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Khosla, A. and Lucas, S.G., eds., 2016, Cretaceous Period: Biotic Diversity and Biogeography. New Mexico Museum of Natural History and Science Bulletin 71.
LAS HOYAS: A UNIQUE CRETACEOUS ECOSYSTEM
ÁNGELA D. BUSCALIONI AND FRANCISCO JOSÉ POYATO-ARIZA
Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, c/ Darwin 2,
Cantoblanco, 28049-Madrid, Spain, e-mails: angela.delgado@uam.es, francisco.poyato@uam.es
Abstract—The Konservat-Lagerstätte of Las Hoyas, from the Barremian of Cuenca, in central Spain, has
provided a large amount of data whose integration reveals the uniqueness of this locality. Evidence provided by
sedimentology, taphonomy, and paleobiology indicates that the paleoenvironment of Las Hoyas corresponds to
that of a freshwater carbonatic lentic ecosystem without any marine inuence, regulated by a seasonal subtropical
climate, in a lacustrine to palustrine wetland subsystem. Las Hoyas has recorded a wide array of species of diverse
plant and animal taxa. At present the biodiversity count comprises 118 families and 201 species. Its fossil record
has provided the nest temporal adjustment for many groups, and the oldest records of aquatic angiosperms,
nemestrinine nectar-feeding dipterans, gonorynchiform teleostean shes, xenoanuran pipids, tapejarid pterosaurs,
gobiosuchid crocodyliforms, and gobiconodontid eutriconodont mammals. The multiple occurrences of closely
related taxa of plants (Atopochara), insects (Allopteridae) and tetrapods (Tapejaridae) shared by the biotas
from Las Hoyas and Jehol suggest that the Early Cretaceous widespread distribution of these groups was due
to dispersive processes that took place from west to east during the Early Cretaceous. The record of Las Hoyas
is a frozen window into a very specic paleoecosystem that can serve as a reference for comparison with other
Cretaceous assemblages and biotas.
INTRODUCTION
The outcrop of Las Hoyas was discovered and began to be
excavated during the decade of the 1980’s (Sanz et al., 1988). It is
located in the province of Cuenca, in Central Spain, some 200 km
northeast of Madrid. It is considered late Barremian in age. Thirty
years of systematic eld work and integrative research have revealed
the uniqueness of this locality among the Konservat-Lagerstätten of
the Cretaceous Period. The most recent, updated information of the
Las Hoyas research group is synthesized in a collective monograph
(Poyato-Ariza and Buscalioni, in press).
The present paper aims to summarize the most recent data on the
research on this locality by focusing, in the rst place, on its relevance in
the context of the known Cretaceous continental Konservat-Lagertätten.
After that, the environmental context of Iberia during the Cretaceous
will serve to frame the environmental context of the locality, which,
in turn, introduces the factors explaining the process of preservation
of the fossil remains. The fossil record from Las Hoyas will be briey
presented; it has provided not only many relevant, rare specimens in an
excellent state of preservation, but also a unique ensemble that offers
the opportunity to integrate an impressive amount of paleoecological
data that reveal the peculiarities of this Cretaceous wetland.
LATE JURASSIC-CRETACEOUS
KONSERVAT-LAGERSTÄTTEN
An important concentration of continental deposits with fossils
of exceptional preservation was produced worldwide during the
Kimmeridgian-Albian time interval (Retallack, 2011). The concurrence
of so many localities in such a relatively short geological time interval
has been related to biochronological stage boundaries, mass extinctions,
oceanic anoxic events, spikes of high atmospheric carbon dioxide, and
transient warm-wet climates (Retallack, 2011). Nonetheless, it is not yet
well understood what were the precise paleoecological, paleoclimatic,
and geological conditions responsible for favoring the abundance and
exceptional preservation of fossil assemblages.
These Kimmeridgian-Albian exceptional localities are located in a
paleolatitudinal band between 30ºN and 60°N along the Tethys realm.
Their ecosystems are hot-spots of biodiversity distributed within a
subtropical belt with high seasonality and warm temperatures (Skelton,
2005; Hay, 2008). These localities include: the coal mine of Guimarota
(Portugal; Martin and Krebs, 2000); the Purbeck Limestones (England;
Watson and Alvin, 1996; Milner and Batten, 2002); the large wetland
of the Iberian Plate ranging from the Iberian Chain to the Pyrenees,
Los Cameros and Montsec basins in Spain (Martín-Chivelet, 2002); the
western European Wealden localities in England and Germany (Batten,
2011); the Yixian and Jiufotang formations of western Liaoning,
northern Hebei, and adjacent Inner Mongolia, gathering the so called
“Jehol Biota” found also in localities from northwestern China,
southwestern China, Siberia, Korea, and Japan (Pan et al., 2013; Zhou,
2014); other famous Asiatic biotas are Tetori from Japan (Matsukawa
et al., 2006), and the exceptional localities, such as Baissa, in the
Transbaikalia area of Siberia (Zherikhin et al., 1999); the Morrison and
the Cedar Mountain formations (North America) (Turner et al., 2004;
Dunagan and Turner, 2004; Garrison et al., 2007), and the Brazilian
Santana formation (Maisey, 1991). In this context, Las Hoyas represents
another paradigmatic example of such exceptional Late Jurassic-Early
Cretaceous deposits. The extent to which these signicant ecosystems
may have channeled the early Mesozoic biotas and their evolution is
still far from accurately understood. The research on the site of Las
Hoyas, carried out for over 30 years, has continued to contribute to the
resolution of this challenging enigma.
IBERIA IN THE BARREMIAN
The paleogeography and sedimentology of the Iberian Plate
were inuenced by the opening of the North Atlantic and its counter-
clockwise rotation with respect to the rest of Europe during the earliest
FIGURE 1. Paleogeography of the Iberian Peninsula (Iberian Massif
and Ebro Massif) for the Berriasian-Aptian time interval (modied
form Gerdes et al., 2010). Las Hoyas is marked with a circle in the
Iberian Massif.
52
Cretaceous. In this context, the Iberian Plate comprised an oceanic
chain of islands in the Tethys Ocean with a low topography placed at
the limit of the subtropical belt, with a paleolatitude of 25-30ºN (Fig.
