ICHTHYOLITES OF THE OLD RED SANDSTONE

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SARCOPTERYGII

Osteolepiformes, Rhizodontiformes and the Panderichthyida  

Osteolepiformes

 Osteolepiform fish are thought to the ancestors of the tetrapods because of the structure of their paired fins and also because they may have had choanae ( an intrabuccal opening – possible posterior nostril (excurrent)) possibly shared with the tetrapods. Despite there being numerous species, structurally they where quite homogeneous. There are two recognised clades, the Tristichopteridae (Eusthenopteridae) and Megalichthyids, although the early cosmine covered ‘osteolepid’ fish  (inc Osteolepis, Gyroptychius and thursius) are unable to fit into a clade and are probably paraphyletic.   They are first seen in the Emsian or Eifelian as Cosmine covered osteolepids, reached their maximus diversity in the Mid/Late Devonian and by the Late Carboniferous only the  large megalichthyids remained.

 The Tristichopteridae include the much celebrated Eusthenopteron foordi from Miguasha in Canada made famous by Jarvik, who spent almost quarter of a century studying it. Eusthenopteron has lost the cosmine of the osteolepids, has a somewhat diphyceral tail  (shared with Gyroptychius) compared to epiceral tail of Osteolepis,  has more angular posteriorly placed dorsal fins and an elongated snout (although juveniles have a shorter snout).

Osteolepiformes in my collection (click on thumbnails to see larger images)

Tristichopterus alatus Egerton,    Mid Devonian,  Scotland

 Osteolepis macrolepidotus,    Mid Devonian,   Orkney,    Scotland

      Lethen-bar, Scotland

  Images from Catalogue of Fossil Fishes AS Woodward 1891

Gyroptychius agassizi,    Mid Devonian,   Orkney,    Scotland

Thursius pholidotus     Mid Devonian,   Murkle Bay,    Scotland

Eusthenopteron foordi     Upper Devonian,      Escuminac Formation,  Miguasha, Quebec, Canada

     Cannibalism plate

   Megalichthys sp.  tooth, Carboniferous, Bearsden, Scotland

Rhizodontiformes

Known from fossils within the Coal measures since the middle of the last century, the name is derived from ‘root – tooth’ because of the the huge fangs that extend deeply into their jaws.  Their pectoral fins were strong and shared the basic structure of having a humerus, ulna and radius with the osteolepiformes and later tetrapods. They first appear in the Mid Devonian but peaked in diversity and size in the Carboniferous. Rhizodus from Scotland reached 6 – 7 m in length with a 1 m jaw and 22 cm fangs.

      Rhizodus hiberi     Carboniferous,   Cowdenbeath,    Scotland

      Rhizodopsis,  Scales and skull,  Carboniferous, Bearsden    

Panderichthyida

  Made up of only 5 species these fish seems to lie between the early tetrapods and the Osteolepidiformes. They are characterised by thickening of the postfrontal plates median to the eyes, the median rostral does not contail the premaxilla, they have a large media gular plate and a lateral recess in the nasal capsule. Panderichthys from Lode in Latvia and Elpistostege from Miguasha in Canada are the best know examples. They have many Tetrapod-like features such as a large, broad, flattened skull (25 pct of the total length of the fish) with closely spaced eyes on the top, labyrinthodont plicidentine in the teeth, a fused intercranial joint, a long and strongly ossified humerus (and many more). They do however retain many fish-like features as well.