Reinforcing the idea of an early dispersal of Hippopotamus amphibius in Europe: Restoration and multidisciplinary study of the skull from the Middle Pleistocene of Cava Montanari (Rome, central Italy)

Abstract

A skull of Hippopotamus recovered from the area of Tor di Quinto, within the urban area of Rome (central Italy) is here redescribed. Despite being one of the most complete specimens of hippopotamuses of the European Pleistocene, the Tor di Quinto skull did not attract much research interest, due to long-standing uncertainties on its provenance. This work begun in 2021, when the skull was restored, within a large renovation project on the vertebrate exposed at the Earth Science University Museum of Sapienza University of Rome. Original sediments were found inside the cranial and mandible cavities during the restoration work, which were sampled for petrographic analyses. By combining a review of the old paleontological, archeological and geological literature published during the 19th and 20th century on the Rome basin and the correlation of these new sedimentological and petrographic information with the lithostratigraphic and synthemic units of the national geological cartography, we clarify that the Hippopotamus skull was most likely to have been collected from a quarry called Cava Montanari, from a formation dated between 560 and 460 ka. Morphological and biometric analyses clearly support an attribution of the Cava Montanari specimen to the extant species Hippopotamus amphibius. The reassessment of the stratigraphic and geological data on Cava Montanari implies that the studied specimen is the earliest confirmed occurrence of Hippopotamus amphibius in the European fossil record.

Fig 1. Selected cranial and mandible remains of late Early to Late Pleistocene hippopotamuses from Europe.

1–108, skull from Collecurti in left lateral (a), ventral (b) and right lateral (c) views; 2—IQW 1991/23 (Mei 23 438), cranium of Untermassfeld in right lateral view (a); 3 –MPUR/V 1950, skull from Sant’Oreste in left lateral view (a); 4 –No catalogue number, mandible from Ortona in right lateral view (a); 5—MPUR/V 52, mandible from Vallinfreda in left lateral view (a); 6 –Crania from La Maglianella; 321, cranium in left lateral (a) and ventral (b) views; 322, cranium in right lateral (c) and occlusal (d) views; 601, skull in left lateral (e), occlusal (f) and right lateral (g) views; 7 –MG3665, hemimandible from Condeixa in right lateral view (a); 8 –Crania and mandibles from Barrington; D3980, skull, D13938 in right lateral view (a); D13938, cranium in left lateral view (b); D3975, mandible right lateral view (c). Images modified from [8, 9, 15, 16, 21, 124, 125]. Images are not in scale. Colors: light blue—Hippopotamus antiquus; light red—Hippopotamus amphibius.

Fig 2

Location of the studied area of Tor di Quinto (A [modified by 100]-B [taken by USGS National MAP Viewer]. Sketches of the historical geological and topographic maps by [126] (C), [127] (D), [128] (E), and [35] (F). In the blue bounded square it is reported the location of the former military riding school (“Scuola di Equitazione”).

Fig 3

Historical picture of the Tor di Quinto area from the archive of the Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA, A, B) and the same area today (taken by USGS National MAP Viewer, C) and from topographic maps by [127] (D).

Fig 4. Geological sketch map of the middle and downstream Tiber River drainage basin (modified after [67]).

Location of the upstream and downstream modern and ancient Tiber River samples used for petrographic analysis are reported.

Fig 5

3D model of the left hemimandible of Hippopotamus amphibius from Cava Montanari (A-C) and the depth-map (D). Legend of the depth map: blue is the bone surface and the red pits are where pebbles had been pushed against and into the bone forming surface depressions.

Fig 6

Geological map of the studied area (Tor di Quinto, Rome) after [35], with the location of the riding school (“Scuola d’Equitazione”), sampled outcrops, and the ancient quarry (A). Stratigraphic logs of the sample outcrops in this work (B). Chronostratigraphic scheme of the Geological Formation of the Rome basin (C).

Fig 7

Compositional biplots [63] displaying data from modern Tiber upstream and downstream sand and ancient sand samples from the Vitinia (VTN), Valle Giulia (VGU), and Fosso della Crescenza (FCZ) Formations (A and B). Rays represent the parameters in the datasets (Q, total quartz; K, K-feldpsar; P, plag; Lc, carbonate lithics; Lm, metamorphic lithics; Lv, volcanics; Hm, dense minerals). The length of each ray is proportional to the variability of the parameter in the data set; the angle between two rays reveals whether the corresponding parameters are well correlated (0°), uncorrelated (90°), or inversely correlated (180°) (from [64]). Note in A) that the modern upstream samples cluster with the Hippopotamus, VGU, and FCZ samples; conversely volcanoclastic downstream fluvial samples cluster with VTN sand. LmLvLs ternary diagram comparing the ancient sand lithic fragments composition. Note that the hippopotamus sediment samples overlap the lithic signature of the VGU sand (C).

Fig 8. The skull of Hippopotamus amphibius of Cava Montanari.

Cranium in dorsal (A), ventral (B), right lateral (C), left lateral (D) and posterior (E) views. Mandible in occlusal (F) right lateral (G) and left lateral (H) views. Scale bar 10 cm.

Fig 9. Cranial morphological features of selected extant and fossil hippopotamus specimens.

The following numbers indicate the main morphological features proposed in literature used to discriminate Hippopotamus antiquus and Hippopotamus amphibius (as reported in Table 2). Colours: blue—H. antiquus from Figline (Upper Valdarno, 1a-1c, modified from [65]); light blue—H. antiquus from La Maglianella (2a-2c, modified from [15]); red–H. amphibius from Barrington (3a-3c, modified from [65]); orange—extant specimen of H. amphibius (4a-4c); yellow—H. amphibius from Cava Montanari (5a-5c). Cranium in dorsal (a), ventral (b) and left lateral (c) views.

Fig 10. Mandibular morphological features of selected extant and fossil hippopotamus specimens.

The following numbers indicate the main morphological features proposed in literature used to discriminate Hippopotamus antiquus and Hippopotamus amphibius (as reported in Table 2). Colours: blue—H. antiquus from Figline (Upper Valdarno, 1a, 1b, modified from [65]); light blue—H. antiquus from La Maglianella (2a, modified from [15]); red—H. amphibius from Barrington (3a, 3b, modified from [65]); orange–extant specimen of H. amphibius (4a, 4b); yellow—H. amphibius from Cava Montanari (5a, 5b). Mandible in left lateral (a) and occlusal (b) views.

Fig 11. Scatter plot of the first two principal components of Principal Component Analysis (PCA).

Fig 12. Paleogeography of the middle Tiber Valley Basin during the uplift dominated phase (latest Early Pleistocene-Late Pleistocene, from 1.3 to 0.1 Ma).

To the left, location of the paleo Tiber fluvial system within the Paglia Tevere graben developed at the west Apennines foothill (in green with axial drainage). To the right, volcanic activity of the Vulsini Mts., Vico, Sabatini Mts. and Albani Hills Volcanic Districts (in pink) and their impact on the Tiber fluvial drainage system. The main pyroclastic flows are dated and reported with red arrows (redrawn and modified after [66]).