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EOCENE, MARINE OSTEOGLOSSOMORPHS AND IMPLICATIONS
FOR THE DISTRIBUTION OF THIS GROUP IN THE CRETACEOUS
Niels BONDE 1 & Jesús ALVARADO-ORTEGA 2
1 Inst. Geography & Geology (Copenhagen University), Øster Voldgade 10, DK-1350 Copenhagen K and
Fur Museum, DK-7884 Fur, Denmark; nielsb@geo.ku.dk; 2 Instituto de Geología, Universidad Nacional
Autónoma de México, Ciudad Universitaria, Coyoacán, D.F., 04510 México; alvarado@geologia.unam.mx
Living osteoglossids and for that matter all Osteoglossi (new crown group taxon comprising
Mormyriformes and Osteoglossiformes) have traditionally been considered prime examples of
primary freshwater fishes. In the marine Eocene one osteoglossid like fish has been recognized
(Brychaetus) in the Eocene London Clay, and it could be considered an ‘anomaly’. But by now 9
more marine Eocene forms have been found. Six were described from the earliest Eocene of
Denmark, where two new genera (Heterosteoglossum and Xosteoglossid) occur with Brychaetus
in basal Eocene, and the former is found also in the overlying ‘Mo-clay’ (Fur Fm.) together
with three more genera. Most common is Heterosteoglossum (which is also identified in basal
London Clay). Opsithrissops from Turkmenistan has the same age, and three marine forms
(Monopteros, Trissopterus, Foreyichthys) occur in Monte Bolca.
Analyses of the phylogenetic relationships of marine osteoglossomorphs shows that these
genera are distributed in different positions in the tree, none of them appear to be closest relatives
of each other. The most primitive is Furichthys, probably high in the stemgroup of Osteoglossi,
while Brychaetoides is basal in the stemgroup of Osteoglossiformes. Xosteoglossid has
a very high position in this stemgroup or is inside the living crowngroup. The arapaimid subgroup
of this crowngroup has Trissopterus low in its stemgroup, while the osteologlossid subgroup
has Opsithrissops, Brychaetus, Foreyichthys and Heterosteoglossum as successively
more advanced members of its stemgroup. Arapaimids and osteoglossids clearly had a marine
last common ancestor, and the last common ancestor of Osteoglossi was probably also marine,
indicating mormyriform origin in the sea. The entire Osteoglossomorpha had a marine origin. So
the hiodontids must be another independant invasion into freshwater. Accordingly, there must
have been a number of Cretaceous, marine ‘stemforms’ of that group, the Mormyriformes and
probably arapaimids, while osteoglossids might be a younger lineage. Where did these ‘missing
Cretaceous ancestors of hiodonts, mormyriforms, arapaimids and perhaps osteoglossids live?
May be in the sea surrounding the mythical ‘Pacifica’ and now subducted, and with ‘stemmormyrifoms’
in the Indian Ocean. Now these expected marine osteoglossomorphs may have
turned up as tiny somewhat ‘Lycoptera-like’ fishes from Early Cretaceous in Tlayúa, México.
They seem to be very primitive within the group, quite likely sistergroup of all the rest, as they
have retained some ‘elopiform-like’ features otherwise lost in osteoglossomorphs.
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