Content uploaded by Francisco Ortega
All content in this area was uploaded by Francisco Ortega
Content may be subject to copyright.
Science Progress (2002), 85 (2), 113–130
The birds from Las Hoyas
JOSÉ LUIS SANZ AND FRANCISCO ORTEGA
Information on the ﬁrst steps of the avian evolutionary history has dramati-
cally increased during the last few years. The fossil record provides a general
view of the morphological changes of the avian ﬂight apparatus from non-
volant ancestors (non-avian theropod dinosaurs) to the ﬁrst derived ﬂiers
of the Early Cretaceous. The Las Hoyas bird record includes three genera:
Iberomesornis, Concornis and Eoalulavis. This fossil material has yielded
information about the early avian evolutionary history. These Early
Cretaceous birds (some 120 Myr old) had a wingbeat cycle and breathing
devices similar to those of extant birds. The function of the rectricial fan
was also similar. In the evolutionary transition from cursorial ancestors to
derived ﬂiers it is possible to verify a trend to increase lift. Primitive wing
aspect ratio morphotypes were elliptical ones, other derived morphotypes
appeared, for example, in the Neornithes (extant birds). Some primitive
ﬂiers, like the Las Hoyas genus Eoalulavis, had an alula (feathers attached
to the ﬁrst digit of the hand) similar to that of present day birds, indicating
braking and manoeuvring skills similar to those of their extant relatives.
Primitive avian life habits are poorly understood. Some evidence from the
Las Hoyas bird record indicates that Early Cretaceous birds were present
in the trophic chains.
Keywords: avian evolutionary history
José Luis Sanz is Professor of Paleontology at the Unidad de
Paleontologia. Departamento de Biología, Universidad Autónoma
de Madrid 28049, Cantoblanco, Madrid, Spain. His research ﬁelds
are systematics and evolutionary history of several dinosaur
lineages, namely ornithopods, sauropods and birds.
Francisco Ortega collaborates with the Unidad de Paleontología of
the Universidad Autónoma de Madrid (Spain) in several Las Hoyas
research projects. His research ﬁelds are systematics of archosaurs
(crocodilians, dinosaurs and birds, between others).
SP/Ortega 9/7/02 10:06 am Page 113
Some 120 million years ago (Early Cretaceous) a shallow lake existed
in the Serranía de Cuenca (East–Central Spain). The sediments of this
paleolake, known as Las Hoyas, consist of rhythmically ﬁne-grained
laminated limestones preserving such delicate structures as dinosaur
muscle tissues, gut contents or feathers
(Figure 1). The Las Hoyas
limestones have yielded a diverse ﬂora and fauna emerging from
both aquatic and terrestrial biota. The ﬂoral assemblage includes
charophytes, bryophytes, ﬁlicales, bennettitales, gnetales, coniferales
and angiosperms. Animal remains belong to bivalves, gastropods,
crustaceans, insects, myriapods, bony ﬁshes, albanerpetontids,
anurans, urodelans, turtles, lizards, crocodiles, dinosaurs and birds
The current bird assemblage from Las Hoyas includes three species:
Iberomesornis romerali, Concornis lacustris and Eoalulavis hoyasi.
Our knowledge about the avian evolutionary history has dramati-
cally improved during the last two decades. This increase of infor-
mation about the ﬁrst steps of bird evolution comes from two sources:
our better knowledge of the Mesozoic fossil record, and the phylo-
genetic systematics (cladistics) approach. Cladistics has allowed
more robust and feasible genealogical hypotheses which are the nec-
essary ﬁrst step for understanding the evolutionary history of every
lineage. Twenty years ago a gap in the record existed between the
oldest known bird (Archaeopteryx, Upper Jurassic) and the Upper
Cretaceous toothed birds (Hesperornithiformes and Ichthyornithi-
formes). During the last few years signiﬁcant discoveries have been
made in the Early Cretaceous of Spain (Las Hoyas) and China
(Yixian Formation, Liaoning Province) This new evidences along
with other data from Argentina and Madagascar, have radically
114 José Luis Sanz and Francisco Ortega
Fig. 1. Fluorescence-induced ultraviolet photograph of the
ornithomimosaur theropod dinosaur Pelecanimimus polyodon from the
Las Hoyas lithographic limestones. Arrow indicates a soft tissue crest at
the posterior side of skull.