1). The climate of Iberia during the Cretaceous was largely subtropical,
semi-arid to humid, inuenced by a maritime climatic regime with
strong NE trade winds (Martín-Chivelet, 2002).
Coincident with the continental break-up at the start of the
Alpine cycle (Triassic-Jurassic), four main basins were formed: Betic,
Pyrenean, Basque-Cantabrian and Iberian. The Iberian Basin originated
in the Triassic-Jurassic period and evolved by generating diverse
subsiding sub-basins during the Early Cretaceous continental rifting
phase. The South Iberian sub-basin is a 300 km long basin located at
the southeast of the Peninsula (Fig. 2). The lower part of the South
Iberian succession is mainly developed as continental and lacustrine
facies. Specically, the Barremian sequences consist of siliciclastic
alluvial deposits (El Collado Formation) and shallow lacustrine
carbonate facies (La Huérguina Formation) (Fig. 3A); this second
lithostratigraphic unit records late Hauterivian-Barremian continental
sedimentation and contains the fossiliferous beds of the locality of Las
Hoyas (Fregenal-Martínez and Meléndez, 2000). The fossil site was
formed in a small basin, the Las Hoyas micro-basin (50 km2), associated
with the rifting phases that induced the uplift of the extended Jurassic
carbonate ramps and the activation of Triassic extensional faults that
generated a set of individual, half graben-like micro-basins (Fig 3B).
The Jurassic limestones, as exposed, underwent pedogenic and karstic
processes, both previously and during continental Early Cretaceous
sedimentation, and the depressed areas were lled by upper Barremian
continental sediments. The Barremian continental deposits of the La
Huérguina Formation unconformably overlie the Middle Jurassic
marine limestones (Fig. 3A).
The Las Hoyas micro-basin was lled with 400 m of lacustrine
sediments arranged in four stratigraphic units separated by local
unconformities (Fregenal-Martínez, 1998; Fregenal-Martínez and
Meléndez, 2000). The fossiliferous beds occur within the second of
these units, called Rambla de las Cruces II, and they consist of nely
laminated limestones composed almost entirely of calcium carbonate,
with only a small fraction of clays and organic matter (Fregenal-
Martínez and Meléndez, 2000). The laminated limestones were
deposited in an extensive, perennial, shallow water carbonate lake
system without marine inuence (Fregenal-Martínez, 1998, Poyato-
Ariza et al., 1998; Buscalioni and Fregenal-Martínez, 2010; Bailleul
et al., 2011). The lake system was drained by carbonatic rich water
fed by groundwater and/or by karstic aquifers, regulated by a seasonal
subtropical climate that controlled water-level oscillations (Fregenal-
Martínez et al., 2014). The fossiliferous beds of Las Hoyas are dated
as latest Barremian (127-125 Ma.) on the basis of their charophyte and
ostracode content (Diéguez et al., 1995; Martín-Closas and Diéguez,
1998).
ENVIRONMENTAL CONDITIONS AND PRESERVATION
The regional environment of the La Huérguina Formation
comprises a complex mosaic of facies, including alluvial plains,
marshy and swampy palustrine plains, several types of channels, and
ponds, plus shallow permanent lakes, that were formed in a subtropical
seasonal regime (Fregenal-Martínez and Buscalioni, 2009). The
warm conditions for the Barremian, as proposed for the Iberian Plate,
consist of high temperatures estimated between 15ºC (60Fº) and 48ºC
(120Fº) with high precipitation rates during the winter season of 16
mm/day (Haywood et al., 2004). The intensity of precipitation was
a consequence of the very warm surface temperatures that favored
oceanic evaporation, and in turn would provoke ash oods and
frequent electric storms and res. However, the resulting models do not
indicate the presence of a pronounced dry season during the Barremian
(Haywood et al., 2004). The environment was a landscape with at and
smooth topography, where the ridges of the Jurassic limestones would
have bordered depressions such as that of the Las Hoyas micro-basin.
In this landscape, the Las Hoyas locality would correspond to a
marginal pool that was integrated into a freshwater complex system.
The water chemistry at Las Hoyas is supported by detailed isotopic
analyses indicating that coelacanths and pycnodont shes (traditionally
considered as marine groups) grew in an entirely freshwater environment
(Poyato-Ariza et al., 1998). All subsequent evidence has been also fully
consistent with this interpretation, as evidenced by the analysis of rare
earth elements in anuran and reptilian bone, including that of dinosaurs
(Bailleul et al., 2011). Furthermore, the Las Hoyas pond was extremely
shallow, as the isolated tetrapod footprint record and desiccation cracks
in particular layers strongly indicate. The pond area was covered by
mats of microbial communities that prompted the production of the
laminated sediments and favored the exceptional preservation and
abundance of fossils (Briggs et al., 1997; Gupta el al., 2008).
Taphonomically, these microbial mats are especially relevant
as a mechanism promoting exquisite preservation and abundance of
fossils because they protect carcasses from progressive degradation
and induce mineral precipitation leading to the formation of lithied
layers (Iniesto et al., 2013, 2015). Fossils unearthed from Las Hoyas are
usually preserved complete and fully articulated, so that the specimens
maintain their body integrity and show ne anatomic details. These
occasionally include remains of soft tissues, such as mineralized
muscle and skin (Briggs et al., 1997; Cuesta et al., 2015; Navalón et al.,
2015; Martin et al., 2015). Such ne preservation even provides cellular
details, such as the structure and composition of hair and skin in the
mammal Spinolestes xenarthrosus (Martin et al., 2015).
Taphonomic analysis comparing the fossil assemblages of the
sedimentary facies indicates that the layers with microbial mats,
which correspond to dry periods, contain abundant remains. However,
the fossil content of the facies that characterize the wetter periods is
limited in number but shows a high diversity (Buscalioni and Fregenal-
Martínez, 2010). In general, the fossil associations of Las Hoyas are
characterized by: 1) instantaneous events such as daily activity (e.g.,
digestion, mass mortality, Pinardo-Moya et al., 1995); 2) series of
ontogenetic stages of aquatic and terrestrial organisms (one of the best
examples is represented by amiiform shes, Martín-Abad, 2015, see also
Poyato-Ariza, 2005); 3) fossils accreted in short-scale events of fossil
accumulation, all elements being of similar size and preservation type,
and 4) the body integrity of fossils, including soft-bodied organism.
