The material consists of in situ specimens that expand over an exposed area of approximately 42 m² and hand samples preserved as compressions–impressions.

Photography and examination of specimens were made with an Olympus Zeiss stereomicroscope and measures with a Leica MZ 16 A. In addition, the material was examined with a Quanta 200 scanning electron microscope at 25 kV.

Hand samples were photographed with a Canon Eos 300D digital camera under low-angle incident light.

The studied area is placed in the Iberian Range (eastern central Spain, Fig. 1).

This study is focused on the Escucha Formation [1] deposited in the extensional Oliete sub-basin during Late Aptian–Albian (Fig. 2A and B). This sub-basin, which measures 1500 km², was active from the Early Barremian to the Early–Middle Albian [27] and [46]. In this area, after the development of carbonate Urgonian platforms during the Aptian, a strong change in sedimentary conditions resulted in high siliciclastic input, and sedimentation took place under continental and coastal conditions.

Plant remains have been recorded from an outcrop of the Escucha Formation (Fig. 3) in the southern sector of the Oliete sub-basin. It is a heterolithic unit dominated by very well sorted fine sand and clay, with coal-bearing deposits in its lower part. Pardo [31] has dated this unit in the Oliete sub-basin as Late Aptian. The Escucha Formation lies upon limestones and marls of the Urgonian Platform (Aptian), and is covered by sandstones of the Utrillas Formation (Fig. 2A) [31] and [38]. The Escucha Formation has been traditionally ascribed to deltaic-estuarine environments [39]. However, recent studies have demonstrated that transgressive sedimentary environments (e.g., barrier-island systems) developed in the Oliete sub-basin at least during the lower part of the Escucha Formation's deposition [42]. The Escucha Formation has been divided into three different lithologic sub-units, from base to top: E1 (heterolithic with coal), E2 (mainly sandstones), and E3 (claystones) (Fig. 2B). At the upper part of the sub-unit E2, a grey-silty lithosome has been recognized, corresponding to a low-energy sedimentary environment related to a sandy shoal. Plant remains have been recovered from silty and muddy grey sediments deposited in the shoreline of a restricted bay-lagoon [41].

The Escucha formation was deposited in extensional sub-basins bounded by palaeogeographic highs produced by listric and block-rotation faults [31] and [38]. Both location of the major faults and depocentre distribution for this unit have been established from isopach maps and stratigraphical correlations, leading to the conclusion that block rotation by listric faults played an important role [39]. Outcrop-scale syn-rift features demonstrate that syn-sedimentary extensional tectonics acted at a great variety of scales during the deposition of the Escucha Formation [44].

During the deposition of the Escucha Formation, the studied area was located at a palaeolatitude of 25°N to 30°N [26] and [48], i.e. at the boundary of the Northern Hot Arid belt (NHA) and of the Northern Mid-latitude Warm humid belt (NMW) [7] and [47]. The Escucha Formation deposition occurred during a significant climate change in the Iberian Basin [43]. This climate change towards the mid-Cretaceous to more arid and hot conditions is marked by the transition from humid intervals with coal-bearing deposits and coarser siliciclastic deposits at the base of the Escucha Formation to an arid environment [43]. This palaeoclimate change has been related to a process in which the connection of the North and South Atlantic initiated an equatorial humid climate belt that forced arid belts towards higher latitudes [7] and [43].

The colonization surface developed during an arid stage in the shoreline of a restricted by-lagoon, in which silty and muddy deposits developed (grey-silty lithosome in Fig. 2 and Fig. 3) [41]. Deposits display three different sedimentary facies: Facies A is formed by lenticular stratification. Grey to black mudstones display sandy lenses of white fine-grained sandstones. Lenticular stratification can be developed in a great variety of environments, in which fluctuations of energy occur [17] and [40]. The occurrence of non-rhythmically laminated mud resembling tidal deposits described by [6] and the occurrence of tidal rhythmites as those described by [16] point to a tidal influence during sedimentation that favoured alternating traction (sand) and suspension (mud) deposition.

Facies B is formed by dark-grey to black massive mudstones, from which bioturbation and fossils are completely absent. Massive mudstones were deposited in quiet water under stagnant conditions. The occurrence of undisturbed lamination suggests that bottom conditions were periodically anoxic and/or with an absence of biota [21] and [30].

Facies C is composed of sand-streaked to laminated mudstones, with laminae maximally a few millimetres thick; sandy laminae become more frequent upwards; animal fossils and bioturbation are completely absent. The sandy streaks and lamination of Facies C probably represent material transported into the muddy area during high-energy events (e.g., storm weather: [30]).

The occurrence of liverworts colonization mat in the upper part of Facies A (Fig. 4) suggests that periods of temporal subaerial exposure took place in a shoreline of a relatively shallow water body. The complete absence of both aquatic faunal remains and bioturbation was probably due to high rates of sedimentation, bottom anoxia and variable salinity.

The liverwort-rich horizon (Fig. 3) is comprised of specimens that have a flattened, typically prostrate dorsal-ventral orientation and spreading rosette-like thalli, which are almost identical to marchantialean gametophytes, except the absence of midrib.

Specimens (Fig. 5) present ecostate, ribbon-shaped thalli, with straight margins up to 4.5 mm in length and 1.5 mm in width, dichotomously branched. Branches are about 2 mm long and 1.5 mm wide and rounded at the tip.