SP/Ortega 9/7/02 10:06 am Page 114
changed our ideas about early avian history with respect to important
topics as historical diversity, origin and development of ﬂight,
feather origin, or life habits of the primitive birds. Nearly half of the
present day Mesozoic genera have been described after 1990
we know that upper Mesozoic avian diversity was much larger than
previously supposed, and the present day birds (Neornithes) are a
particular lineage coming from a Cretaceous bird lineage. On the other
hand, modern avian ﬂight appears during the Early Cretaceous, and
feathers are an evolutionary novelty that appeared in the bird ances-
tors, the non-avian theropod dinosaurs. Early Cretaceous birds were
constant components of the terrestrial ecosystems all around the world.
This paper deals with the information yielded by the Las Hoyas birds
related to some of these aforementioned research topics, especially
concerning the development of the modern ﬂight and life habits of the
The Las Hoyas bird Record
The ﬁrst avian species found at Las Hoyas was Iberomesornis romerali,
a sparrow-sized small animal (Figure 2) whose mass has been esti-
between 15 and 20 g. The specimen is almost complete, lack-
ing the skull and the anterior cervical vertebrae. Iberomesornis is
characterised by a singular combination of traits, including some
primitive ones, present in the non avian theropods, like a sacrum
composed of ﬁve vertebrae, tarsal not fused to the tibia or to the
metatarsals, and lacking any evidence of metatarsal fusion. Along with
these symplesiomorﬁc features, Iberomesornis shares with modern
www.scilet.com The birds from Las Hoyas 115
Fig. 2. Iberomesornis romerali.
SP/Ortega 9/7/02 10:06 am Page 115
birds a number of evolutionary novelties: a derived, modern avian
furcula, with a low interclavicular angle, strut-like coracoids, and a
series of free caudal vertebrae (eight in number) and a large pygo-
style composed by the fusion of 10–15 vertebrae
(Figure 3). This
combination of primitive and derived traits led Sanz et al., 1988 to
propose a phylogenetic hypothesis for this Las Hoyas bird, placing it
in an intermediate position between Archaeopteryx and modern
birds. The introduction within the avian ingroup of the Early
Cretaceous Chinese genus Confuciusornis, along with the reinter-
pretation of some characters, has recently modiﬁed this early phylo-
genetic hypothesis for Iberomesornis. The present proposal is that this
primitive Spanish bird is more closely related to the enantiornithine
birds than previously supposed
(see phylogenetic hypothesis of
The Enantiornithes are an extinct group of Cretaceous birds
described, for the ﬁrst time, by Walker in 1981, in the North of
Argentina. We now know that this avian lineage have a world-wide
distribution, whose remains have been discovered in both Americas,
China, Mongolia, Australia and Spain. The Spanish enantiornithine
record comes from Las Hoyas and the Montsec
(Province of Lleida).
Two enantiornithine genera have been described in Las Hoyas:
Concornis and Eoalulavis. The ﬁrst one (type species C. lacustris) is
about twice the size of Iberomesornis, and it is recognised by an almost
complete skeleton lacking the skull and neck with some feathers evi-
. One of the most singular features of Concornis is the sternal
morphology (Figure 5). The sternum has a carina developed just in its
caudal half, with two crests diverging from its anterior region. The
116 José Luis Sanz and Francisco Ortega
Fig. 3. Iberomesornis romerali. Fluorescence induced ultraviolet photo.