This evidence largely suggests that the Las Hoyas fossil assemblages
are autochthonous and were demic (i.e., produced in the same habitat
where the organisms lived).
The fossil associations of Las Hoyas t perfectly into the
limitations of the denition of biota: 1) a particular geographic region,
2) a specic time, and 3) a certain ecological habitat (Chapin et al.,
2002). The singular environmental conditions have recorded a highly
representative biota, both in space and in time, consisting of a very
high diversity of taxa; a mixture of aquatic, amphibian, and terrestrial
plants and animals (abridged in Sanz et al., 1999, 2001b; Ortega et
al., 2003; Poyato-Ariza, 2005; Buscalioni et al., 2008; Buscalioni and
Fregenal-Martínez, 2010; Poyato-Ariza and Buscalioni, in press). The
multidisciplinary evidence provided by sedimentology, taphonomy, and
paleobiology consistently indicates that the paleoecology of Las Hoyas
corresponds to that of a freshwater carbonatic lentic ecosystem without
any marine inuence, regulated by a seasonal subtropical climate, in a
lacustrine to palustrine wetland subsystem.
FOSSIL RECORD
The fossil record from Las Hoyas has provided an important
temporal adjustment for many groups, with the oldest records
of aquatic angiosperms (Gomez et al., 2015), the nectar-feeding
nemestrinine dipterans (Mostovski and Martínez-Delclòs, 2000),
gonorynchiform teleostean shes (together with El Montsec, Poyato-
Ariza, 1996), xenoanuran pipids (Baez, 2013), tapejarid pterosaurs
(Vullo et al., 2012), at the time of its discovery the ornithomimosaurs
(Pelecanimimus polyodon, Pérez-Moreno et al 1994, Mackovicky et
al., 2009) and carcharodontosaurids (Concavenator corcovatus, Ortega
et al., 2010), gobiosuchid crocodyliforms (Ortega et al., 2000), and
gobiconodontid eutriconodont mammals (Martin et al., 2015), among
the best examples.
Las Hoyas has recorded a diverse array of plant and animal taxa.
The biodiversity count currently comprises 118 families and 201 species
(Fig. 4). The plants of Las Hoyas are known from palynomorphs of
Chlorophyta and Streptophyta (Zygnematales and Embryophyta),
and from meso- to macrofossils of aquatic Charales and terrestrial
Embryophyta (de la Fuente and Zetter, in press; Martín-Closas et al.,
in press). Animals constitute 77% of the total diversity at specic
level. Their higher rank diversity consists of Annelida-Nematoda
(Timm et al., in press), Mollusca, Arachnida, Myriapoda, Ostracoda,
Malacostraca, Hexapoda, and Vertebrata. Hexapod arthropods (insects)
form, by far, the most diverse group, as they represent 36% of the total
number of species recorded in the locality. The next most abundant
group comprises the vertebrates (29%), with the actinopterygian shes
as the most diverse of these.
The fossil record from Las Hoyas strongly supports the wetland
environment proposed for this ecosystem. The high production of
53
FIGURE 2. A, Simplied tectonic map of the Iberian Peninsula showing
the present location of the South Iberian sub-basin (modied from
Martín-Chivelet et al., 2000). B, Simplied geographic and geological
map of the Southwestern Iberian sub-basin showing the location of Las
Hoyas (modied from Sopeña and De Vicente, 2004).
periphyton, vegetal diversity, insect diversication, and the dominance
of aquatic organisms linked to a strong environmental seasonality are
characteristics typical of modern wetlands (Van der Valks, 2006). These
conditions are accompanied by a heterogeneous fauna of terrestrial
and aquatic organisms, suggesting a high degree of mosaicism in
the ecosystem, with tightly arranged interconnected microhabitats.
Furthermore, many organisms have a variety of morphological
adaptations and inferred life habits.
Plants
Representatives of almost all plant groups thriving in Europe
are found at Las Hoyas. Over 17 groups with about 70 species are
represented: Chlorophyta (4), Charophyta (4), Marchantiophytina
(1), Lycophyta (1), Monilophyta (14), Cycadalaes-Bennettitales (1),
Coniferophyta (10), Gnetales (1), and Magnoliophyta (6) (Diéguez et
al., 2010; Martín-Closas et al., in press). Most of the oral richness is
formed by conifers and ferns (Fig. 5). The spatial distribution of the
ora is not clear because most of the fossils are fragmented and were
transported as small to medium-sized remains. The ora can be divided
into several structural types of vegetation according to hydro-edaphic
(soil saturation) conditions and growth forms: 1) freshwater ponds, 2)
ooded marshes and swamp forest, and 3) savannah-like conditions.
1) The hydrophytic vegetation consists of a wide variety of
charophytes: Atopochara trivolvis triquetra, Clavator harrisii,
Clavatoraxis robustus, C. diaz-romerali, and Palaeonitella
vermicularis, the rst two being particularly abundant. The aquatic
owering plant Montsechia vidalii is a very characteristic and abundant
plant widespread in the shallow, alkaline and oligotrophic temporary
lake environments (Gomez et al., 2015).
2) Marshes and swamps were occupied by Frenelopsis ugnaensis,
a representative of the extinct conifer family Cheirolepidiaceae.
Frenelopsis ugnaensis was probably a shrub that formed largely
monospecic belts of vegetation in temporary ooded areas (Gomez et
al., 2002; Barral et al., 2016). It also had a number of leaf adaptations
to cope with water loss due to high alkalinity and/or water-table
uctuations caused by wet/dry seasonality. Because of these adaptions,
a xeromorphic nature has been suggested for the ora of Las Hoyas
(Diéguez and Meléndez, 2000).
3) The savannah-like vegetation formed the most widespread
terrestrial landscape. The matoniacean tree fern Weichselia reticulata
dominated these open landscapes along with other ferns, many of
them herbaceous, such as members of Cyatheales (e.g., Onychiopsis
psilotoides), Osmundales (e.g., Cladophlebis spp.), and Schizaeales
(e.g., Ruffordia goepperti) (Martín-Closas et al., in press).
