lit. pid. Such high-crowned teeth are typically Phil. Soc., 79(4):35-48. osteodont, suggesting that this tooth pattern Brown, C. is derived among Hybodontidae. 1900. Uber das Genus Hybodus und seine sys- 5. At least two taxa of high-crowned Hy- tematische Stellung. Palaeontographi- bodontidae are recognizable (Hybodus, Eger- ca, 46:149-174. tonodus). Buckland, W. 6. Although irregular osteodentine may 1837. Geology and mineralogy considered with represent a more derived condition than co- reference to natural theology. (Bridge- lumnar osteodentine (see 3 above), it is cla- water treatises), 2 vols. Philadelphia distically more parsimonious to regard the (fossil fishes, vol. 1, pp. 202-204). Cappetta, H. presence of columnar osteodentine as a syn- 1980. Les selaciens du Cretace superieur du apomorphy ofAcrodus and Asteracanthus. Liban I: Requins. Palaeontographica 7. This phylogeny predicts that Hybo- Abt. A, 168:69-148. dontidae ancestral to the Hybodus/Egerton- Cappetta, H., and G. R. Case odus and Acrodus/Asteracanthus groups 1975. Contribution a l'etude des selaciens would have possessed low-crowned osteo- du Groupe Monmouth (Companien- dont teeth, lacking the columnar variety of Maestrichtien) du New Jersey. Palae- osteodentine. Such a tooth histology does not ontographica Abt. A, 151:1-46. seem to have been described, although irreg- Casier, E. are in some 1947a. Constitution et evolution de la Racine ular nondenteonal canals present Dentaire des Euselachii. I. Note pre- orthodont teeth (e.g., Patterson, 1966, pl. 5, liminaire. Bull. Mus. Hist. Nat. Belg., fig. 1). 23(13):1-15. 8. The "anaulacorhize" architecture of 1947b. Constitution et evolution de la Racine hybodontid teeth is probably apomorphic, Dentaire des Euselachii. II. Etude com- rather than a primitive pattern as suggested parative des types. Ibid., 23(14): 1-32. by Casier (1947a-c). 1947c. Constitution et evolution de la Racine Dentaire des Euselachii. III. Evolution LITERATURE CITED des principaux caracteres morpholo- giques et conclusions. Ibid., 23(15): 1- Agassiz, L. 45. 1833-44. Recherches sur les poissons fossiles. Charlesworth, E. Neuchatel, 5 vols., 1420 pp. 1839. Illustrated zoological notices. On the re- Alberti, F. A. von mains of a species of Hybodus from 1834. Beitrag zu einer Monographie des bun- Lyme Regis. Mag. Nat. Hist., new ser., ten Sandsteins, Muschelkalks und Keu- 3:242-248. pers, und die Verbindungen dieser Ge- Compagno, L. J. V. bilde zu einer Formation. Stuttgart and 1973. Interrelationships of living elasmo- Tiubingen, xx + 366 pp. branchs. In P. H. Greenwood, R. S. 1987 MAISEY: CRANIAL ANATOMY OF HYBODUS RETICULATUS 37
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