Palaeos Vertebrates: Sarcopterygii: Rhipidistia

The Rhipidistians used to be thought of as Porolepiformes plus Osteolepiformes. Gradually, it became accepted that the porolepiforms were more closely related to lungfishes (Dipnoi) than to osteolepiforms, that rhizodonts fit into the picture as well, and the whole group was probably paraphyletic with respect to the Tetrapoda. Accordingly, the Rhipidistia as now understood is now a vast group including everything from lungfishes to lions. In fact, it can be defined as the crown group lungfishes + lions. However, the archetypal rhipidistian is still a moderately large Devonian sarcopterygian with, as Carroll 1988) says, "many specializations [which] may be attributed to adaptations to a predatory way of life in shallow water."

Descriptions

Characters: snout region sharp downward bend & may be slightly concave [ZY02]; premaxilla anteromedially tall & becoming rapidly shorter laterally [ZY02]; pineal foramen at anterior margin of parietals [ZY02]; postparietal flanked laterally by >2 bones & overlapped posteriorly by median extrascapular [ZY02]; compound cheek bone (squamosal + quadratojugal + preopercular) [ZY02]; otoccipital region well-ossified [ZY02]; vestibular fontanelle absent [ZY02]; large pit for basichranial muscle [ZY02]; well-developed intracranial joint with connecting process on sphenethmoid portion [ZY02]; large notochordal pit [ZY02]; thin postorbital pila (?) forming bridge between top of basipterygoid process and side of braincase wall [ZY02]; relatively broad suborbital ledge [ZY02]; orbital wall with drop-shaped recess behind optic foramen for eyestalk [ZY02]; myodome ventral to eyestalk [ZY02]; anteriorly-placed ethmoidal articulation (as Youngolepis) [ZY02]; internasal cavities small [ZY02]; "fenestra ventralis" choana?) present [ZY02]; very large, long parasphenoid [ZY02]; lower jaw with 3 coronoids [ZY02]; lower jaw with very large adductor fossa taking up >50% of length [ZY02]; prearticular & Meckelian bone forming convex ventral flange which extends beyond ventral border of infradentaries [ZY02]; prearticular with dorsal field of denticles and ventral pattern of undulating parallel ridges [ZY02]; scapulocoracoid tripodal [ZY02]; relatively large-pore cosmine surface layer, with pores often spoon-shaped and arranged in parallel grooves [ZY02].

Characters: maxilla forms sharp posterodorsal angles [CA96$]; cheek and operculum moveable with respect to skull roof [C88]; skull roofing bones similar to tetrapod pattern posteriorly [C88]; parietals with pineal opening, flanked by prefrontals & postfrontals [C88]; postparietals, supratemporals & tabulars present; orbits small [C88]; sclerotic plates numerous [C88]; 1-2 squamosals present and usually large [C88]; 2 supraorbitals (prefrontal & postfrontal) [CA96$]; operculum principally composed of opercular & subopercular (as Actinopterygii) [C88]; preopercular does not contact maxilla, because squamosal contacts quadratojugal [CA96$]; lateral occipital fissure (primitive) present [C88]; large lateral commisure supports two articulations for hyomandibula [C88]; hyomandibular articulations near anterior limit of fontanelle [J+03]; fontanelle dorsal to commisure for epaxial muscles (raise head) [C88]; prootic bridge at anterior terminus of notochord absent, so that notochord abuts against ethmoid portion of braincase [C88]; ventral cranial fissure (at break between trabecular & parachordal skull) extended dorsally to divide braincase into anterior & posterior halves just anterior to Vth cranial nerve exit [C88]; anterior braincase strongly ossified primitively [C88]; nasal capsules large [C88]; choana present, bordered laterally by maxilla & premaxilla, medially by vomer & palatine [C88]; ectopterygoid present and posterior to palatine [C88]; vomers, palatines & ectopterygoids bear fang pairs as well as smaller teeth running parallel to marginal dentition [C88]; large pterygoids sheath palatoquadrates [C88]; parasphenoid covers sphenethmoid from vomers to ventral cranial fissure [C88]; parotic plates cover braincase posterior to parasphenoids [C88]; pterygoid surrounds subtemporal fenestra (where jaw adductors descend to jaw) [C88]; lower jaw with stout dentary and ventral covering of splenials, angular & surangular [C88]; long prearticular [VS91]; Meckelian bone extends to symphysis [VS91]; 3 coronoids with fang pairs [VS91]; branchiostegals and gulars present [C88] [CA96$]; polyplocodont (labyrinthodont) dentition [VS91] [CA96$]; vertebrae with ossified centra, but extent of ossification quite variable [C88] [1]; zygapophyses absent or, at most, incipient [C88]; no specialization of anterior cervicals [C88]; caudal fin supported by neurals, hemals & epihemals [C88]; trend to symmetrical tail, with loss of mineralized scales (as in Actinopterygii) [C88]; basal portions of all fins covered in body scales, with lepidotrichia distally [C88]; anterior dorsal fin with enlarged basal, not articulating with vertebrae [C88]; posterior dorsal & anal articulate directly with neural or hemal arches [C88]; primitively, extrascapular and cleithral series (including posttemporal) articulate with posterior skull roofing bones [C88]; pectoral dermal (cleithral) series (from ventral to dorsal) includes cleithrum, anocleithrum, supracleithrum & posttemporal [C88]; pectoral girdles joined by large clavicle and small interclavicle [C88] [CA96$: interclavicle]; dorsal end of cleithrum broad & rounded [CA96$]; ascending process of clavicle wraps around anterior edge of cleithrum, overlapping it laterally & mesially [CA96$]; large cleithrum supports scapulocoracoid [C88]; limb endoskeletons with clear central axis, segmented repeatedly, with secondary radials on one or both sides [C+02]; humerus, ulna & radius present [C88] [2]; pelvic fins smaller than pectorals [C+02]; pelvic girdle with two elements probably joined medially by cartilage [C88] [3]; pelvic fin without sacral attachment [C+02]; acetabulum faces posteriorly [C88]; marine, marginal marine, brackish and fresh-water aquatic predators.