Invertebrates
Aquatic invertebrates played an important role in the Las
Hoyas biota. The presence of periphyton in the ponds and small
lakes would have provided high food availability and dissolved
oxygen concentrations. Major groups of aquatic invertebrates include
crustaceans, molluscs, and an important number of insect families (Fig.
6). Among decapod crustaceans, only one species each of craysh
(Austropotamobius llopisi), freshwater prawn (Delclosia martinelli),
and spelaeogriphacean (Spinogriphus ibericus) have been found
(Garassino, 1999; Jauma et al., 2013). Nonetheless, other crustaceans
such as ostracods are fairly diverse, especially at the regional scale of
the La Huérguina wetland. Ostracods are known from three families,
Cyprideidae, Cyprididae, and Candonidae (Rodriguez-Lárazo, in
press). Molluscs are dominated by gastropods belonging to the
Viviparidae and the Pulmonata Planorboidea, and to a lesser extent
Unionoidea (Fig. 6) and Trigonioidoidea bivalves. The malacofauna is
largely conspecic with those from other European localities (Munt et
al., 2012; Delvene and Munt, in press). The buttery position (death
position) of the fossil bivalves supports the inference that they lived
close to the sediment surface; unionoids have been interpreted as very
shallow to semi-infaunal organisms (Delvene and Munt, in press).
Gastropods are represented by small and medium-sized species, their
association indicating freshwater conditions. The aquatic insects include
two dominant groups: the heteropteran families Belostomatidae and
Corixidae, and the carnivorous aquatic beetle family Coptoclavidae, all
three extraordinarily diversied in this locality. Fully aquatic insects
include, in addition, pupae of dipterans (Chaoboridae, Chironomidae),
larvae of Ephemeroptera (Euthyplociidae, Leptophebiidae), odonatans,
and other coleopterans such as Dytiscidae, and Hydrophilidae.
Semiaquatic insects such as chresmodids and the gyrinid beetles are
comparatively less abundant (Delclòs and Soriano, in press).
The recorded entomofauna is taxonomically diverse, and it
comprises 14 orders, with 40 families, and 38 species. Among all the
orders, the aquatic and terrestrial coleopterans are considered to have
been one of the most diverse worldwide in Early Cretaceous localities,
composed of several different families from the three major suborders
(Archostemata, Adephaga and Polyphaga) (Soriano et al., 2007a).
The terrestrial families of coleopterans encompass very different
feeding strategies: xylophagous (Cupedidae, Buprestidae, Elateridae);
saproxylic (Parandrexidae); herbivorous (Byrrhidae); carnivorous
(Trachypachidae); possible malacophagans (Trachypachidae),
and carrion feeders (Staphyllinidae) (Soriano et al., 2007a). Other
terrestrial insects include termites, cockroaches, crickets, dipterans, and
neuropterans. About 73% of the terrestrial insects have herbivorous-
frugivorous trophic habits, playing a fundamental role in the process of
transforming the vegetal matter into detritus.
Other terrestrial arthropods have also been reported, including
Araneae of the sub-horizontal orb web builders Tetragnathidae (Selden
and Penney, 2003) and a fairly abundant association of Myriapoda
(Fig. 6) assessed to the superfamily Xyloiuloidea (Selden and Shear,
in press).
Fishes
The shes from Las Hoyas include rare Chondrichthyes and
Coelacanthiformes and abundant Actinopterygii, which constitute
the vast majority of the ichthyofauna in terms of both taxonomy and
ecology (see Poyato-Ariza and Buscalioni, in press, for further details).
Las Hoyas is the youngest Cretaceous locality with a Jurassic-like
relict ichthyofauna as discussed by Poyato-Ariza and Martín-Abad
(present volume). Long-term relatively stable conditions prevented
relict ichthyofaunas from suffering the “mid-Cretaceous” turnover that
started in the Aptian. In this sense, the sh fauna from Las Hoyas, very
much like those of the Jurassic and the earliest Early Cretaceous, was
dominated by non-teleostean actinopterygians in secondary consumer
niches (e.g., Pycnodontiformes, Semionotiformes, Macrosemiiformes,
Amiiformes), whereas teleosteans (including the basal ostariophysans
Gonorynchiformes) were limited to primary consumer niches regardless
of their taxonomic diversity. Fishes played a key role in the ecosystem,
being trophically related with aquatic, amphibious, and terrestrial
organisms, both as prey and as predators.
Albanerpetontidae and Batrachia
Amphibians from Las Hoyas consist of a diversied array of
Albanerpetontidae, Salientia, and Urodela (Fig. 7). Albanerpetontids
and salamanders are relatively abundant in the locality. The bizarre
albanerpetontids are represented by the species Celtedens ibericus.
The holotype preserves scaly skin and eyelids, suggesting a wholly
terrestrial habit (McGowan and Evans, 1995). Celtedens is a basal
Western Europe albanerpetontid spanning the interval from the
54
Kimmeridgian to the Early Cretaceous, sister taxon of Wesserpeton and
Albanerpeton (McGowan, 2002; Sweetman and Gardner, 2013).
The anurans of this locality have furnished relevant evidence
to shed light on the morphological evolution of various lineages of
“archaeobatrachian” (non-neobatrachian) frogs. Two of the species are
endemic to Las Hoyas: the xenoanuran pipoid Gracilibatrachus avallei
and the costatan Iberobatrachus angelae, whereas the third taxon,
?Eodiscoglossus sp., has a more basal phylogenetic position (Baez,
2013). Gracilibatrachus avallei (Fig. 7) forms part of the earliest
known radiation of Laurasiatic pipoids (Baez, 2013; Cannatella, 2015).
It possesses some notable skeletal features, such as a relatively short
rostrum, short and compact vertebral column, and long hind limbs,
pes and hands. These characters are considered to be early steps in the
evolution of the fully aquatic way of life of modern pipids.
Salamanders (Urodela) are the most common tetrapods in the
locality. They are represented by two species, Valdotriton gracilis,
and cf. Hylaeobatrachus (Evans and Milner, 1996; Evans, in press).