Scattered at irregular distances over the dorsal epidermis of the thalli, rounded outline structures were observed, which correspond to pits of about 0.2 mm (Fig. 6) in outer diameter. These structures have been interpreted as the air pores that open into the characteristic air chambers of the Marchantiales. Each air chamber contains columns of photosynthetic cells and facilitates gas exchange and water relations [12]. The air chambers’ walls protect the assimilatory tissue for undue evaporation. Unlike the stomata of vascular plants, which close in dry weather, the air pores of liverworts have a limited ability to regulate the opening size, since a certain capacity of movement was observed [50].

The most noteworthy aspect is the presence of distally rounded or cup-shaped structures with fringed margins about 0.7 to 0.9 mm of diameter in the outer part and up to 0.4 mm in its inner part, with entire margins (Fig. 7). These represent scyphulae in which gemmae have developed. These structures are splash cups that are the best-known method for vegetative dispersal, since they can disperse their carried contents up to 60 cm [3].

Gemmae cups of the studied material showed some differences in shape and size. There existed three typologies: •

rounded about 0.9 mm in diameter, with a central pore of 0.5 mm in diameter;

Within living Marchantiopsida, only Marchantia and Lunularia produce gemmae cups. Morphological differences between gemmae cups of Marchantia (bowl-shaped cups) and Lunularia (crescent-shaped cups) have led us to conclude that the fossil material is related to the first extant genera.

Included in the sediment amongst the specimens, dispersed lenticular gemmae of 20 μm in diameter occurred (Fig. 9).

Lenticular gemmae are produced and dispersed by wind or raindrops from gemmae cups to form new individuals, which allow the plant to reproduce asexually in places where sexual reproduction is unsuccessful and to colonize an area very rapidly, because they are generally are produced earlier and faster than spores.

Asexual reproduction in extant liverworts is more common than sexual reproduction. In many taxa, the maintenance of populations depends, almost exclusively, on asexual reproduction, since it is an adaptative strategy in severe conditions [10].

The specimens spread over an area of unusual size and under similar environmental conditions, in which no other plants occurred.

The bryophyte community was formed by solitary plants, plants interlocked by their tips and plants interwoven (Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10 and Fig. 11). They were physiologically independent of and genetically identical to their mother plants, since they proceeded from gemmae. They were thus stands, even though individuals are not interconnected [20] and [22]. The loose arrangement of specimens indicates an early stage of colonization. The majority of the living Marchantiaceae shows a life form consisting of mats according to Magdefrau's [25] classification. In this life form, the growth of plants gives rise to colonies in which their thalli and branches lie mainly in one plane, whether interlocked or parallel and extending horizontally over the substratum [11].

Palaeogeographical [7] and [47] and sedimentary data [43] indicate that the studied colonization mats were formed under hot and dry environmental conditions. Morphological characteristics of the specimens and type of reproduction, along with the traits of life form, corroborate the fact that mats makers underwent severe conditions. The xerophytic characteristics shown are succulent thallus, presence of air pores and air chambers, and exclusive asexual reproduction that implies a reduction in the amount of water required, concomitant with the partial reduction of sporophytes. In addition, it has been demonstrated [4] that the production of gemmae is an adaptative strategy in severe conditions.

No fossil record of liverwort-rich horizons exists in the palaeogeographic low-latitude zones located in arid climatic belts (Fig. 12). However, liverwort mats were described [5] in sediments from the Late Albian of Alexander Island, Antarctica, a zone included in the Southern High-Latitude Temperate humid belt.

Comparison between the marchantiacean mats here described and those from Alexander Island reveals great differences, as much in the individuals as in the structure of population.

The first difference refers to systematic assignation. Due to their morphological traits, to the presence of air chambers and gemmae cups, the here-studied mat-producers are assignable to family Marchantiaceae without any doubt. In the case of liverwort communities from Alexander Island, the mats were formed by Thallites bicostatus, Thallites sp. and Marchantites rosulatus. The form genus Thallites was proposed [51] for those specimens that showed characters that agree equally with algae and hepatics, namely incertae sedis. About Marchantites, it is a form genus established for specimens with obscure characters included in Marchantiales. In relation to the population, this is not described, but the author interpreted these liverwort-rich horizons as colonization mats because of the occurrence of individual plants that rarely overlap, showing clearly their outline and little evidence of competition. This arrangement of specimens cannot be considered as a mat, in accordance with the Magdefrau's classification of life forms.

The authors also summarize diverse liverwort-rich beds from the Middle Devonian of Podolia [18], from the Triassic of Australia [49] and New Zealand [33], from the Rhaetian–Liassic of Skromberga (province of Scania) [24], from the Berrisian–Valangian of South Africa [2], and from the Albian of southeastern Australia [9]. After comparison, the liverwort-rich horizon from South Africa seems to be close to marchanticean mats from the Escucha Formation, except as regards the palaeoclimate.

The morphological characteristics of the majority of the specimens, such as their growth form and life form, large number, area occupied, and the evident stage of asexual reproduction, combined with the occurrence of gemmae and the absence of other plant remains, allow us to recognize this liverwort-rich horizon as an in situ clonal colonization mat, formed by the growth of marchantiacean plants.

The arid palaeoclimates pointed out by means of available sedimentological data are in accordance with the results of the study of palaeobotanical data.

The marchantiacean community studied represents a xerotermophylous community, well adapted to dry and hot conditions, in which the loose arrangement of individuals indicates an early stage of colonization.