SP/Ortega 9/7/02 10:06 am Page 116
sternal caudal zone is deeply notched, in a conﬁguration similar to
that of the Chinese genus Cathayornis
. The furcular morphology is
derived with an interclavicular angle of some 60°. The hand is
slightly shorter than the forearm and the femur clearly shorter than
the tibiotarsus. The fossil remains in which the species Eoalulavis
hoyasi is based consist of the anterior part of an articulated skeleton
lacking the skull and anterior cervical vertebrae (Figure 6). This beauti-
fully preserved specimen includes evidence of wing feathers in position
(both primary and secondary ones) and the alula (feathers attached to
the ﬁrst digit of the hand). Some body feathers are visible around the
humeri and pectoral girdle. Again, the sternal morphology is charac-
teristic, but very different to that of Concornis. The sternum of
Eoalulavis is lanceolated, with a foot-like caudal expansion, possessing
www.scilet.com The birds from Las Hoyas 117
Fig. 4. Cladogram showing a hypothesis of the avian phylogenetic
SP/Ortega 9/7/02 10:06 am Page 117
a faint carina. The anterior sternal third has a narrow cleft, in which
the extremely long hypocleidium (posterior process of the furcula)
. Both Concornis and Eoalulavis present a series of
evolutionary novelties, mainly in the coracoid, humeri and metatarsi,
typical of enantiornithine birds. The internal relationships within this
clade is poorly understood, but most authors agree on its mono-
The avian ﬂight
The evolutionary success of birds is probably related to its skills
for ﬂying. From the ancestral groups (cursorial non-avian theropod
118 José Luis Sanz and Francisco Ortega
Fig. 5. The enantiornithine bird from Las Hoyas Concornis lacustris.
SP/Ortega 9/7/02 10:06 am Page 118
dinosaurs), descendants with ﬂying capabilities appeared. The known
avian fossil record yields general information on the historical morpho-
logical changes produced in this process, from non-volant to volant
forms. Most of the evolutionary novelties of this sequence of morpho-
logical changes are related to the avian ﬂight apparatus, composed of
bones, muscles and feathers. The bird fossil record offers information
on bones and feathers, but the phylogenetic functional morphology
presents information about muscles and associated ligaments. Thus,
from the most primitive bird (Archaeopteryx) to modern extant birds
(Neornithes) a series of evolutionary novelties related to the ﬂight
apparatus improved the ﬂight skills of this group of volant theropod
The morphological change sequence of the ﬂight apparatus has to
be analysed in terms of physical forces. Flight requires a combination
of lift and thrust. In the avian ﬂight the wings, by means of a set of
upstroke and downstroke movements, provide both forces. Weight
generated by the pull of gravity has to be balanced by the lift pro-
vided by the wings. But lift is not the only aerodynamic effect of the
wings: a drag force is also generated, that has to be balanced by the
propulsion forces (thrust). Besides the wings, other elements of the
avian ﬂight apparatus are signiﬁcant elements in the volant locomotion.
For example, the appearance of a rectricial fan is very important in
www.scilet.com The birds from Las Hoyas 119
Fig. 6. Eoalulavis hoyasi. Top, ﬂuorescence induced ultraviolet photo of
the specimen before preparation. Bottom, reconstruction. A: alula; PR:
primary remiges; SR: Secondary remiges.
SP/Ortega 9/7/02 10:06 am Page 119
order to permit steering and braking. On the other hand, a complex
combination of evolutionary novelties is characteristic of the avian
respiratory system (lungs and aerial sacs) which are relevant in the
conquest of the aerial environment. Thus, after obtaining some infor-
mation on the ﬂight skills of Archaeopteryx we will see that the
information provided by the Las Hoyas birds is related to three topics:
wing and tail biomechanics, wing evolutionary history, and function
of the respiratory system.
Wing and tail biomechanics
The skeleton and feather systems of Archaeopteryx are very similar
to those of the most derived non-avian theropods. The retention of a
primitive cursorial locomotor module composed by the decoupling
of tail and hindlimbs
, is combined with the presence of some avian
synapomorphies like the relative elongation of the forelimb or the
angle between the scapula and coracoid. However, the glenoid cav-
ity (pectoral girdle humeral articulation region) is oriented in a way
similar to that of non-avian theropods, implying a smaller efficiency
in the wingbeat cycle. The present consensus is that Archaeopteryx
was not merely a glider, but could probably perform some kind of
powered ﬂight, although not as efficiently as extant birds.
Archaeopteryx was probably capable of taking off from the ground,
because of the added thrust provided by its hind limbs
its capabilities of performing a low speed ﬂight and its manoeuvr-
ability skills were probably limited.
From the forerunner condition present in Archaeopteryx, a series
of evolutionary novelties appeared in the avian lineage, which
implies more efficiency in the volant locomotion. This synapo-
morphies related with the improved ﬂying skills are present in birds
120 million years old, as the Chinese genus Confuciusornis or the
Las Hoyas genera Iberomesornis, Concornis and Eoalulavis.