Valdotriton is a fully metamorphosed species with a lifestyle similar
to that of living pond salamanders. A recent phylogenetic analysis
places it on the stem of salamandroids close to their divergence
with cryptobranchoids (Gao and Shubin, 2012; Anderson, 2012).
Hylaeobatrachus is perennibranchiate, therefore a fully aquatic
organism. The study of the phylogenetic position of this taxon is
complicated by the fact that perennibranchiate salamanders show
paedomorphic features (Evans, in press; Wiens et al., 2005).
Reptiles
Reptiles are represented by turtles, lizards, crocodiles, dinosaurs,
FIGURE 3. A, Chrono-lithostratigraphic chart of the South Iberian Sub-basin during the Late Jurassic-Early Cretaceous, showing the depositional
environments, depositional sequences and lithostratigraphic units (modied from Martín-Chivelet et al., 2000). B, Simplied geological map of
Las Hoyas according to Fregenal-Martínez and Meléndez (2000).
FIGURE 4. Chart of the biodiversity record in Las Hoyas (upper
Barremian, La Huérguina Formation, Cuenca) showing the percentage
of families of each group among the total number of families recorded.
55
FIGURE 5. Plants from Las Hoyas (upper Barremian, La Huérguina Formation, Cuenca, Spain). A, Charophyta: Clavatoraxis robustus MCCMLH
16112, paratype; scale bar 20 mm. B, Clavatoraxis robustus MCCMLH 6111, holotype; scale bar 5 mm. C, Dicksoniaceae fern Sphenopteris
MCCMLH 17262; scale bar, 5 mm. D, Osmundaceae fern Cladophlebis MCCMLH 26061; scale bar 5 mm. E, Cheirolepidiaceae conifer
Frenelopsis MCCMLH 31338a; scale bar, 5 mm. F, Angiosperm Montsechia vidalii, MCCMLH 26463b; scale bar 10 mm. G, Angiosperm
Iterophylum lobatum, MCCMLH 26740b; scale bar 5 mm. H, Detail of G showing healed bite mark on the left border; scale bar 2 mm. Photos J.
A. Gracia.
56
FIGURE 6. Invertebrates from Las Hoyas, (upper Barremian, La Huérguina Formation, Cuenca, Spain). A, Decapod family Atyidae: Declosia
martinelli, MCCMLH219, holotype (modied from Garassino, in press); scale bar 5 mm. B, Bivalve family Unionoidea: Unio cf. turgidulus
(modied from Delvene and Munt, in press) MCCMLH 28321; scale bar 10 mm. C, Insect family Allopteridae: Allopterus mayorgay MCCMLH
18571, paratype; scale bar 5 mm. D-E, Insect family Aeschnidiidae: Angloaeschnidium montevili MCCMLH-036R holotype, part and counterpart;
scale bar 5 mm. F-G, Insect family Aeschnidiidae: Sinaeschnidia martinezdelclosi LHMCCM-173362, holotype, part and counterpart; scale bars
5 mm. H, Insect family Scoliidae, Archaeoscolia hispanica, holotype, LH-13827 scale bar 5 mm. C and H modied from Delclós and Soriano, in
press. D-G Photos J.A. Gracia.
and pterosaurs (Fig. 7). The paleoherpetofauna is characterized by the
relative abundance of crocodiles and by the high diversication of
lizards. A single species of eucryptodiran chelonia was attributed to
the endemic genus and species Hoyasemys jimenezi (Pérez-García et
al., 2012). The squamate herpetofauna is composed of the widespread
Jurassic-Cretaceous family Paramacellodidae and the unspecialized
Iberian-UK genus Meyasaurus diazromerali, plus a set of endemic taxa:
Hoyalacerta sanzi, Scandensia ciervensis and Jucaraseps longipes
(Bolet and Evans, 2011, 2012; Bolet, 2014; Evans and Bolet, in press).
The phylogenetic position of the non-paramacellodid squamates is
labile, but Meyasaurus and Scandensia might have afnities with
Anguimorpha and Iguania (Conrad, 2008; Bolet, 2014).
Crocodylomorphs are relatively abundant and morphologically
diverse. Such diversity corresponds to a phylogenetic broad spectrum
that evolutionarily includes very basal crocodyliforms such as
Gobiosuchidae and different groups of mesoeucrocodylians, including
the atoposaurid Montsecosuchus, plus new taxa of Eusuchia on the stem-
group of modern Crocodylia (Buscalioni and Chamero, in press).The
57
FIGURE 7. Vertebrates from Las Hoyas (upper Barremian, La Huérguina Formation, Cuenca, Spain). A, Ginglymodi sh, new genus, juvenile
specimen MCCMLH 15119b; scale bar 5 mm. B, Ginglymodi sh, new genus, specimen MCCMLH 11171 showing soft tissue preservation
in the orbital and abdominal regions; scale bar 10 mm. C, Xenoanura pipimorph Gracilibatrachus avallei, MCCMLH 21171b, holotype; scale
bar 20 mm. D, Skull in ventral view of the eusuchian Crocodyliform, MCCMLH 13370; scale bar 20 mm. E, Skull in lateral aspect of the
Ornithomimosauria Pelecaniminus polyodon MCCMLH 7777, holotype; scale bar 20 mm. F, Pectoral region and forelimbs of the Enantiornithes
Eoalulavis hoyasi, MCCMLH 13500, holotype; scale bars 20 mm. Photos A-B, F. Maher; C-F, J. A. Gracia.
unnamed basal crocodyliform is a fully terrestrial species (Buscalioni
et al., 1996; Buscalioni and Chamero, 2009), but Montsecosuchus
is a singular example of specialized aquatic adaptations. A powerful
tail, a robust axial skeleton with three sacral vertebrae, and a broad
thoracic cage are particular traits that suggest diving capabilities in
Montsecosuchus, in analogy to Recent marine iguanas.
