Kras, in a strict tectonic sense, can be defined as the Trieste–Komen plateau (synclinorium) or the Komen thrust sheet, but, in a wider geotectonic sense, this area of southwestern Slovenia belongs to the Outer Dinarides.

Platy and laminated limestones with cherts that occur as a special facies unit within different Late Cretaceous platform formations are characteristic for Kras 〚5〛, 〚6〛, 〚16〛, 〚17〛 and 〚26〛. The major part of these bituminous (carbon-rich) beds belongs to the Komen limestone, whereas a minor part of the platy and laminated limestones, termed the Tomaj limestone, occurs within the thick-bedded Santonian–Campanian Lipica formation.

The strata of the Tomaj limestone were deposited in a lagoon environment and, contrary to the Komen limestone, they show no trace of intertidal conditions. A good connection of the sedimentary environment with the open sea is indicated by abundant pelagic micro- and megafossils 〚14〛, 〚15〛, 〚27〛, 〚28〛, 〚29〛 and 〚30〛. Mass mortality of fishes 〚7〛 and saccocomids is observed at certain levels and is connected with the mixing of the otherwise well-stratified waters of the lagoon. During quiet periods, above the seafloor where anoxic conditions prevailed, the water column allowed the existence of nektonic and planktonic organisms 〚7〛. The Tomaj limestone also contains a rich megaflora 〚10〛 derived from a landmass that began to rise south of the ‘Tomaj Lagoon’ during the Late Santonian.

The fossil feather was collected in the Tomaj limestone at Križ, southwest of Tomaj (Fig. 1). Based on the abundant foraminifers Murgella lata (Luperto Sinni) in the underlying strata, and Calveziconus lecalvezae Caus & Cornella in overlying beds, it is placed in the Upper Santonian–Lower Campanian.

The feather from Križ (Fig. 2) is 28.5 mm long and its maximum width is 7 mm. It is preserved as a dark film showing little relief on a slab of Tomaj limestone, and is apparently an instance of preservation as a carbonised trace, the most common type of feather fossilisation 〚9〛. Carbonisation is a common type of fossilisation in the Tomaj limestone. The specimen shows relatively few details, but there is no doubt that it is a feather (interpretation as a plant fossil was ruled out by Inna Dobruskina during her study of the fossil flora from the Tomaj limestone). The calamus is not preserved, and there is no indication of a plumaceous region at the base of the vane. The rachis is well visible, and barbs, most of them undisturbed, can easily be seen, but no barbules can be distinguished; however, the fact that the barbs have mostly kept their original position suggests that they were connected by barbules and hooklets. The barbs are attached to the rachis at an angle of about 20° to 30°. The vane, which seems to have largely kept its original outline, is asymmetrical, the rachis being somewhat offset. There is no noticeable clip at the distal tip, which is rounded rather than angled.

Feathers were long considered as characteristic of birds, but recent discoveries in the Early Cretaceous Yixian Formation of northeastern China have revealed the existence of feathers or feather-like integuments in various non-avian theropod dinosaurs, as discussed in detail in Wellnhofer’s recent review 〚31〛. Although the exact status of some of the Chinese specimens is still disputed (Caudipteryx, which has advanced feathers, is now generally regarded as an oviraptorosaur, but according to a recent analysis this whole group should be regarded as secondarily flightless birds 〚20〛), there is no doubt that some non-avian dinosaurs possessed ‘modern’ feathers 〚25〛. Therefore, the fact that the Slovenian feather on the whole closely resembles a modern bird feather is not sufficient to definitely refer it to a bird. Its occurrence in a marine environment, where it may have been dropped by a flying or swimming bird, may suggest that an avian origin is more likely than a non-avian one, but a clear-cut identification is obviously difficult.

It is difficult to determine the exact position of an isolated feather on the body of an unknown bird-like animal. Despite its asymmetry, the feather from the Tomaj limestone, which is not clipped distally, does not seem to a primary or secondary flight feather; it is somewhat reminiscent of coverlets from the wing of Columba figured by Griffiths 〚12〛, but its exact position must remain uncertain.

The occurrence of this feather in a marine deposit is noteworthy, as the majority of fossil feathers are from lacustrine settings 〚11〛. However, as noted above, the Tomaj limestone was deposited in a lagoon setting, and fossil feathers are known to occur in such restricted marine environments, the Archaeopteryx feathers (whether isolated or associated with skeletons) from the Late Jurassic Plattenkalk of Franconia 〚12〛 and the isolated feathers from the Early Cretaceous Crato Formation of Brazil (references in Martill and Davis 〚19〛) being well-known examples. The anoxic conditions at the bottom of the Tomaj lagoon must have facilitated the preservation of the feather as a carbonised trace.

The feather from Križ is an addition to the short list of known Late Cretaceous feathers. While a relatively large number of feather localities of Early Cretaceous age have been recorded, very few are known from the Late Cretaceous (see the recent list of Cretaceous feathers in Martill and Davis 〚19〛). The Santonian/Campanian feather from the Tomaj limestone can now be added to the meagre record from the Santonian Niobrara Chalk of Kansas 〚32〛, from Santonian amber in southern Alberta (Foremost Formation) and Japan (Taneishi Formation), and from Turonian amber (Raritan Formation) in New Jersey 〚13〛 and 〚19〛.

If it does belong to a bird, the feather from Slovenia also complements the scanty record of Late Cretaceous birds from Europe, which so far consisted only of skeletal remains. Various enigmatic fragmentary specimens from the Maastrichtian of Transylvania have been ascribed to birds, but they may in fact belong to small theropods (see the review by Csiki and Grigorescu 〚8〛), although the status of some of them is still problematical. Volgavis marina, a possible limnofregatid, is known from jaw elements from the Uppermost Maastrichtian of the Don River Basin in European Russia 〚21〛 and 〚23〛. A few specimens from the Campanian of southern Sweden, including Parascaniornis 〚18〛 have been referred to archaic diving birds of the order Hesperornithiformes 〚22〛 and 〚24〛. Remains of enantiornithine birds 〚1〛 and 〚4〛 and of the giant flightless bird Gargantuavis philoinos 〚2〛 and 〚3〛 are known from several Late Campanian or Early Maastrichtian localities in southern France. To this rather short list may now be added the feather from Slovenia, if it is indeed from a bird. A single feather is of course not a sufficient basis for a discussion of palaeobiogeographical affinities, but it is hoped that in the future more information will be forthcoming about the birds that lived on the Dinaro-Adriatic carbonate platform in the Late Cretaceous. This platform, located in the Tethys Sea, was at least temporarily and locally emergent, but its biogeographical links with other European landmasses are still poorly understood.