Palaeos Vertebrates 70.200 Chondrichthyes: Crown Group Chondrichthyes

*Palæos:*		Unit 70: Chondrichthyes
*The* *Vertebrates*		200: The Crown Group

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Chondrichthyes: The Crown Group

Taxa on This Page

The Crown Group and the Cladoselachida

This is the chondrichthyan crown group, defined as ratfish + rays. On one side we have odd symmoriids, strange stethacanthids, and downright bizarre holocephalans, including the extant chimaeriforms. On the other side, we find the elasmobranchs, a conservative and dignified lot consisting of the living sharks and rays (Neoselachii) and their equally stuffy and conservative ancestors. Somewhere near the intersection of these two incompatible groups, trying desperately to hold the family together, are the Cladoselachida.

Despite the name, the Cladoselachida may not be sharks, as the word is normally used, and are certainly not a clade of cladodont sharks. It isn't clear what it takes to be a "cladodont." The word is sometimes used to denote one of four vague grades of elasmobranch: cladodont, xenacanth, ctenacanth, and hybodont. It also describes a type of chondrichthyan tooth morphology which has no useful phylogenetic implications. In fact, the group is named for a genus, Cladodus, which is a nomen nudum, "a form genus sustained only by our ignorance of skeletal remains unequivocally related to the rather diverse tooth form known as 'cladodont.'" Schaeffer (1981); Maisey (2001a) (quoting Schaeffer).

For what it may be worth, the typical cladodont tooth, as shown in the image, features a large, relatively skinny main cusp flanked by 2-4, roughly symmetrical subsidiary cusps on each side. The outermost cusp is generally the largest of the subsidiary cusps. The main cusp, in particular, bears longitudinal striations. All cusps bulge slightly on the buccal side, leaving a broad lingual shelf. Williams (1998). The shelf overlies a bulge of attachment bone referred to as a "torus." Maisey (1977, 1983). However, this type of dentition is not restricted to the classical Cladoselachida. In fact, they may be more typical of ctenacanths (whatever this means -- see Ctenacanthidae). Nor do these teeth diagnose any reasonably defined clade of chondrichthyans. In fact, now that we think of it, we've seen conodont elements which could be described in practically the same terms.

About Cladodoides:

Consequently, there's not a great deal we can say about Cladoselachida or Cladodus. However, we have entirely too much to say about one particular putative cladoselachian, Cladodoides. Cladodoides is one of two isolated braincases recovered from the Late Devonian of Wildungen, Germany. Both braincases were found stratigraphically and geographically near some jaw material on which Jaeckel (1921) erected the genus Cladodus and the species wildungensis. Both braincases were originally referred to C. wildungensis, although there was no diagnostic basis for doing so. In 1937, the two braincases were separately described by Eric Stensiö and Walter Gross. Gross confused things further by referring Stensiö's specimen to a new species, C. hassiacus. There matters stood for about 60 years, until Dr. John Maisey of the American Museum of Natural History decided to have another look at this famous braincase (Gross's specimen). In view of the uncertain affinities of the specimen and the vagueness of Cladodus, Maisey redesignated Gross's specimen Cladodoides. Maisey (2001a). A full redescription followed in Maisey (2005).

Cladodoides internal braincase surface. Maisey (2005)

By way of background, Maisey started producing a series of remarkable shark brain papers in about 1999 -- a series which continues to this day. Something obviously happened which sparked a sudden and pronounced intensity in Maisey's work. The closest analogy we can recall at the moment is Kobe Bryant of the Los Angeles Lakers. Bryant was always one of the NBA's best supporting players, with solid, consistent play and strong defense. Then, quite suddenly, in mid-career, Bryant went from being a top supporting player to producing legendary individual performances, such as his 81 points against Toronto in January, 2006. True, Dr. Maisey will probably never score 81 points against Toronto; but, then again, Kobe Bryant is probably quite weak on Paleozoic shark anatomy.

But tiresome sports analogies aside, Maisey (2005) is a classic paper for two reasons. First, it touches so many of the big issues in early vertebrate paleontology. So, for example, it speaks in some detail about some recent startling changes in the developmental biology of the jaw (Cerny et al., 2004), the relationship between the orbital articulation of sharks and the basicranial articulation of tetrapods, the relationship between the extrinsic eye muscles in the basal gnathostome groups, the comparative anatomy of the semicircular canals in chondrichthyans and other early vertebrates, the origins of the metotic fissure -- in fact, just about all of the things that make early vertebrate paleontology interesting. Second, it provides a wonderful example of what technology can do for paleontology. Maisey not only made extensive use of the computerized tomography facilities at the University of Texas, but went on to process the CT data with additional software which made it possible for him to visualize internal "surfaces" in three dimensions. For example, the image shows the internal surface of the braincase, as if the specimen had been cut in two and prepared on the inside. Actually the braincase is intact and the inner surface is both invisible and covered with matrix.

