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H.-P. Schultze, T. Marss. Revisiting Lophosteus
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Systematic position
Most researchers considered Lophosteus as a primitive osteichthyan, but difficult to be
assigned systematically. Rohon (1893: 70) was the first to recognize its osteichthyan
relationship; he placed the genus close to the “Cyclodipterinen” (Holoptychius,
Glyptolepis etc.) of Pander (1856). Gross (1969, 1971) identified Lophosteus, together with
Andreolepis as basal osteichthyan in the family Lophosteidae and order Lophosteiformes.
This was based on plesiomorphic histological characters, and he left the question open,
if the family and order are closer to the Actinopterygii or the Sarcopterygii. Schultze (1977)
separated these genera and placed Andreolepis at the base of Actinopterygii, because he
described ganoine in the latter genus, and identified Lophosteus as a basal osteichthyan
between the actinopterygians and sarcopterygians. Janvier (1978) agreed with the
placement of Andreolepis in Actinopterygii. He suggested assignment of the order
Lophosteiformes at the base of the Actinopterygii, preceding the Palaeonisciformes;
nevertheless he left open the question whether both genera belong „to a hitherto unknown
class of fishes which became extinct at the end of the Silurian and possessed characters
in common with the actinopterygians, the struniiformes and the acanthodians“ (Janvier
1978: 94). Long (1989) considered the lophosteiforms as the probable sister group to all
osteichthyans. Schultze (1992) employed Lophosteus as an outgroup to arrange early
actinopterygians phylogenetically, whereas Janvier (1996) placed the genus at the base
of the Actinopterygii. Märss (2001) agreed with Schultze (1977) that Andreolepis is an
actinopterygian and that Lophosteus is different from that genus; she placed Andreolepis
in the new family Andreolepididae because the lateral line canals are situated differently in
the dermal skeleton in the genera Andreolepis and Lophosteus. Lophosteus has rather
wide and deep open canals whereas in Andreolepis they are closed as in actinopterygians.
Otto (1991) suggested an intermediary position between acanthodians and osteichthyans.
Burrow (1995a) indicated similarities with placoderms, but those were considered superficial
by Märss (2001).
A limited number of characters are known only in Lophosteus (Appendix 2). The
main characters are those of the scales, their shape, ornament and histology. In addition,
few characters can be extracted from spines, teeth and tesserae. We have analysed
these characters in comparison with early osteichthyans, one acanthodian and one
arthrodiran placoderm (Appendix 3). Phylogenetic analysis (PAUP 3.1.1: DELTRAN)
with Climatius and Sigaspis as outgroups, and Terenolepis, Naxilepis and Orvikuina
deleted because of missing characters, results in a shortest tree of 42 steps, a consistency
index of 0.714 and a homoplasy index of 0.452. This analysis consistently places
Lophosteus with Climatius in the outgroup outside the osteichthyans. The analysis
separates the actinopterygians from the sarcopterygians within the osteichthyans (Fig.
7). Andreolepis forms a sister group of all other actinopterygians, and the Chinese
sarcopterygians at the base to all other sarcopterygians except Achoania.
Interrelationships within the sarcopterygians, including Psarolepis and Achoania, are
unresolved. The database is too restricted to achieve a resolved cladogram for the
sarcopterygians, which in any case is not the goal of this analysis. Interrelationships
within actinopterygians are still resolved with the inclusion of Orvikuina and Naxilepis;
only the additional inclusion of Tenerolepis results in an unresolved interrelationship
of all actinopterygians above Andreolepis.