Orthocormus: a bony fish with an odd reversal: a shark face

Everyone agrees
that Orthocormus roeperi (Weitzel 1930; Arratia and Schultze 2013; Early Jurassic; 45cm; BSPG 1993 XVIII-VFKO B1) is a pachycormiform fish. In the large reptile tree (1781+ taxa) Orthocormus nests with a more derived, Late Jurassic pachycormiform with a longer snout, Aspidorhynchus (Figs. 3, 4), the famous Rhamphorhynchus nibbler.

For reasons unknown,
Aspidorhynchus (Figs. 3, 4) is not traditionally considered a pachycormiform fish.

Arratia and Schultze 2013 consider 
“The fish described here is the best-preserved pachycormiform from Bavaria, Germany, as well as from the Upper Jurassic worldwide.”

Figure 1. Orthocormus, a small, bony fish with a shark-like face.

Figure 1. Orthocormus, a small, bony fish with a shark-like face. Shown 1/3 actual size on a 72dpi screen.

According to Wikipedia
“Pachycormiformes were characterized by having serrated pectoral fins (though more recent studies demonstrated that fin shape diversity in this group was high, reduced pelvic fins and a bony rostrum.”

Figure 2. The shark-like skull of Orthocormus, a ray-fin bony fish.

Figure 2. The shark-like skull of Orthocormus, a ray-fin bony fish. The extended rostrum is a reversal from shark ancestors.

As you can see, this is less than accurate description.
The pelvic fins are not reduced in Orthocormus nor are the pectoral fins serrated. Pachycormus does not have a bony snout. Wikipedia lists several synapomorphies (“Pulling a Larry Martin“) and does not list Aspidorhynchus as a pachycormiform. Better to just run the analysis and find the last common ancestor to determine clade membership.

Rhamphorhynchus entangled with Aspidorhynchus

Figure 3. Rhamphorhynchus entangled with Aspidorhynchus. Both complete and articulated. Inside the belly of Rhamphorhynchus are several smaller fish. Inside its throat is another. Image from Frey and Tischlinger (2012).

Dentition according to Arratia and Schultze 2013:
“There are small conical teeth on the upper jaw, a large tooth on the posterior part of the premaxilla, and both large and small teeth on the maxilla; the lower jaw carries large teeth anteriorly and smaller ones posteriorly.”

Notochord
“The vertebral column is formed by a persistent notochord without chordacentra,”

The tail
“The scaly caudal apparatus, formed by large modified scales with a precise arrangement, is interpreted as an adaptation to fast swimming comparable to that of modern tunas.”

Spiral valve
Another apparent reversal is the presence of a soft tissue spiral intestine, as found in sharks and several other various taxa without apparent pattern, including sarcopterygians. This is the only time Arratia and Schultze 2013 mention the term, ‘shark’ in their text.

Figure 2. The face of the Wyoming Dinosaur Center CSG 255 specimen of Asphidorhynchus + Rhamphorhynchus with facial bones identified using DGS.

Figure 4. The face of the Wyoming Dinosaur Center CSG 255 specimen of Asphidorhynchus + Rhamphorhynchus with facial bones identified using DGS.

Wkipedia reports on Pachycormiformes,
“Their exact relations with other fish are unclear, but they are generally interpreted as stem-teleosts.”

By adding taxa, the LRT provides exact relations with other fish, in this case, within bony fish, close to the base, not in the stem. Again, a wider view provided by the LRT supplements and aids the more focused view of those describing the fossil firsthand. Reversals and convergent traits can be tricky. Let the unbiased software do the decision-making.


References
Arratia G and Schultze H-P 2013. Outstanding features of a new Late Jurassic pachycormiform fi sh from the Kimmeridgian of Brunn, Germany and comments on current understanding of pachycormiforms. Pp. 87–120 in Mesozoic Fishes 5 – Global Diversity and Evolution, Arratia G, Schultze H-P and Wilson MVH (eds.).
Weitzel K 1930. Drei Riesenfische aus den Solnhofener Schiefern von Langenaltheim. – Abh. Senckenberg. Naturforsch. Ges. 42 (2): 85-113.

wiki/Orthocormus
wiki/Pachycormiformes

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