Thelodonti ├─Placodermi └─Eugnathostomata ├─Chondrichthyes │ ├─Eugeneodontida │ └─Chondrichthyes (Crown) │ ├─┬─Cladoselachida │ │ └─Symmoriida │ │ ├─Symmoriidae │ │ └─Holocephali │ └─Elasmobranchii │ ├─Xenacanthida │ └─Ctenacanthiformes │ ├─Ctenacanthidae │ └─Euselachii │ ├─Hybodontiformes │ └─┬─Synechodontiformes │ │ ├─Mucrovenator │ │ └─Palaeospinacidae │ │ ├─Paraorthacodus │ │ └─Synechodus │ └─Neoselachii └─Teleostomi
While it now seems firmly established that the neoselachians did not originate from among the hybodonts, few would argue with the proposition that the living sharks are fairly closely related to hybodonts. Certainly, modern sharks diversified in a world dominated by hybodonts, which first appeared in the Viséan were easily the dominant chondrichthyans of the Triassic and Early Jurassic. The origins of the Neoselachii are uncertain, but they had certainly become worldwide by the Middle Triassic. They increasingly dominated marine environments from the Jurassic and apparently had some role in pushing the hybodonts to extinction by the end of the Cretaceous.
Cuny et al. (2001) reconstruct the adaptive sequence along the following lines. Synechodonts and other early stem Neoselachii first developed in the Triassic. During this time, the shark world was dominated by hybodonts who seem to have been largely benthic, durophagous sharks, with a few large, relatively slow predators. The proto-neoselachians made a niche for themselves as quick, generally small opportunistic feeders with generalist diets who lived along the coasts. As teleosts developed who were capable of higher speeds, the Neoselachii were able to move into more dominant predator roles, since they were preadapted for this sort of feeding style. This cannot have been the only factor in the extinction of the hybodonts, since the hybodonts adapted to rivers and estuarine environments during the Jurassic and Cretaceous, and dominated these zones in which the neoselachians have never done well. Rees 1998), Underwood (2002).
Palaeospinacidae: Welcommia, "Hybodus" minor
Range: Early Jurassic to Paleocene Eocene?)
Phylogeny: Synechodontiformes: Mucrovenator+ *: Paraorthacodus + Synechodus.
Characters: triple-layered enamel on teeth; teeth cladodont, central cusp moderately high [C+01]; with small outer cusps; main cusp never flanked by low blades [C+01]; deep open vascular canals in central depression of root [C+01]; characteristic vascularization of cusps [C+01].
Images: various palaeospinacid teeth. The 4 on the left are identified former site) as Paraorthacodus. The sample on the upper right is identified former site) as Synechodus.
Links: kazak2.htm (very good tooth site); Canada.htm; L’ICHTHYOFAUNE DU PLIENSBACHIEN (JURASSIQUE INFERIEUR) DE ... bibliography); List of Literature bibliographic).
References: Cuny et al. 2001) [C+01] ATW030925.
Range: Early Jurassic to Late Cretaceous
Phylogeny: Palaeospinacidae: Synechodus + *.
Characters: teeth with relatively large accessory cusps enamel layer of main cusp does not reach the tip.
Image: from Canada.htm Nakagawa Collection). Could be Synechodus, but they are rather worn and its hard to tell.
Links: Cretaceous Shark Teeth #1; PALAEOSPINACIDAE; Canada.htm; MGS: Art/Humor; Paris Basin Liassic Fishes 2; Mesozoic elasmobranchs; ELASMO.COM Fossil Genus- Paraorthacodus; Paraorthacodus recurvus page1; L’ICHTHYOFAUNE DU PLIENSBACHIEN (JURASSIQUE INFERIEUR) DE ...; PALAEOSPINACIDAE; Natural Canvas Fossils - Sharks; Sharks from the mid-Cretaceous Gearle Siltstone, Southern .... ATW031205.
Synechodus: Woodward 1888 = Palaeospinax Egerton 1872. P. priscus Agasiz 1843, P. egertoni Woodward 1889) S. andersoni Case 1978; S. dubrisiensis Woodward 1888 (= "Hybodus" dubrisiensis Mackie 1863); S. egertonia (Toarcian); S. enniskilleni (Hettangian-Sinemurian); S. lerichei Herman 1977; S. occultidens (Hettangian-Sinemurian); S. paludinensis Delsate et al. 2002 (Hettangian); S. pinnae (Sinemurian); S. plicatus Underwood 2002 (Kimmeridgian); S. prorogatus Kriwet 2003 (Callovian of Poland); S. riegrafi Thies 1983 (Albian); S. streitzi Delsate et al. 2002 (Hettangian); S. striatus Case 1978; S. turneri Case 1987.
Range: Late Triassic to Eocene? or Paleocene [K03]; We've seen a credible abstract reporting a possible Middle Triassic occurrence in Japan. There is a theory that marine life recovered very slowly from the end-Permian event and that some classical Tethys fauna were recruited from the Panthalassic Ocean. This would seem to fit that story. Probably best known from the Albian-Aptian of Europe.
Phylogeny: Palaeospinacidae: Paraorthacodus + *.