Notes: [1] As several writers have pointed out, there is no obvious phylogenetic pattern to the degree of ossification. [2] Carroll [C88] discusses some pectoral fin homologies with the tetrapod forelimb which are no longer generally accepted. It is safe only to say that the pectoral fin has a humerus, radius, ulna, and probably an intermedium. The pelvic fin likewise has a femur, tibia, fibula, and probably the intermedium. As we will see the tetrapod digital arch was probably an innovation at the tetrapod level. This arch gave rise to the carpals/tarsals, metacarpals/tarsals, and digits. The matter is not free from doubt, since we must then believe that the rhizodonts either developed a digital arch convergently or are more closely related to tetrapods than is generally supposed. Homoplasy seems the more probable conclusion for reasons we may discuss below. [3] The two elements are probably homologues of the ilium and pubis. The medial symphysis, if present, was formed by an anteroventral cartilaginous extension of the pubes, i.e., a pubic symphysis.

Stout-bodied fish; blunt snout; snout with rostral tubules; eyes small; parietals do not contact supraorbitals (not sure what this means -- Porolepis, for example, has no supraorbitals and most dipnomorphs have parietal fused into parietoethmoidal shield); supraneural spines (bones? spine would imply a enamel + dentine structure) articulating with certain neural arches; epicercal tail; elongate, leaf-shaped pectoral fins; pectoral fins with numerous mesomeres bearing pre- and postaxial radials [S95]; second dorsal fin has branching posterior radials.

Porolepiformes (= Holoptychiids): Glyptolepis, Holoptychius, Porolepis, Quebecius (?).

Introduction: The Porolepiformes were large deep bodied predators, with long fins, short broad heads, and small eyes. They represent a somewhat primitive lineage of rhipidistians. Their name comes from the characteristic rows of pores visible on the scales of some genera (e.g. Porolepis). The lower jaw tooth whorl contains several rows of large teeth on the whorl. The scales vary from thick rhombic cosmine types in primitive forms, to large, coarsely ornamented rounded scales in holoptychioids.

Distribution: The group is largely marine and uncommon during the Early Devonian. During the Middle Devonian they colonized brackish and fresh-water environments. They were especially common during the Late Devonian (especially the cosmopolitan genus Holoptychius) and their distinctive scales are useful as stratigraphic markers.

Ecology: These fish, especially the large brackish and fresh-water forms, may have been ambush predator which lay in wait for passing smaller fish, catching them with a quick forward lunge, rather like the pike does today. (MAK 991221)

Short, blunt anterior skull; strongly folded teeth filled with attachment bone ("dendrodont"); lower jaw massive with coronoid tusks, parasymphysis with large teeth (tooth whorls); parasphenoid large with numerous teeth; unclear whether choana is present (ventral nares may be under palate); palatoquadrate articulates with braincase through ethmoid and basipterygoid processes (possible suprapterygoid process); nares large and widely spaced; small anteriorly placed eyes; relatively few rostral tubules; pineal opening absent; very short parietals and longer post-parietals; main lateral line passes through center of post-parietals; extra cheek (squamosal) bones; hyomandibula short, robust and articulates with palatoquadrate via symplectic; intracranial joint present; opercular series small; unpaired fins have large dorsal plates; no radials in anterior dorsal fin (lepidotrichia articulate with basal plate); pectoral has >10 mesomeres with 2 sets of segmented radials (lifted head up when on bottom?); pelvic fin short, few mesomeres & 1 set of radials; scales & dermal bones covered with cosmine; surface enamel covers entire interior of pores in pore canal system; odontodes buried in cosmine present; near-shore marine deposition.

Image: modified from numerous sources, including Long 1995). A palatal views of some of the same structures may be found at The Sphenethmoid.

Notes: The arrangement shown in the figure is fairly typical of basal sarcopterygians other than dipnoans. The adductor chamber is restricted to the space lateral to the palatoquadrate (derived forms beat this limitation in various ways). However, the force of the mandibular adductors is amplified by simultaneous contraction of the underlying subcephalic muscle. The subcephalic is anchored on the otico-occipital region which is held in place by the notochord. Contraction of the subcephalic thus flexes the ethmoid portion of the braincase -- and thus the entire anterior skull -- downward at the intracranial joint between the ethmoid (in purple) and otico-occipital (in yellow) portions of the braincase. The double action of the jaws creates a powerful snap at the front edge of the mouth. This arrangement may explain why sarcopterygians tend to have wide, strong rostra and why they develop anterior fangs, shorter jaws, symphysial tooth whorls and the like. 020117.

Less than 30 cm; small, non-standard bones flanking paired dermal skull bones; sutures always visible between premaxilla and the rest of the frontoethmoidal shield; orbits small; posterior face of ethmosphenoid facing posteroventrally; ethmoid relatively large and not fully separated from otico- occipital; intracranial joint situated at the level of the trigeminal exit; cosmine covering with enamel penetrating pores (a porolepiform trait); odontodes buried in cosmine present.

Note: The name is derived from an acronym for Prince of Wales Island, located in arctic Canada.

Sarcopterygii: Rhipidistia

Styloichthys and Powichthys

Taxa on This Page

Rhipidistia

Descriptions