The Terre Rosse are red-soil deposits trapped in palaeokarstic fissures dissolved into Mesozoic limestone, extending primarily on the northwestern slopes of the Gargano promontory between the towns of Apricena and Poggio Imperiale in the Apulia region. The fissures were first brought to scientific attention in the late 1960s and early 1970s by Dutch researchers led by Matthijs Freudenthal, who began exploring the extraordinarily rich vertebrate assemblages that have since become known as the Mikrotia fauna, named after the most common representative taxon, a giant endemic murid rodent. The amphibians, by contrast, had been noted only in passing: a doctoral thesis and a recent review mentioned discoglossids, hylids, bufonids and ranids, but no dedicated study had ever been published.

The new work surveyed all recognised amphibian remains held in three collections, at the University of Florence, the University of Turin and the Naturalis Biodiversity Centre in Leiden. The researchers counted the number of identifiable specimens and estimated the minimum number of individuals for each fissure, then mapped how the taxa were distributed across the biostratigraphic phases defined for the Terre Rosse on the basis of mammal assemblages, from phase 0 to phase 4 with subphases. In total, 62 fissures yielded anuran fossils, with a maximum of 45 identifiable specimens in a single fissure and a minimum number of individuals that never exceeded nine. The mean was 8.4 specimens and 2.5 individuals per fissure, so amphibians are scarcely represented in the fillings overall.

One frog towers over the rest of the assemblage. Latonia, a genus of large discoglossid frogs whose sole living relative survives in a small marshy valley in the Levant, accounts for roughly 61 percent of the identifiable remains and about 60 percent of the estimated individuals. It is present in almost every fissure that produced anurans at all: only seven of the 62 fissures lacked Latonia, and in most cases it was either the most abundant or the only frog identified. The Gargano material is assigned to the Latonia seyfriedi species group on the strength of a suite of diagnostic characters, including pustular sculpturing on the dermal skull bones that became secondarily ossified to the bone itself, a palatine process on the maxilla, a long and slender pterygoid process, a large posterior depression on the maxilla, and a completely fused frontoparietal bearing a divided thickening on its ventral surface.

The phylogenetic analysis reinforced this placement. The team added the Gargano frog as a new operational taxonomic unit to a published morphological matrix and ran a maximum parsimony analysis, replicating two iterations of a previous study. When two Mesozoic discoglossid genera were excluded, the analysis returned a single most parsimonious tree in which the Gargano Latonia forms a polytomy with Latonia seyfriedi and Latonia kolebabi, supporting the attribution to the seyfriedi species group. The consistency index of that tree was 0.82 and the retention index 0.92.

The other three taxa are far scarcer. The green toad, tentatively labelled aff. Latonia gr. seyfriedi in the formal treatment of the bufonid material, represents about 21 percent of individuals and appears in roughly 44 percent of the fissures; the tree frog and the green frog each account for around 5 percent of remains and 8 percent of individuals. Bufonids, hylids and ranids were the dominant or sole anuran in only four, three and one fissures respectively. Notably, the study found no clear evidence of the true toad genus Bufo, contrary to earlier accounts. The toad fossils mix features of different living genera: several bones support referral to Bufotes, but the long, anterolaterally directed occipital canal of one fused frontoparietal and prootic-exoccipital complex recalls Epidalea, and the urostyles show medial contact between the condyloid fossae more typical of that genus.

What makes the assemblage scientifically striking is not its diversity but its oddity. The Gargano Latonia shows significant variation in characters that recent work treats as diagnostic at species rank, even among specimens from the same fissure, and its skull bones carry a degree of exostosis, with dense sculpturing and a strongly developed orbital roof, more in agreement with older, Middle Miocene populations than with Late Miocene ones elsewhere in Europe. The green toads combine features of different genera, and the urostyles of both the toads and the tree frogs bear rare traits such as an unfused dorsal suture line and, occasionally, a small intercondylic process. The authors suggest these peculiarities may reflect evolution in isolation on the palaeoisland, either through diversification into endemic species or through bottlenecks that spread usually rare features through small populations.

The community structure also appears remarkably stable through time. Across the biostratigraphic phases for which fissure attributions are currently possible, Latonia dominates almost constantly, with a possible exception at the beginning of phase 2 where representation is more equal. The authors caution, however, that this picture is preliminary: many fissures cannot yet be assigned to a phase, some subphases rest on a single fissure, and even the best-represented phase counts only 11 fissures. Clear diversity trends over time cannot yet be extrapolated, and apparent patterns such as the disappearance of the tree frog after subphase 2a may be spurious and strongly biased.

The absence of salamanders is itself informative. Living Mediterranean islands are overwhelmingly inhabited by frogs rather than newts and salamanders, a pattern probably linked to the lower overseas dispersal potential, lower mobility and lower tolerance of saline environments in urodeles, and the fossil record mirrors this: urodele remains are known from only a handful of Mediterranean island localities, chiefly the Early Miocene of Lesbos and the Early Miocene to Pleistocene of Sardinia. The Gargano anurans fit the same trend, reinforcing the interpretation that the fissures preserve genuinely insular communities. The researchers also note that earlier, less insularised stages of the amphibian palaeocommunity are missing from the record, since the oldest phase, phase 0, has so far yielded no amphibians at all, so what the fissures capture is a picture of communities already shaped by island life.

Where the frogs came from remains an open question. All four taxa belong to clades that were widely distributed across Europe during the Miocene, and the few other Italian Miocene occurrences are limited to Messinian localities whose remains are too incomplete to establish direct relationships. The strong sculpturing of the Gargano Latonia hints at a Middle Miocene ancestry, which would parallel suggestions made for the island’s giant hedgehog Deinogalerix, but would sit awkwardly with the murid-dominated mammal faunas, since murids only entered western Eurasia in the Late Miocene. One possible resolution is that insular lineages decouple from continental evolutionary trends: in a stable island environment buffered from ongoing climate change, the Latonia population may simply have retained its heavy cranial exostosis while continental relatives reduced it. Previous work on the Terre Rosse herpetofauna has suggested that rafting, rather than land bridges, was the most likely mode of colonisation for the palaeoisland’s amphibians, and the new anuran record adds further evidence that the Gargano palaeoisland served as an incubator for insular evolution, a small isolated world where a Latonia-dominated, highly adaptable frog community persisted across the phases of the Terre Rosse faunal complex.

Subject of Research: Late Miocene anuran fossils from the Gargano Terre Rosse fissure fillings of southern Italy

Article Title: Frogs and toads in the holes: Late Miocene amphibians from the Gargano Terre Rosse fissures (Southern Italy)

Article References: Villa, A., Marchitelli, D., Pavia, M., Rook, L., van den Hoek Ostende, L. W., & Delfino, M. (2026). Frogs and toads in the holes: Late Miocene amphibians from the Gargano Terre Rosse fissures (Southern Italy). Swiss Journal of Palaeontology, 145, 1081-1110. https://doi.org/10.3897/sjp.145.e206406

Image Credits: AI Generated

DOI: 10.3897/sjp.145.e206406

Keywords: Latonia, Gargano, Terre Rosse, Late Miocene, anurans, palaeontology, insular evolution, Mikrotia fauna, discoglossid frogs, Mediterranean islands, fissure fillings, Bufotes