Dinosaurs are medium-sized organisms that have been preserved
articulated and showing integumentary impressions. A basal iguanodont
was attributed to Mantellisaurus athereldensis on the basis of the
pes bones (Llandres et al., 2013). The non-avian theropods are the
multi-toothed ornithomimid Pelecanimimus polyodon (Fig. 7) and
the bizarre carcharodontosaur Concavenator corcovatus, the former
known from the anterior part a body, and the latter from an almost
complete individual (Pérez-Moreno et al., 1994; Ortega et al., 2010;
Cuesta et al., 2015). Pelecanimimus, placed phylogenetically within
Ornithomimosauria (Choiniere et al., 2012), forms part of a Valanginan
to Barremian European fauna (Allain et al., 2014). It is characterized by
supernumerary dental series (Pérez-Moreno et al., 1994), rendering the
interpretation of its trophic biology rather conicting (Barrett, 2005).
Pterosaurs are represented by fragmentary fossils: isolated teeth
58
attributed to the families Istiodactylidae and Ornithocheiridae, and
cranial remains representing the rst unambiguous record of a tapejarid
outside Brazil and China (Europejara olcadesorum) (Vullo et al., 2009;
Vullo et al., 2012).
Aves
The exceptional preservation of the Las Hoyas birds has provided
important evidence on the evolutionary transformations of the skeleton
and the ight musculature during the evolutionary history of Aves (e.g.,
Sanz et al., 2002; Navalón et al., 2015). The avifauna is characterized
by several enantiornithine taxa (Fig. 7), all endemic: Iberomesornis
romerali, Concornis lacustris, and Eoalulavis hoyasi. As with other
Early Cretaceous enantiornithines, these species are relatively small
in size in comparison with coeval ornithurines and other basal birds
(Zhou, 2004). The interrelationships within the clade Enantiornithes
remain largely unsolved. Some authors argue that Iberomesornis is
more basal than both Concornis and Eoalulavis (O’Connor et al., 2009).
Iberomesornis has been placed outside Euenantiornithes (Chiappe,
2002), whereas the other two genera have been clustered as sister taxa
(Chiappe and Walker 2002) or as relatives of Asiatic Euenantiornithes
such as Holbotia and Qiliana (O’Connor et al, 2009; Zelenkov and
Averianov, 2015). Further work is needed to describe several skeletons
included in the rst record of a bone accumulation interpreted as a
fossil pellet (Sanz et al., 2001a). A number of isolated feathers of a
relative small average size indicate that these enantiornithines would
have had different color patterns, such as stripes or patches (Marugán-
Lobón and Vullo, 2011).
Mammals
This group is represented at Las Hoyas by a single but complete,
articulated individual of a eutriconodont mammal, with extraordinary
preservation of skin and pelage (Martin et al., 2015). Phylogenetically,
Spinolestes xenarthrosus is placed within the Gobiconodontidae as the
sister taxon of the clade formed by Gobiconodon plus Repenomamus.
This small mammal shows a mosaic of functional features, suggesting
a terrestrial locomotion with ambulatory gait and potential digging
abilities.
DISCUSSION: LAS HOYAS IN THE CRETACEOUS
From a paleobiogeographic perspective, the biota of Las Hoyas
is characterized by its peculiar combination of taxa. As described
above, during the Barremian the Iberian Plate formed a set of small
oceanic islands situated in the Western Tethys, in the European
Archipelago, halfway between North America and Africa. Such an
arrangement of islands in proximity to large continental masses favored
the conguration of a complex biota whose oristic and faunistic
composition is characterized by: 1) the presence of ancient late
Jurassic relicts, 2) the linkage between eastern and western Laurasian
landmasses and northern Gondwana, 3) the emergence of evolutionary
endemism, and 4) the occurrence of new clades greatly relevant to our
understanding of the evolutionary history of modern lineages. These
features are discussed in the context of Early Cretaceous cosmopolitism
and the Cretaceous Terrestrial Revolution, with the Jehol Biota as a
clear reference for comparison.
Early Cretaceous Cosmopolitism
It is worth considering that the taxonomic assemblages of the
Chinese Jehol Biota (as dened by Pan et al., 2013) contain a mixed
oral and faunal composition that is, in many aspects, taxonomically
equivalent to that of Las Hoyas. Jehol, as an ensemble, has been
considered paleobiogeographically as a “refugium, cradle or both”
(Zhou et al., 2003; Zhou, 2014), suggesting that ancient Jurassic oras
and faunas, which apparently became isolated in Asia, would have
boosted the evolutionary processes of terrestrial endemic speciation
during the Early Cretaceous (Fig. 8, refugium hypothesis). However,
the same authors also indicate an exceptionally high number of
cosmopolitan taxa in the Jehol Biota (Fig. 8, dispersal hypothesis).
It must be mentioned that Las Hoyas and the Jehol Barremian biotas
share closely related taxa. In this sense, the study of the Las Hoyas
biota is quite relevant in order to understand the paleobiogeographic
complexity of Early Cretaceous oras and faunas, no matter how
distant they can be.
It is commonly accepted that latest Jurassic and Early Cretaceous
terrestrial assemblages appear to show both temporal and geographic
similarity worldwide (Buffetaut and Suteethorn, 1998; Manabe et
al., 2000; Zhou et al., 2003; Barrett and Wang, 2007; Barrett et al.,
2011). The information provided by the biota of Las Hoyas strengthens
this hypothesis of Early Cretaceous cosmopolitism by showing the
relationships between Asiatic and Gondwanan assemblages for an
important number of continental plant and animal taxa during the
Barremian. The freshwater charophyte Atopochara trivolvis triquetra
occurrs in a large numbers in Chinese basins and in a number of
European localities, including Las Hoyas (Martín-Closas and Wang,
2008). The ora of these freshwater environments with charophytes
were completed with very basal angiosperms; their earliest freshwater
record corresponds to Archaefructus from the Barremian Yixian
Formation and Montsechia (Fig. 5) from Iberia (El Montsec and Las
Hoyas; Coiffard, et al., 2007; Gomez et al., 2015). A similar situation
also occurs with the Chinese eudicot Leefructus mirus and Iterophylum
lobatum (Fig. 5) (Barral et al., 2013). Even though the phylogenetic
relationships of these early angiosperms are still unsolved, the
parallelism is quite clear.