Characters: no enlarged cephalic scales in males [M77]; lateral cephalic aortae not straight, but swing laterally to meet efferent hyoidean and internal carotid arteries [G91]; jaw not hyostylic [G91]; palatoquadrate with moderately strong otic process [M77]; palatoquadrate otic process with steep anterior margin [M77]; palatoquadrate otic process with small articular facet [M77]; palatoquadrate also supported by hyomandibulars [M77]; palatoquadrate anteriorly slight, with short symphysis [M77]; palatoquadrate covers significant part of orbit [M85]; elongated occiput extending behind otic capsules [M84] [M85]; occipital condyles with large flat, posteromedially directed articular surface [M85]; postorbital braincase much broader than anterior braincase and >50% total length [M85]; elongate dorsal endolymphatic fossa [M85]; hypophyseal duct closed externally [G91]; weak postorbital process without jugular canal [M85]; postorbital process above anterior part of otic capsule [M85]; postorbital articulation for palatoquadrate present, but articular surface confined to "primary" postorbital process as in hexanchiforms [M04]; moderate supraorbital shelf [M85]; suborbital shelf absent [M85]; ethmoid region with strong anteroventrally sloping articular facet for palatoquadrate [M85]; small rounded precerebral fontanel [M85]; neurocranium curved downwards anteriorly (pedomorphic) [M84] [M85] [d96]; ectethmoid process absent [d96]; ethmoid area weakly calcified [M85]; olfactory capsules widely separated [M85]; nasal & ethmoid regions, ventral midline without cartilaginous keel [M04]; Meckel's cartilage elongated & moderately deep [M77]; lower jaw with long symphysis [M77]; tooth cusps striated and enameled [M77]; lateral cusplets not sharply separated from main cusp (differs from Paraorthacodus) [K03] [KK04]; labial face of tooth cambered and bulges to overhang crown/root junction (differs from Paraorthacodus) [K03] [KK04]; root base dentine with numerous foramina [M77]; tooth roots with corrugated labial base [C+01]; $ tooth roots with lingual torus absent (reversal) [G91] [contra M77]; teeth with shiny outer enamel layer [M77]; teeth with parallel fibered (= bundled) enamel [G91]; teeth without haphazardly-fibered enamel (!?) [G91] [M85] [M77 says "very little"]; hyomandibular attaches to palatoquadrate next to orbit [d96]; hyomandibulars meet braincase near foramen magnum [M77]; ceratohyals meet medially [M77]; basihyal absent (!??) [M77]; basibranchials close to basihyal [G91] [M84]; epibranchials and ceratobranchials very long & slender [M85]; divided occipital demicentra flanked by prominent condyle & nerve X fossa  [M85]; occipital demicentra incorporated into occiput [M85]; clacified vertebral centra [M77]; neural spines also calcified [M77]; $ loss of calcified ribs [G91] [M77 says this is also typical of Neoselachii]; dorsal fins with spines [M77] or without? [M85]; fin spines triangular in cross-section [G91] [M77]; dorsal spines gently recurved [M77]; dorsal spines with enamel only on anterodistal portion [M77]; dorsal spines with distinct proximal (trunk) and distal (mantle) sections and a sharp division between the two [M77]; dorsal spines with thick enamel and no posterior denticles [M77]; anterior dorsal fin does not have complete set of radials [G91]; some species may lack one or both dorsal fin spines [KK04]; anal fin present [G91]; coracoids not fused [d96] [M84]; pectoral fin aplesodic, with ceratotrichia [G91]; pelvic metapterygium spans entire fin base [G91]; pelvic metapterygium separated from main clasper mixopterygial) cartilage by <2 vertebral segments [G91]; clasper denticles present [M77]; placoid scales[G91]; scales with small rhomboidal base, posteriorly recurved, spatulate crown stiffened by a keel and paired striae [M77];
Notes:  the text indicates that "the glossopharyngeal and vagus nerves emerged from the braincase within the [occipital] embayment." Maisey (1985:13). However, the figures consistently show the glossopharyngeal nerve ("IX" in the left figure) exiting anterolateral to the embayment, in fact from the lateral surface of the lateral otic process ("lotpr" in the left figure). While this is a regrettably technical matter, the issue is of some serious phylogenetic significance, since this foramen is quite large and -- if not the glossopharyngeal nerve -- could be a major arterial foramen (but which?).
Links: PALAEOSPINACIDAE; hajtaender7; Miscellaneous Shark Teeth Catalog; Paris Basin Liassic Fishes 1; hajtaender7 more tooth photos); Cretaceous Shark Teeth #1 (many more teeth); APP 45 (2) 2000 (Early Jurassic Swedish fauna); Origin of Modern Sharks nontechnical summary of phylogeny); SyrenCRL French -- technical description of teeth); 25 ALBERTIANA Triassic stratigraphic succession in British Columbia); Artist- Alain Beneteau [The Dinosauricon] art); Elasmobranch remains from the Taho Limestone possible !Middle Triassic specimen); Image- Synechodus and Hybodus [The Dinosauricon].
References: Cuny et al. 2001) [C+01]; deCarvalho 1996) [d96]; Gaudin (1991) [G91]; Kriwet (2003) [K03]; Kriwet & Klug (2004) [KK04]; Maisey (1977) [M77]; Maisey 1984) [M84]; Maisey (1985) [M85]; Maisey (2004) [M04]. ATW080218.