Mene oblonga Agassiz, 1833
Monte Bolca near Verona
This is not the "normal" Mene rhombea Volta, 1796, but a Mene oblonga Agassiz, 1833. For a menid it has a very shallow body, especially in comparison with the sympatric Mene rhombea (click here for comparison). Its fossils are very rare in Monte Bolca. The length of this fish is about 15cm; adult fish can reach almost 20cm.
The location is known as Pesciara or "Fishbowl" due to its many marine fossils.The sediments from Monte Bolca are from the Eocene epoch, the Lutetian, so the fish is about 50 million years old.
The entire, abundantly fossiliferous formation consists of 19 metres of limestone, all of which contain fossils, but interspersed in which are the lagerstätten layers that contain the highly preserved specimens. Within these layers, the fish and other specimens are so highly preserved that soft tissue preservation can is observable and even the skin colour pattern can sometimes be determined (see for example the photo of Paranguilla tigrina in this album).
There are several other related outcroppings in the general vicinity of Monte Bolca that also carry fossils, such as Monte Postale and Monte Vegroni. The term "Monte Bolca" is often used interchangeably to refer to all the sites collectively.
The Monte Bolca deposits where formed in a lagoon which was separated from the open sea by coral reefs. During that time, there was considerable volcanic activity. The volcanoes on the nearby mainland erupted periodically, their lavas and ashes covering the land for some time, locally causing mass extinctions. The sediment is very fine grained limestone; the preservation suggests when the fish died it was rapidly buried in fine sediment that was anoxic.
Mene rhombea and Mene oblonga are both closely related to the living moonfish Mene maculata that lives in small schools in and around reefs in the Indo-Pacific region. Mene maculata inhabits deeper coastal waters near the bottom on both the continental shelves and around major island groups; it is very likely that the same behavior applies to Mene rhombea and Mene oblonga. The Menidae belong to the Perciformes, however the affinity of Mene remains obscure. Recent studies suggest a relationship with the pomfrets, dolphinfish and the jacks. Aipichthyoides from the Cretaceous of Lebonon is believed to be close to Mene's ancestry.
The earliest accepted Mene species are Mene purdyi from the Paleocene of Peru and M. phosphatica from Tunisia; they already resemble later species such as both M. rhombea and oblonga from Monte Bolca and even the living species, M. maculata.
Moonfish have a very deep, almost triangular shaped-body that is very much flattened from side-to-side, a large triangular tail fin and conspicuously long rays in the ventral fins. Mene maculata is a planktivore; the small, upturned mouth of Mene rhombea suggests the same feeding behavior whereas Mene oblonga picked out edible detrius and organisms out of the sediment.
M. Friedman and D. Johnson, Journal of Vertebrate Paleontology 25(4):770-783. 2005 A NEW SPECIES OF MENE (PERCIFORMES: MENIDAE) FROM THE PALEOCENE OF SOUTH AMERICA, WITH NOTES ON PALEOENVIRONMENT AND A BRIEF REVIEW OF MENID FISHES