A second set of examples of Early Cretaceous cosmopolitism
can be outlined for insects. The terrestrial Chrysopoidea (including
the extinct families Allopteridae, Liassochrysidae, Mesochrysopidae,
Tachinymphidae and Limaiidae, Nel et al., 2005) provide a typical
case-study. The genus Allopterus (Fig. 6) is shared by the Valanginian-
Barremian of Iberia (El Montsec and Las Hoyas) and the Early
Cretaceous Chinese Laiyang entomofauna (Nel et al., 2005; Makarkin
et al., 2012). Different species of Tachinymphes are described from
the Late Jurassic of Transbaikalia (Rusia) and the Barremian of Jehol
(China) and Las Hoyas (Nel et al., 2005; Makarkin et al., 2012).
Furthermore, the evolution of the insect family Aeschinidiidae (Fig. 8)
comprises sister species of the genus Sinaeschnidia (Fig. 6) shared by
the Early Cretaceous of Yixian and Las Hoyas (Fleck and Nel, 2003),
as well as close relationships between the Chinese Linaeschnidium
sinensis and the two Western European genera Angloaeschnidium
(Fig. 6) and Lleidoaeschnidium. (Huang et al., 2008).The beetle genus
Coptoclavella provides yet another example of the occurrence of
closely related species in Mongolia (Bon Tsagan, Barremian-Aptian)
and Iberia (El Montsec, Barremian) (Soriano et al., 2007b).
Tetrapods constitute a third, ample set of cosmopolitism evidence.
Examples include eodiscoglossid frogs, paramacellodid lizards,
iguanodontian ornithopods, dromaeosaurid and carcharodontosaurid
theropods, enantiornithine birds, and ornithocheirid pterosaurs that
are well-known to have a worldwide distribution during the Early
Cretaceous (Zhou et al. 2003; Barrett et al., 2002). For instance,
scincomorph “paramacellodids” (i.e., Paramacellodus, Becklesius,
Pseudosaurillus, Saurillus, Sharovisaurus, Mimobecklesius,
Atokasaurus, and Tepexisaurus) are distributed from the Kimmeridgian
to the Barremian of China, Japan, Russia, Iberia, North America, and
Morocco (Evans and Searle, 2002; Apesteguía, 2007; Bolet and Evans,
2010). Pterosaurs provide further examples of close relatives in China
and Iberia, as Las Hoyas shares two pterodactyloids with the Barremian
to early Aptian freshwater deposits of Liaoning: istiodactylid and
ornithocheirid (Vullo et al., 2009). Europejara from the late Barremian
of Las Hoyas and Sinopterus from the Jiufotang Formation are closely
related (Vullo et al., 2012). The gobiconodontid mammals have a
broad distribution from Asia to North America during the Early
Cretaceous (Barremian to Albian), so that the discovery of Spinolestes
xenarthrosus (Martin et al., 2015), together with two gobiconodontid
teeth from the Early Cretaceous of Spain (Cuenca-Bescós and Canudo,
2003) and Britain (Sweetman, 2006), has extended the distribution
of this family to Western Europe. This is consistent with the broad
interchanges proposed for other groups between Europe and Asia
during the Early Cretaceous (e.g., Baryonychinae, Ornithomimosauria,
basal Titanosauriformes, Carcharodontosauria, Allain et al., 2014).
The occurrences of closely related taxa shared by the biotas of Las
Hoyas and Jehol suggest that the Peritethyan widespread distribution of
so many taxa in the Early Cretaceous was due to dispersive processes
that took place from west to east (Fig. 8). Most of the widespread
Laurasia taxa would have been distributed along a peritethyan
paratropical belt represented by a large ensemble of nonmarine Early
Cretaceous basins with a similar climatic regime, characterized by a
winter-wet paleoclimate (Saward 1992; Martín-Closas and Wang, 2008;
Willis and McElwain, 2002). These paleoclimatic and paleoecological
circumstances would explain the rapid expansion of the charophyte
Atopochara lineage that had colonized Eurasia by the Valanginian-
Hauterivian, the northern Gondwanan margins from Western Europe
during the Barremian, and North America during the Aptian (Martín-
Closas and Wang, 2008). Furthermore, the closest relatives of many of
the latest Jurassic-Early Cretaceous groups from the Barremian-Aptian
59
of eastern China are suggested to be found in Western Europe (Wang
et al., 2005) (Fig. 8). Thus, as in the Atopochara-dispersive model
(Martín-Closas and Wang, 2008), other continental groups would have
colonized the eastern freshwater lentic environments. Interestingly,
the colonization of northern Gondwanan areas by the Atopochara
lineage coincided with that of other groups, including insects and
tetrapods. The Las Hoyas insects of Chrysopoidea, Aeschnidiidae, and
dipteran Tipulidae (Nel et al., 2005; Ribeiro and Lukashevich, 2014)
are phylogenetically related to Brazilian Aptian taxa; and because
the earliest member of the Tapejaridae is Europejara, the view that
tapejarids may well have expanded rst throughout Eurasia and later into
Gondwana (Aptian of Brazil) also reinforces the Atopochara-dispersive
model. Curiously enough, this process did not affect ichthyofaunas,
which were consistently and signicantly different in Europe and Asia
since the Jurassic (compare, for instance, Poyato-Ariza et al., 1999 with
Chang and Miao, 2004).
FIGURE 8. A, Area cladograms showing the relationships between Iberia and Jehol during the Barremian, summarizing the refugium and dispersal
hypotheses. The refugium hypothesis would imply the differentiation of independent lineages in Asia and Europe during the Early Cretaceous. The
dispersal hypothesis would imply closely related taxa occurred in Asia and Western Europe during the Early Cretaceous. B, Cladogram showing
the relationships of aeschnidiid insects based on Fleck and Nel (2003). The geographic distribution is included for each terminal taxon; areas are
indicated by the following numbers: 1, Western Europe; 2, North America; 3, Russia; 4, China; 5, Brazil; 6, Japan; 7, Gondwana. Internal nodes
have been optimized using the Fitch method to show the geographic placement of the common ancestors. Note that the stems of the Chinese
species are Western European in origin. C, Map showing the arc of islands along the Peritethys during the Early Cretaceous (modied from Blakey,
2006); phases 1 and 2 correspond to sequences of dispersive processes that involved different lineages.