Thus, we will see that the functional interpretation of the three
skeletal zones (pectoral girdle, furcula and pygostyle) is modiﬁed
regarding the conditions present in a primitive ﬂyer as Archaeopteryx.
The coracoid of Archaeopteryx is short, and subquadrangular in
shape. The derived coracoidal morphology (strut-like) is typical of
modern birds, but is also present in the primitive birds from Las
. This type of derived coracoid indicates, as in Neornithes,
the existence of a complex biomechanical system (triosseum foramen)
formed by the scapula, furcula and coracoids. This structure pro-
vides the mechanism for the ligament of the supracoracoideus
muscle, working as a pulley-like system during the upstroke phase of
120 José Luis Sanz and Francisco Ortega
SP/Ortega 9/7/02 10:06 am Page 120
the wingbeat cycle. Recent analysis has concluded that this system
not only abduces the wing, but also produces a horizontal rotation of
the humerus, to position the wings for the downstroke
. The presence
of a triosseum foramen in Las Hoyas birds suggest that in these genera,
there is a wingbeat cycle mechanism similar to that of extant birds.
The furcular morphology of Archaeopteryx is primitive, with inter-
clavicular angle near 90°. This primitive morphology is retained by
the basal pygostylian birds, Confuciusornis and Changchengornis
). Iberomesornis presents a derived avian fur-
cula, with a low interclavicular angle and a developed hypocleidium.
This type of modern furcula is very important in ﬂight performance,
because of the large amount of oxygen needed during the powered
ﬂight. The main function of the furcula is contribute to the ventila-
tion of the lungs and air sacs during the active ﬂight
. Thus, it is
very probable that some primitive birds, like the Las Hoyas genera,
had a similar breathing mechanism to that of their extant relatives.
The clade Pygostylia (see Figure 4) is characterised by an evolu-
tionary novelty that dramatically changes the primitive tail morphol-
ogy present in Archaeopteryx. Basal pygostylians, like the
Confuciusornithidae, or more derived, like Iberomesornis, have fused
caudal vertebrae into a single element (pygostyle). In Archaeopteryx,
the relative long feathered tail could have helped to increase the lift,
but the aerodynamic efficiency seems to be low. The presence of a
pygostyle implies the existence of a Parson’s nose, with rectrice
feathers along with a rectricial bulb (that in extant birds facilitates
the opening and closing of the fan of caudal feathers). This structure
is relevant during the ﬂight since allows a greater manoeuvrability,
improves the steering and braking capabilities, and increases the
In summary, the ﬂying skills of some Early Cretaceous birds, such
as Iberomesornis, would be improved with respect to those of
Archaeopteryx. Iberomesornis had better wingbeat cycles and
breathing mechanisms, as well as caudal fan functions. Neverthe-
less, this Las Hoyas bird cannot be considered as a volant form with
ﬂying capabilities equivalent to those of its extant relatives. In pre-
sent day birds, locomotion is characterized by the relationships
between three locomotor modules: pectoral, hind limbs and tail
There is a decoupling between the hind limb and tail modules (the
tail is no longer involved in cursorial locomotion) and there is a new
allegiance between the pectoral and tail ones for performing ﬂight.
In Iberomesornis, the presence of derived wing proportions, cora-
coid and furcula, correlated with a large pygostyle, suggest that the
allegiance between the pectoral and tail modules began to be formed,
www.scilet.com The birds from Las Hoyas 121
SP/Ortega 9/7/02 10:06 am Page 121
whereas the decoupling between hind limb and tail modules was not
complete. In conclusion, Iberomesornis ﬂight skills have to be con-
sidered as intermediate between the primitive volant condition rep-
resented by Archaeopteryx and the sophisticated ﬂight capabilities of
the extant birds.
122 José Luis Sanz and Francisco Ortega
Fig. 7. Life reconstruction of a couple of ﬂying Iberomesornis.
SP/Ortega 9/7/02 10:06 am Page 122
Wing evolutionary history
During the avian ﬂight locomotion, lift depends on several factors:
wing size, air speed, air density, and the angle of attack (angle of the
wing with respect to the trajectory)
. In the evolutionary transition
from non-avian dinosaurs to birds an improvement of the ﬂight skills
is expected. In fact, the morphological transformations of the skele-
tal ﬂight apparatus allow us to propose a sequence composed of three
(1) non-avian dinosaurs showing the cursorial condition;
(2) primitive ﬂiers, such as Archaeopteryx, making up an intermedi-
ate stage; and
(3) derived ﬂiers, such as some primitive birds like Confuciusornis,
and basal ornithothoracines like Iberomesornis.