Best of all, the paper is free for the downloading at the AMNH website. This is particularly fortunate, since it provides us with an iron-clad excuse not to attempt the impossible job of summarizing the entire paper. Instead, we will take on the more congenial (if equally lengthy) task of wandering down just one of the legion of scenic anatomical trails which radiate from the braincase of Cladodoides. Specifically, we will look at a vessel bearing the following preposterous name:

The Efferent Pseudobranchial Artery ("EPA")

As usual, we must begin with a careful study of nomenclature, using well-established rules of anatomical deconstruction. In approaching any new item of cranial anatomy, we should first apply Przynowitzkyinskya-Krzybblyskoumi's Rule: the complexity of an anatomical label is inversely proportional to the size and importance of the object to which the label adheres, unless the label includes the name of some guy who might possibly be Italian. So, the EPA is clearly a small feature, perhaps of limited functional significance. Second, we make use of a lemma of Law of Maximum Perversity, namely: all recognizable roots in an anatomical name will be misleading. Since we recognize the root terms branchial (relating to gills) and artery (a vessel which supplies blood from the heart) we can be reasonably certain that the EPA has nothing to do with gills and is probably a vein. Finally, we will add that there exists a fairly broad literature on the structure of the EPA. This should suggest the relevance of the Canon of Reciprocal Befuddlement, an analogue of the famed Uncertainty Principle. The Canon holds that: for each anatomical structure, there exists a point (Ð_¿) after which any increased knowledge of structure proportionately decreases our understanding of how the hell the thing can possibly perform its supposed function -- and vice-versa.

Combining these rules, we may infer that the efferent pseudobranchial artery (i) is a relatively small and/or unimportant vessel which (ii) is probably a vein with no obvious connection to gills, and (iii) performs a function which is not well understood. These conclusions are largely negative, but -- as we will see -- fundamentally sound.

Descriptions

Range: From the Early Devonian. Last appearance depends on what is included and whether the group is paraphyletic. Certainly to the Late Devonian under any definition.

Phylogeny: Condrichthyes (Crown) : Symmoriida + *. Cladodus is a nomen nudum. If we give this group a reasonable phylogenetic definition, say Cladoselache > Cobelodus, it may be synonymous with Crown Group Chondrichthyes, or an even more inclusive clade.I

Characters: Mouth terminal; teeth cladodont (3-pointed); palatoquadrate firmly connected to braincase; braincase short, with long post-orbital process; 5 gill slits; notochord unrestricted; 2 dorsal fins; short spine anterior to 1st (& 2nd?) dorsal fin; fin margins supported by ceratotrichia; paired fins also contain unjointed radials articulating directly on girdle or metapterygium; fin spines lack enameloid (c.f. enameloid ontogeny) (all dentine); no anal fin; tail large & lunate, with upper (chordal) lobe reenforced by cartilagenous plates; no pelvic claspers; few scales; scales only on fin margins and around orbit. Perhaps similar to modern fast predators (e.g. tuna); known to have fed on actinopterygians & perhaps conodonts.

Characters: 0.1-1.5 m; body fusiform; mouth terminal or somewhat ventral; rostrum usually short; cladodont teeth with large median cusp; conspicuous, wide supraorbital roof; postorbital process of braincase (an extension of the "roof") then extends ventrally to support otic process of palatoquadrate laterally & anteriorly; shape and proportions of braincase elements highly variable; usual hatchet-shaped palatoquadrate; ventral calcified sheath over notochord in some forms; no calcified ribs; single dorsal fin; $ posterior dorsal fin with arrow-shaped or triangular plate posterior to fin [CS01*]; anal fin absent; tail fins often superficially symmetrical and deeply forked; very long, jointed metapterygium with free whip at end; $ proximal articular facet of metapterygium directed anteriorly [CS01*]; all fin radials clearly divided into proximal and distal sets (?); enlarged pelvic plate; $ myxopterygial claspers present (not homologous in detail with claspers of Recent sharks) [CS01*]; $ few scales except adjacent to lateral lines [CS01*].

Notes: "[CS01*]" indicates synapomorphies which were calculated from the data given in Coates & Sequeira (2001) using very different software (PHYLIP). Using the full Coates & Sequeira data set, this analysis yields a single most parsimonious tree identical to that found by the authors, but requires 3 more steps to reach it. The actual data used to generate these synapomorphies is slightly different and gives slightly different results.