60
This evidence of dispersive processes supports the hypothesis
of migration of Laurasian taxa towards Gondwana during the Early
Cretaceous (Hauterivian-Aptian) versus the “Eurogondwanan
hypothesis” (Ezcurra and Agnolin, 2011). The connection of areas
located along the northern and southern edges of the Tethys, with
similar paleogeographic and paleoclimatic conditions (i.e., islands and
peninsulas occasionally linked to other land masses, large carbonate
platforms, low topography, and a winter-wet climate in a paratropical
belt; Willis and McElwain, 2002; Fanti, 2012) appears to be supported
by a broad spectrum of groups comprising plants, insects and
tetrapods. The study of other Early Cretaceous oras and faunas along
the Peritethys regions (e.g., Israel and Croatia) are still necessary to
complete a global biogeographic picture. If this is supported, then the
Early Cretaceous continental assemblages should be regarded as an
interesting new by-product of geo-dispersal (Lieberman, 2005).
Endemism and Terrestrial Revolution
About 25% of the species from the fossil record of Las Hoyas
are endemic. Obviously, the recognition of endemic forms must be
taken with caution as the specimens found in exceptional deposits are
sometimes difcult to compare with fossils from other localities found
disarticulated.The endemic community of Las Hoyas can be regarded
as composed of species with a short biostratigraphic time interval, and
by those with special morphological adaptations. The signicance of
the aquatic and terrestrial adaptions found in the amphibian and reptile
groups from Las Hoyas has already been remarked upon. In addition
to this, endemism is a strong indication of geographically isolated
conditions (i.e., it reects the insularity of Iberia during the Early
Cretaceous).
Much of this endemism relates to monogeneric and monospecic
taxa (Stenamara mia, Lepidhoyas microrhys, Hoyasotes tanyrhis,
Gordichthys conquensis, Jucaraseps longipes, Gracilibatrachus
avallei and Concavenator corcovatus among others). Some of these
monospecic taxa are phylogenetically placed on the stem of clades
that subsequently diversied during the Late Cretaceous. Such stem
taxa have tremendous phylogenetic implications in the evolution of
modern lineages, as angiosperms, modern insects, modern teleosts,
lizards, crocodiles, and birds. Thus, the origin of new lineages during
the Early Cretaceous is located not only in Asia as suggested (see Zhou
et al., 2003; Benton et al., 2013) but also in Iberia.
In sum, the refuge hypothesis for the Jehol Biota does not explain
the two common geographical features of both Barremian sites:
Cretaceous cosmopolitism plus the origin of evolutionarily important
endemism. The refuge hypothesis predicts the differentiation of faunas
and oras in Iberia and China (Fig.8) from relict Jurassic taxa in each
area, as shown. This has been falsied by the presence of closely
related species. Conversely, a dispersive process along arcs of islands
(island model) would explain both dispersion in concert with speciation
(Ronquist and Sanmartín, 2011).
CONCLUSIONS
The three major attributes of the Las Hoyas fossil assemblage are:
1) its quality of preservation; 2) its oral and faunistic compositional
delity; and 3) its evolutionary relevance. The assemblage of Las Hoyas
encompasses a large number of articulated individuals, about 70%,
preserved in exquisite morphological detail. It is a record of a distinct,
momentary geologic window suggesting a highly representative
ecosystem. Specically, vertebrates are emblematic exponents of the
Cretaceous Terrestrial Revolution (Lloyd et al., 2008) that largely
started to recongure the ecosystems of our planet towards their modern
organization. Relevant macroevolutionary phenomena included also
the critical actinopterygian faunal turnover in the aquatic ecosystems of
the Early to Late Cretaceous that may arguably represent an equivalent
“Cretaceous Aquatic Revolution” towards the modern organization.
The upper Barremian Spanish locality of Las Hoyas has yielded a
set of unique vertebrate taxa that have provided signicant data on
the evolutionary biology (i.e., origin, phylogenetic relationships, and
biogeographic distribution) of many major clades. Interestingly, the
diversity of the Las Hoyas vertebrate fauna is characterized by the
presence of endemic species with singular adaptations representing
all the vertebrate groups that dwelt in this particular ecosystem,
characterized as a seasonal freshwater subtropical wetland. Our most
recent research has aimed at reconstructing the ecological interactions
in this paleoecosystem, focusing on its trophic organization. The
data collected during 30 years of excavation are the basis for current
research into the network that connects plant and animal diversity
(Chlorophyta, Zynematales and Charales, Marchantiophytina,
Licopodophytina, Spermatophytina, Mollusca, Arachnida, Myriapoda,
Ostracoda, Malacostraca, Hexapoda, and Vertebrata) in the form of a
trophic network of very complex relationships that we are just starting
to explore.
The fossil record of Las Hoyas shows that this locality is as
an exceptional Konservat-Lagerstätte that actually preserves the
completeness of a whole Early Cretaceous wetland ecosystem.
The Jehol Biota has been considered unique due to the absence of
comparable Lagerstätten outside Asia (Luo, 1999). Nonetheless, there
are no doubts that Las Hoyas represents a single ecosystem in time and
in space, its fossiliferous beds packed in tight sedimentary continuity
within a very restricted area. As a consequence, the Spanish locality
has made, and continues to make, a clear and signicant impact on
our knowledge of the origin, biogeography and evolution of Early
Cretaceous biotas and ecosystems. Moreover, the Chinese Jehol
Biota sensu Pan et al. (2013) is a mixed ensemble of oras and faunas
from distinct, non-coeval ecosystems, whereas Las Hoyas provides a
complete single paleoecological reference for comparison with other
Cretaceous assemblages and biotas, and, in this sense, is quite unique
among Cretaceous Lagerstätten.
ACKNOWLEDGMENTS
This synthesis forms part of the research contributions carried
out by a large team of specialists in sedimentology, taphonomy, and
paleobiology, consolidated by Funds provided by the MINECO
reference CGL2013-43642 P, and the support of the Junta de
Comunidades de Castilla-La Mancha. We are in debt with Susan Evans,
and Romain Vullo for their valuable comments.
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