In the evolutionary transition from cursorial ancestors to derived
ﬂiers, one of the locomotory physical requirements is for lift to be
increased. The increase of lift in primitive ﬂiers can be evaluated in
terms of the wing loading. The wing loading (WL) is the estimation
of the lift of a wing by means of the ratio W/S, in which W= weight
and S= wing area, the latter is the surface projected by the fully
extended wings and the portion of body situated between them
Thus, it is expected that the decrease of the wing loading from mem-
bers of Stage 1, i.e. feathered dinosaurs such as Protarchaeopteryx
or Caudipteryx, to Archaeopteryx, and from the latter to the mem-
bers of Stage 3.
In order to check this hypothesis we need to know or estimate both
the weight and the wing area of taxa involved in the origin and
development of avian ﬂight.
Weight estimation is based on the equation: FL = 0.5659 ⫻
, resulting from a regression analysis of femur length (FL) on
weight in 65 species of extant birds representing 13 different
. In the same way, wing area is estimated using the equation:
S = 0.052 + 0.008 ⫻ W, for a regression analysis of weight on wing
area from a 120 extant birds based on ref. 25.
The results support the formulated hypothesis: wing loading
decrease from probable non-ﬂying forms, like Caudipteryx and
, to primitive ﬂiers, like Archaeopteryx and
from this to the ﬁrst derived ﬂiers (Confuciusornis and basal
ornithothoracine birds). However, an unexpected increase of the
wing loading began with the clade Ornithurae. If the hypothesis we
propose is correct, then large wing loading values in extant birds are
www.scilet.com The birds from Las Hoyas 123
SP/Ortega 9/7/02 10:06 am Page 123
a reversal of the derived character states of the primitive condition,
probably related to different types of ﬂight performance. For exam-
ple, cruising speed depends on wing loading. The greater the value
of this ratio, the faster the bird is able to ﬂy
Another factor determining the ﬂight performance is the shape of
the wing or aspect ratio. This is deﬁned as the ratio between length
(span) and width (chord) of the wing. Based on the aspect ratio, there
are ﬁve basic types of wings
. The elliptical morphotype is
characterised by a slenderness, that is, span dimension is small
relative to chord. This morphotype is characteristic of most Passeri-
formes and doves, which have high manoeuvrability. Long soaring
(gliding) wings are morphotypes with a high aspect ratio, and are pre-
sent in albatrosses and gulls, that is, high-speed soarers
. Short soar-
ing wings are morphotypes with intermediate aspect ratios between
long soaring and elliptical morphotypes. They are characteristic of
vultures, storks, and eagles, that is, low-speed soarers. Swallows and
martins have high-speed wings, which are characteristically bowed
and acuminated. Finally, there is the hovering morphotype, which is
typical of hummingbirds. Hovering ﬂight is characterized by the
absence of a thrust component, only a lift component. This type of
ﬂight performance requires a large energy input.
The fossil evidence of wing aspect ratio is scarce. Only excep-
tionally well-preserved specimens, i.e. the Berlin Archaeopteryx,
show its wing aspect ratio. In most fossils, wing aspect ratio is not
preserved and a reconstruction exercise is necessary. In these condi-
tions, it is difficult to propose even a general evolutionary pattern of
wing aspect ratio. Nevertheless, some conclusions can be made. The
elliptical wing morphotype seems to be primitive within Aves. It
shares several conditions such as being present in Archaeopteryx and
probably in basal ornithothoracine birds, and being the least special-
ized and most frequent type of wing. This primitive wing aspect ratio
could be the condition widespread among Enantiornithes. Derived
aspect ratio morphotypes (long and short soaring, high-speed, hov-
ering morphotypes) appeared, at least, in the Neornithes, and proba-
bly within more inclusive clades. As in the case of wing loading,
aspect ratio diversity can be related to different bird life habits asso-
ciated with different ﬂight requirements.
One of the most important features in the evolutionary history of
avian ﬂight is a trend towards reducing drag. When a bird is trying to
reduce speed, it extends the alula generating an additional air ﬂux
that reduces the eddying that occurs at the posterodorsal zone of the
wing, reducing or even eliminating problems of lift maintenance.