Image: Cobelodus aculeatus, with several modifications, after Zangerl (1981), citing Zangerl, R & GR Case (1976), Cobelodus aculeatus (Cope), an anacanthous shark from Pennsylvanian black shales of North America. Paleontographica Abt. A 154: 107-157. Original length, about 1 m.

Characters: $ glossopharyngeal (IX) nerve exits through metotic fissure [CS01*]; dorsal surface of head with large denticles; $ only one dorsal (reversal) [CS01*]; $ fin spine not composed of vascularised osteodentine (reversal) [CS01*]; $ fin spine does not precede dorsals (reversal) [CS01*]; $ caudal neural and supraneural spines not extended (reversal) [CS01*]; caudal fin usually symmetrical; $ procoracoid directed posteriorly [CS01*]; pectoral fins long; $ pelvic plate semicircular with anterolateral concavity [CS01*]; $ claspers with clawed terminus [CS01*].

Characters: narrow suborbital shelf [CS01]; amphistylic jaw suspension; $ "basicranium encloses Y-shaped canal enclosing division of dorsal into lateral aortae" [CS01]; hyomandibula short and crescent-shaped; $ whorls of fused, 3-cusped tooth families occur close to the level of the jaw articulation [CS01]; oropharynx lined with denticles; prominent caudal neural arches [CS01]; elaborate spine & brush structure on first dorsal [CS01]; $ anterior dorsal fin with triangular osteodentine spine, having distinct anterior shoulder and seated on baseplate containing globular calcified cartilage (see image at left) [CS01]; $ baseplate continuous with spine-brush complex [CS01]; posterior dorsal supported with radials and triangular cartilage [CS01]; caudal axis strongly upturned [CS01]; hypochordal lobe supported by long, distally splayed radials [CS01]; pectoral fins with long metapterygial whip [CS01]; scales with single crowns [CS01]; scales non-growing and restricted to discrete areas [CS01]; lateral line scales C-shaped [CS01]; strong sexual dimorphism, with highly modified anterior dorsal fin.

Note: Coates & Sequeira (2001) sometimes avoid calling the spine-brush complex a "dorsal fin." It is certainly unclear whether it is homologous to the dorsals of more conventional sharks. These authors are also rather unhappy with the odd results of their parsimony analysis and offer an alternative cladogram (the details of which are to be published), with "arguably more conventional" results. However, lacking the details of the alternative analysis, the pure parsimony results have been adopted here.

Image: (right) Akmonistion life reconstruction from [CS01]. Scale bar is 5 cm.

Characters: $ loss of persistent metotic-occipital fissure (reversal) [CS01*]; $ glossopharyngeal (IXth) nerve does not exit through metotic fissure (reversal) [CS01*]; $ loss of hyoid articular area on posterolateral angle of otic capsule (reversal) [CS01*]; $ loss (reversal) of high aspect-ratio lunate heterocercal caudal fin (hypochordal radials short and/or segmented) [CS01*]; $ notochord terminates almost horizontally [CS01*]; $ pectoral articular surface stenobasal (reversal) [CS01*].

Notes: [1] "[CS01*]" indicates synapomorphies which were calculated from the data given in Coates & Sequeira (2001) using very different software (PHYLIP). Using the full Coates & Sequeira data set, this analysis yields a single most parsimonious tree identical to that found by the authors, but requires 3 more steps to reach it. The actual data used to generate these synapomorphies is slightly different and gives slightly different results. [2] All these reversals are not as unlikely as it may appear. They merely reflect the increasing consolidation of the palatoquadrate with the braincase and concommitant adaptation to a duraphagous, slow-moving, benthic habit -- a tendency finding its logical endpoint in the holocephalians. 011209.

Damocles: another stethacanthid-type shark with a particularly elaborate head-dress.

Characters: pointed snout and crown of head and neck denticles, females naked and with shorter snout; orbits large; males with a large forwardly pointed dorsal spine; $ Loss of second dorsal fin (reversal) [CS01*]; nearly symmetrical high aspect ratio tail, denoting some cruising ability; $ pectoral fins elevated [CS01*]; pectoral fin with trailing metapterygial whip, otherwise thick and relatively stiff.

Notes: "[CS01*]" indicates apomorphies which were calculated from the data given in Coates & Sequeira (2001) using very different software (PHYLIP). Using the full Coates & Sequeira data set, this analysis yields a single most parsimonious tree identical to that found by the authors, but requires 3 more steps to reach it. The actual data used to generate these synapomorphies is slightly different and gives slightly different results. 020223.

Chondrichthyes: The Crown Group

Abbreviated Cladogram

Contents

Taxa on This Page

The Crown Group and the Cladoselachida

About Cladodoides:

The Efferent Pseudobranchial Artery ("EPA")

Descriptions