History of Paleomammology: The Taphonomy of Hoplitomeryx

"The species of ruminants are the most difficult to distinguish from one another. Although they are sharply distinct from other quadrupeds, they resemble each other so much that to characterize genera one has to use parts such as the horns;…[]"

G. Cuvier "On the Fossil Bones of Ruminants Found in the Superficial Deposits."

The Italian fossil record of terrestrial mammals is relatively poor during the Early and Middle Miocene; it is only in the Late Miocene that the record of terrestrial vertebrates increases. The pre-Messinian sediments document the existence of three distinct bioprovinces, two of them characterised by various endemic species, denominated after the recent geographical regions the Abruzzi-Apulia and the Tusco-Sardinian paleobioprovince. The third, the Calabro-Sicily bioprovince, shows mainly non-endemic mammals with species distributed at these times also in North Africa.

Fig.2. Paleogeografic reconstruction of the Italian peninsula during the Miocene-Pliocene transition with the discussed localities. In green land areas, in brown the first highlands of the Apennines.

During the Miocene and the Early Pliocene sea level variations transformed repeatedly the area of the Italian peninsula in an archipelago with isolated islands of various sizes and a humid and warm climate - an ideal laboratory for evolution.
One of these today lost islands was the modern peninsula of Gargano, on the east coast of South Italy, separated at these times from the main land by a branch of the Adriatic Sea. Practically all fossil mammals of Gargano show extraordinary morphological signs of insularity. This faunal assemblage of the Abruzzi-Apulia bioprovince is known as Mikrotia-fauna, after the common genus of the endemic murid of the region.

A peculiar animal of this fauna is the genus Hoplitomeryx (LEINDERS 1984). This artiodactyl resembles in its reconstructed appearance the Muntjac, a basal representative of the deer family, but displays a unique combination of anatomically characters. Despite some specialisation in the structure of the limbs, the most astonishing features can be found on the skull of the animal - five horns (not antler like in the true deer), a pair of horns above each orbit and one central nasal horn, paired with prominent sabre like upper canines ("moschid" type).
These unusual combination of characteristics make the phylogenetic position of this animal inside the artiodacyls very uncertain, the only described species Hoplitomeryx matthei (LEINDERS 1984) is actually classified in a own family, Hoplitomerycidae, even if the fossil material of Gargano shows almost four to five distinct "size-classes" (from estimated 5 to 50kg body mass), that may represent single species (VAN DER GEER 2007).
If these classes represent in fact species, the uniform anatomical characters between them suggest a rapid adaptation and radiation from a common ancestor, who after the formation of the island of Gargano adapted quickly to the diverse habitats found on the island. The problem of differentiating the fossil species is based on the problem of dating the sediments that contain the fossils. The sediments show no stratification and the bones show no change in size trough the preserved stratigraphic column.
Three explanations for this observation are possible:

- the red clays are to disturbed to display a temporal variations and succession of different species

- different species existed in the same time period and became embedded in the red clays

- the size classes all represent the variation of one species

Despite the insecurities about the number of species, TORRE et al. in 2005 postulated an affinity of Hoplitomeryx to the basal ungulates of the family Choeropotamidae and Dacrytheridae, known from the Eocene of France and closely related to the Suidae. According to VAN DER GEER also an affinity with the ancestors of the modern North American pronghorn (Antilocapridae) is plausible.

The first fossils of Hoplitomeryx were found near the city of Foggia, in homogenous to weakly stratified red clays which partially fill the karst fissures of the underlying limestone substrate. In the late sixties and subsequent years from similar karst fissures of the promontory of Gargano further fossil material could be recovered. The fossils are found in reworked, homogenous to weakly stratified red clays (denominated "terra rossa"), which partially fill the paleo-karstic fissures of the Mesozoic limestone substrate. This karst network is truncated by a disconformity of Late Pliocene to Early Pleistocene sediments of marine origin.
The exact age of the terra rossa is uncertain, the stratigraphic correlation and the mammal chronology suggests a temporal range from the late Miocene to early Pliocene, with preferences from various authors to the Pliocene (VAN DER GEER 2007).

Fig.3. The "Passalacqua" quarry, situated at the north-western margin of the Gargano promontory, close to the village of Apricena in the italian province Apulia. Here in sediments deposited in a karst network, discovered during quarrying of the limestone, a very diverse and now well-documented fossil vertebrate assemblage was discovered.

Fig.4. Molar of Hoplitomeryx.

Until 1990 Hoplitomeryx was believed to be an exclusive genus of the former island of Gargano, but during a field campaign of the Museum of Palaeontology and Geology of the University of Florence a rich assemblage of Miocene vertebrates (crocodiles, turtles, various deer and an otter like animal) was discovered on the eastern ridge of the Mount Civita near Scontrone in the province of Abruzzo.
Found embedded in yellow limestone breccias the bones of various mammalian species were recognized, one of them identified as Hoplitomeryx. This discovery suggests a paleogeographic connection, maybe trough an episodic land bridge or smaller islands of the island of Gargano to the European mainland.

During the Messinian (7Ma) all the endemic species of the Tusco-sardinian bioprovince disappeared and were replaced by a continental fauna with clear European affinities (BONFIGLIO 2005).
This geological period is marked by a pronounced sea level drop of the Mediterranean Sea, causing deposition of thousand of meters thick evaporites. The former isolated islands get reconnected with the European continent, and a dramatic faunal turnover occurred.
Only on still isolated islands some species characteristic for the Abruzzi-Apulia bioprovince survived this dramatic episode until the Pliocene, but with the establishment of the tectonic raised land bridge from the promontory of Gargano to the Italian peninsula, the last Hoplitomeryx went extinct.

Bibliography:

AGUSTI, J. & ANTON, M. (2002): Mammoths, Sabertooths, and Hominids - 65 Million Years of Mammalian Evolution in Europe. Columbia University Press: 313 (Figure 1. is taken from this book)
BONFIGLIO, L. (2005): Paleontologia dei vertebrati in Italia - Evoluzione biologica, significato ambientale e paleogeografia. Museo di Storia Naturale - Verona: 238
VAN DER GEER, A. (2007): The effect of insularity on the Eastern Mediterranean early cervoid Hoplitomeryx. The study of the forelimb. Quaternary International 182 (1): 145-159