The most primitive known fossils showing alula evidence are
124 José Luis Sanz and Francisco Ortega
SP/Ortega 9/7/02 10:06 am Page 124
the Early Cretaceous enantiornithine birds Eoalulavis
. The alula feathers of Eoalulavis have a derived
asymmetric vane, showing no structural differences with the alula
feathers of extant birds. However, it is probable that in Eoalulavis
alula feathers were fewer in number than in extant birds. No alulas
are known outside the clade ornithothoraces. Both, Archaeopteryx
and the well-known Confuciusornis, lack the alula
. Thus, the must
likely hypothesis is that the alula appeared as an evolutionary
novelty in ornithothoraces.
In order to support the large levels of muscular work involved in
flight, birds need a highly efficient respiratory system. The performance
www.scilet.com The birds from Las Hoyas 125
Fig. 8. Pellet, probably from a theropod dinosaur, containing fossil
remains of, at least, three different species of primitive birds.
SP/Ortega 9/7/02 10:06 am Page 125
of ﬂight at high altitude probably was a problem in the evolutionary
history of birds, as there seems to be a decrease of oxygen pressure
with height. The respiratory system of the extant bird is made of
lungs and air sacs
that permit efficient ventilation even in lower
oxygen pressure environments. It seems very probable that aerial
sacs were present in the avian ancestral groups. It is also probable
that some primitive birds like Iberomesornis, with a derived ﬂying
apparatus, had improved the functional levels of their air sacs. But
these primitive birds lacked the pneumatic foramina, present in
extant birds in several bones (vertebrae, humerus, sternum, etc). If
this absence indicates a lesser efficiency of the respiratory system,
then it is possible to formulate a hypothesis about the existence in
bird evolutionary history of a trend to produce forms with increasing
skills to perform ﬂight at lower atmospheric oxygen pressure. If the
oxygen pressure levels of the Mesozoic atmosphere were similar to
those of the present day, birds progressively acquired a better cap-
ability to ﬂy at elevated altitude. Thus, it seems probable that primi-
tive ﬂiers such as Archaeopteryx were able to reach a low ceiling-
height, and a long evolutionary trend occurred until the appearance
of present day high-altitude soarers.
126 José Luis Sanz and Francisco Ortega
Fig. 9. SEM photograph of one of the tiny avian long bones of the pellet
illustrated in Fig. 8.
SP/Ortega 9/7/02 10:06 am Page 126
Primitive bird life habits
The presence of birds (both in terms of population density and diver-
sity) in the terrestrial Early Cretaceous ecosystems is clearly more
important than previously supposed. Nevertheless, the life habits of
these primitive birds are poorly known. The avian fossil record of
Las Hoyas has provided some information related to the position of
some Early Cretaceous birds in the trophic chains.
The thoracic box of Eoalulavis contains organics particles identi-
ﬁed as exoskeletal elements of unidentiﬁed crustaceans: the oldest
known direct evidence of trophic habits for birds
. This evidence
seems to support some kind of life habit related to aquatic environ-
Another avian material found at the Las Hoyas site yielded evi-
dence about the role of birds in the Mesozoic trophic chains. The fossil
is made of a bone assemblage (about 23 cm
in surface area) coming
www.scilet.com The birds from Las Hoyas 127
Fig. 10. Life reconstruction of the probable pellet producer of the fossil
illustrated in Fig. 8.
SP/Ortega 9/7/02 10:07 am Page 127
from four tiny juvenile bird individuals (Figure 8). Some morpho-
logical differences and size divergences indicate that the fossil bone
assemblage comes from at least three different species. The most
parsimonious hypothesis to interpret this fossil is that an organic
agent was responsible for this bird bone assemblage, resulting from
a predator that produced a regurgitated pellet
. This hypothesis is
supported by the SEM observation of the bones: its surface (perios-
teum) shows the characteristic pitting of digested bones of birds and
mammals (Figure 9). But the identiﬁcation of the predator is prob-
lematic. Based on the actual and putative predator fossil record of
Las Hoyas, the most probable producer of this fossil pellet was a
large pterosaur or a small-medium sized theropod dinosaurs (Figure
10). This fossil pellet clearly indicates that birds were usual prey in
the Early Cretaceous terrestrial ecosystems
We thank all the co-authors of our previous publications and co-
workers at the Las Hoyas projects for stimulating the ideas and
research discussed here. We want also thank to M. Anton and
R. Martín for reconstruction of Eoalulavis and Iberomesornis and
G. F. Kurtz for UV photographs. Work in Las Hoyas fossil site has
been supported by funds from Junta de Comunidades de Castilla-La
Mancha, Spanish DGICYT and European Union.
