The Menaspis holotype appears to be missing its skull and gill chambers

Menaspis also appears to be preserved in ventral exposure
(Fig 1) contra previous interpretations (Figs 3–5). Distinct from most fish fossils (e.g. Fig 2), Menaspis is apparently missing the skull and gills. Perhaps they were bitten off. The tail is missing because the substrate that once preserved the tail is missing. Here’s the holotype newly interpreted (Fig 1). The preserved part is about 6 inches or 15 cm in length.

Figure 1. Menaspis in situ appears to be missing its skull and gill chambers and exposed in ventral view. If so the restoration of a skull and gill chambers turns this enigma taxon into a more familiar acanthodian.

With these changes, Menaspis nests with the spiny shark (acanthodian)
Homalacanthus (Fig 2) in the large reptile tree (LRT, 2107 taxa). These are both basal acanthodans. Menaspis is known from the Late Permian (~260mya). In the LRT a few clades moved around at the split between bony fish that sometimes developed lobe fins and bony fish that never developed lobe fins.

Figure 4. Homalacanthus in situ and reconstructed.
Figure 2. Homalacanthus in situ and reconstructed. In the LRT this taxon now nests with Menaspis (Fig 1).

Menaspis armata
(Ewald 1848; Late Permian; > 15cm long) was described as the ‘last known arthrodire placoderm’. The former skull spine is the pectoral fin. The former ‘horns’ are ventral spines. The skull and gill chamber appear to be missing here. The tail is off the rock break. Prior interpretations are provided below (Fig 3–5). Ortlam 1986 confirmed Ewald 1848 who reported the specimen was preserved in ventral view. Others (Figs 3–5) disagreed.

Figure 3. Bendix-Almgreen 1971 dorsal view illustration of Menaspis with orbits misplaced. The actual fossil exposes the dorsal surface in ventral view with barbels descending, not ascending. 

Figure 3. Bendix-Almgreen 1971 dorsal view illustration of Menaspis with orbits misplaced. The actual fossil exposes the dorsal surface in ventral view with barbels descending, not ascending. See figure 1. 

We looked at Menaspis earlier
here in 2019 when the LRT nested it with catfish and placoderms, assuming the fossil was complete. That now appears to be wrong. It’s interesting to see how this strange specimen can be interpreted in various ways with different insights and assumptions. If the specimen retains a skull those giant ‘dorsal’ spines are autapomorphic. If the skull is absent the spines are homologous with spiny sharks (= Acanthodii) spines. Evolution does not usually produce autapomorphies (= unique structures) without precedent or descent. This new interpretation (Fig 1) is therefore more parsimonious, with precedent. If valid, Menaspis spines are now synapomorphies with acanthodian ventral spines.

Lund 1977 reports:
“Bendix-Almgreen (1970) reinterprets the much-interpreted morphology of Menaspis as an arthrodire derivative, with dubious success. His placement of the orbit and orientation of the mouth do not seem implausible.”

Lund thought Menaspis had
“a batoid habitus such as body flattening, loss of the dorsal fin, and development of defensive spines which render Menaspis a uniquely derived form.”

In evolution very fiew traits are unique.

Figure 1. Old lithograph of Menaspis with barbels raised like horns.
Figure 4. Old lithograph of Menaspis with barbels raised like horns. See figure 1.

Some authors report
Deltoptychius Mcoy 1855; Morris and Roberts 1862; 345-298mya) is closely related to Menaspis. Several specimens of Deltoptychius are known. A rare complete NHM specimen looks like a ratfish. We’ll look at Deltoptychius tomorrow.

Figure 5. Menaspis armata from Patterson 1965. See figure 1.

Grogan, Lund and Greenfest-Allen 2004
nested Menaspis between the long-snouted shark, Squaloraja, and the hammerjaw/swordfish precursor, Traquairius. The rest of their cladogram also suffered from massive taxon exclusion.

This is a novel interpretation for Menaspis.
If prior interpretations are correct we’ll have to accept there is nothing else quite like this taxon among LRT tested taxa. The jaws and eyes have not been positively identified. Witness, two other interpretations (Fig 3, 4) do not match each other and the the third (Fig 5) does not identify orbits. Confirmation, refutation or correction is needed here.

References
Bendix-Almgreen SE 1971. The anatomy of Menaspis armada and the phylogenetic affinities of the menaspid bradyodonts. Lethaia 4(1):21–49.
Ewald J 1848. Über Menaspis, eine neue fossile Fischgattung. Berichte Über die zur Bekanntmachung Geeigneten Verhandlungen der Königlich-Preussischen Akademie der Wissenschaften zur Berlin 1848:33-35.
Jäckel O 1890. Über Menaspis, nebst allgemeinen Bemerkungen über die systematische Stellung der Elasmobranchii. Sitzungsb. Ges. nature. Freunde, Berlin 1891: 115–131.
Lund R 1977. xx
Morris and Roberts 1862. Quart. J. Geol. Soc., 18.
Ortlam D 1986. Neue Aspekte zur Deutung von Menaspis armata Ewald (Kupferschiefer, Zechstein 1, Deutschland) mit Hilfe der stereoskopischen Röntgentechnik. Geologisches Jahrbuch Reihe A, Band A 81.
Patterson C 1965. The phylogeny of the chimaeroids. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, 249(757): 101–219.
Patterson C 1968. Menaspis and the bradyodonts. In: T. Ørvig, Current Problems of Lower Vertebrate Phylogeny. (Hrsg.): Nobel Symposium. Band 4. Almquist and Wiksell, Stockholm 1968, S. 171–205.
Schaumberg G 1992. Neue Informationen zu Menaspis armata Ewald. Paläontologische Zeitschrift 66:311.

wiki/Homalacanthus
wiki/Acanthodes

wiki/Menaspis
German wiki/Menaspis

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