1. Pérez-Moreno, B.P., Sanz, J.L., Buscalioni, A. D., Moratalla, J. J., Ortega, F. &
Rasskin-Gutman, D. (1994) A unique multitoothed ornithomimosaur dinosaur
from the Lower Cretaceous of Spain. Nature, 370, 363–367.
2. Briggs, D. E. G., Wilby, P. R., Pérez-Moreno, B.P., Sanz, J.L. & Fregenal-
Martínez, M. (1997) The mineralization of dinosaur soft tissue in the Lower
Cretaceous of Las Hoyas, Spain. Journal of the Geological Society of London,
3. Ortega, F., Sanz, J. L., Barbadillo, L., Buscalioni, A. D., Diéguez, C.,
Evans, S. E., Fregenal-Martínez, M. A., de la Fuente, M., Madero, J., Martín-
Closas, C., Martínez-Delclòs, X., Meléndez, N., Moratalla, J. J., Pérez-Moreno,
B. P., Pinardo-Moya, E., Poyato-Ariza, F. J; Rodríguez-Lázaro, J., Sanchiz, B.
& Wenz, S. (1999). Las Hoyas. La huella del pasado. Fósiles de Castilla-La
Mancha. (Aguirre, E. & Rábano, I., coords.), pp. 195–216. Junta de
Comunidades de Castilla-La Mancha. Toledo, Spain.
4. Sanz; J.L., Fregenal-Martínez, M.A., Meléndez, N. & Ortega, F. (2001) Las
Hoyas. Palaeobiology II (Briggs, D. E: G. & Crowther, P. R., eds), pp.
356–359. Blackwell Science, Oxford.
128 José Luis Sanz and Francisco Ortega
SP/Ortega 9/7/02 10:07 am Page 128
5. Chiappe, L.M. (1997) Aves. The Encyclopedia of Dinosaurs (Currie, P. &
Padian, K., eds), pp. 23–28. Academic Press, New York.
6. Sanz, J. L. & Buscalioni, A. D. (1992) A new bird from the Early Cretaceous of
Las Hoyas, Spain, and the early radiation of birds. Paleontology, 35(4), 829–845.
7. Sanz, J. L., Bonaparte, J. F. & Lacasa, A. (1988) Unusual Early Cretaceous
birds from Spain. Nature, 331, 433–435.
8. Sanz, J. L. & Bonaparte, J. F. (1992). Iberomesornis romerali, a fossil small
bird articulated skeleton from the Early Cretaceous of Spain. Proc. II Int. Symp.
Avian Paleontol., Los Angeles, 1988, pp. 39–49.
9. Sanz, J. L., Perez-Moreno, B.P., Chiappe, L.M. & Buscalioni, A. D. (in press).
The birds from the Lower Cretaceous of Las Hoyas (Province of Cuenca,
Spain). Mesozoic Birds: Above the Heads of Dinosaurs (Chiappe, L. M. &
Witmer, L. M., eds). The University of California Press, Berkeley.
10. Sereno, P. C. (2000) Iberomesornis romerali (Aves, Ornithothoraces) reevalu-
ated as an Early Cretaceous enentiornithine. Neues Jahrbuch fur Geologie und
Palaontologie Abhandlungen, 215, 365–395.
11. Chiappe, L.M. (in press.). Basal bird phylogeny: problems and solutions.
Mesozoic Birds: Above the Heads of Dinosaurs (Chiappe, L. M. & Witmer, L.
M., eds). The University of California Press, Berkeley.
12. Sanz, J.L., Chiappe, L. M., Pérez-Moreno, B.P., Moratalla, J.J., Hernádez-
Carrasquilla, F., Buscalioni, A.D., Ortega, F., Poyato-Ariza, F.J., Rasskin-
Gutman, D. and Martínez-Delclós, X. (1997) A Nestling Bird from the Lower
Cretaceous of Spain: Implication for Avian Skull and Neck Evolution. Science,
13. Sanz, J. L., Chiappe, L. M. & Buscalioni, A. D. (1995) The osteology of
Concornis lacustris (Aves: Enantiornithes) from the Lower Cretaceous of Spain
and a reexamination of its phylogenetic relatioships. American Museum
Novitates, 3133, 23 pp.
14. Zhou, Z. (1995) The discovery of Early Cretaceous bird in China. Courier
Forschunsinstitut Senckenberg, 181, 9–22.
15. Sanz, J. L., Chiappe, L. M., Pérez-Moreno, B.P., Buscalioni, A. D., Moratalla,
J. J., Ortega, F. & Poyato-Ariza, F. J. (1996). A new Lower Cretaceous bird
from Spain: implications for the evolution of ﬂight. Nature, 382, 442–445.
16. Chiappe L. M. & Calvo, J. O. (1994) Neuquenornis volans, a new Upper
Cretaceous bird (Enantiornithes: Avisauridae) from Patagonia, Argentina.
J.Vertebr. Paleontol., 14(2), 230–246.
17. Chiappe, L. M. (1995) The ﬁrst 85 million years of avian evolution. Nature,
18. Padian, K. & Chiappe, L.M. (1998). The early evolution of birds. Biological
Reviews, 73(1), 1–42.
19. Gatesy, S. M. & Dial, K. P. (1996) Locomotor modules and the evolution of
avian ﬂight. Evolution, 50(1), 331–340.
20. Burgers, P. & Chiappe, L.M. (1999). The wing of Archaeopteryx as a primary
thrust generator. Nature, 399, 60–62.
21. Poore, S. O., Sanchez-Haiman, A. & Goslow, G. E. (1997). Wing upstroke and
the evolution of ﬂapping ﬂight. Nature, 387, 799–802.
22. Chiappe, L.M., Ji, S.-A., Ji, Q. & Norell, M.A. (1999) Anatomy and
Systematics of the Confuciusornithidae (Theropoda: Aves) from the Late
Mesozoic of the Northeastern China. Bull. American Mus. Nat. Hist., 242, 1–89.
www.scilet.com The birds from Las Hoyas 129
SP/Ortega 9/7/02 10:07 am Page 129
23. Bailey, J. P. & DeMont, M. E. (1991) The function of the wishbone. Can. J.
Zool., 69, 2751–2758.
24. Norberg, U. M. (1990) Vertebrate Flight. Springer-Verlag. 291 pp.
25. Tennekes, H. (1997) The Simple Science of Flight. From Insects to Jumbo Jets.
The MIT Press, 137 pp.
26. Pennycuick, C.J. (1989) Bird ﬂight performance. A practical calculation
manual. Oxford University Press. 153 pp.
27. Olmos, M. (1988) Investigaciones sobre la alometría funcional en las extremi-
dades de las aves. Ph. Dissertation. Universidad de Barcelona. 258 pp.
28. Ji, Q., Currie, P., Ji, S-A. & Norell, M.A. (1999) Two feathered dinosaurs from
Northeastern China. Nature, 393, 753–761.
29. Saville, D.B.O. (1957) Adaptative evolution in the avian wing. Evolution, 11,
30. Hou, L., Martín, D., Zhou, D. & Feduccia, A. (1999). Archaeopteryx to opposite
birds-missing link from the Mesozoic of China. Vertebrata PalAsiatica, 37(2),
31. McLelland, J. (1989). Anatomy of the lungs and airsacs. Form and Function in
Birds (King, A. S. & McLelland, J., eds) vol. 3, pp. 221–279 Academic Press,
32. Schmidt-Nielsen, K. (1993). Animal Physiology. Adaptation and Environment,
602 pp. Fourth Edition. Cambridge University Press.
33. Sanz; J. L., Chiappe, L. M., Fernández-Jalvo, Y., Ortega, F., Sánchez-Chillón,
B., Poyato-Ariza, F. J., Pérez-Moreno, B. P. (2001) An Early Cretaceous pellet.
Nature, 409, 998–999.
130 José Luis Sanz and Francisco Ortega
SP/Ortega 9/7/02 10:07 am Page 130