A new spalacolestine mammal from the Early Cretaceous Jehol Biota

A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian ‘symmetrodontans’

Authors:

Han et al

Abstract:

‘Symmetrodontans’ are extinct mammals characterized by having a reversed-triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians; it is a key feature in taxonomy of Mesozoic mammals and one to understand mammalian evolution and palaeobiology. Here we report a new genus and species of ‘symmetrodontan’ mammal, Lactodens sheni, from the Early Cretaceous Jehol Biota, represented by a partial skeleton with dentary and upper and lower teeth with dental morphologies well-preserved. The new species has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double-rooted canines, extremely low-crowned and transversely thin premolars, and acute angled molars. The dental morphologies of molars and peculiar deciduous premolars are similar to those of Spalacolestes from North America. The associated upper and lower dentitions from one individual animal helped to clarify tooth identification of some spalacotheriids represented only by fragmentary material. Phylogenetic analyses indicate a close relationship of the new species to North American spalacolestines and faunal interchanges between Eurasia and North America, thus supporting the notion that small-bodied spalacotheriids were diverse and had a pan-Laurasian distribution during the Early Cretaceous. Absence of the Meckelian groove suggests acquisition of the definitive mammalian middle ear in spalacolestines, and deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.

Comparing the Barremian/Aptian Cretaceous Ecologies of the Jehol and Wealden Biotas

A comparison of Barremian–early Aptian vertebrate assemblages from the Jehol Group, north-east China and the Wealden Group, southern Britain: the value of microvertebrate studies in adverse preservational settings

Author:

Sweetman

Abstract:

Vertebrates are a highly significant component of the Jehol Biota of northern Hebei, western Liaoning, and southeastern Inner Mongolia. Furthermore, Jehol vertebrate fossils from these areas are remarkable for their abundance and diversity, and for their taphonomy which has resulted in the preservation of large numbers of complete skeletons and of soft tissues including the feathers of birds and nonavian dinosaurs. In contrast, and also for taphonomic reasons, skeletons are extremely rare in the approximately coeval (Barremian–earliest Aptian) Lower Cretaceous Wealden Group of the Isle of Wight, southern England, UK. However, collection of isolated macro-remains over a period of almost 200 years has demonstrated that the Wealden Group, and in particular the Barremian Wessex Formation, contains Europe’s most diverse dinosaur assemblage. This assemblage, until the relatively recent discovery of China’s feathered nonavian dinosaurs and birds, appeared to be of comparable diversity to that of the Jehol Biota. Isolation of microvertebrate remains in an ongoing study commenced in 2002 has demonstrated that not only is the Wealden Group dinosaur assemblage more diverse than apparent from macro-remains and that it includes birds but also that it includes a high diversity of other aquatic, amphibious and terrestrial vertebrates. It is, therefore, now possible to make meaningful comparisons between the Jehol Biota and that of the Wealden Group of the Isle of Wight. Similarities and marked differences are apparent that reflect geographical separation, palaeoenvironmental factors and the remarkably small outcrop area available for the Wealden Group.

Two new Species of Gobiconodon Mammals From Aptian/Albian Cretaceous China

Two new species of Gobiconodon (Mammalia, Eutriconodonta, Gobiconodontidae) from the Lower Cretaceous Shahai and Fuxin formations, northeastern China

Authors:

Kusuhashi et al

Abstract:

Two new gobiconodontid mammals, Gobiconodon tomidaisp. nov. and Gobiconodon haizhouensissp. nov., from the Lower Cretaceous (Aptian to Albian) Shahai and Fuxin Formations, respectively, in Liaoning Province, northeastern China, are described. Gobiconodon tomidaisp. nov. is a small-sized species of the genus characterised by the lower dental formula 2.1.2.5, a double-rooted p2, the molariform crown being mesiodistally longer than tall, and a distinct and almost continuous lingual cingulid on m2–m4. Gobiconodon haizhouensissp. nov. is of similar size to Gobiconodon tomidaisp. nov. and is characterised by the lower dental formula 2.1.3.5, the molariform crown being slightly taller than the mesiodistal length, and a well-developed and almost continuous lingual cingulid on m1–m3. The new materials indicate that the upper molariform count of most species of Gobiconodon is likely to be four, one less than the lower molariform count. Gobiconodon is the second mammalian genus common to the Jehol and Fuxin mammalian faunas.

Did a Jehol Biota get Buried by a Volcano Like Pompeii?

The Chinese Pompeii? Death and destruction of dinosaurs in the Early Cretaceous of Lujiatun, NE China

Authors:

Rogers et al

Abstract:

The Lujiatun Unit (Yixian Formation) yields some of the most spectacular vertebrate fossils of the Jehol Group (Lower Cretaceous) of NE China. Specimens are preserved both articulated and three-dimensional, unlike the majority of Jehol fossils, which are near two-dimensional compression fossils. The site has been referred to as the ‘Chinese Pompeii’ because the dinosaurs and other animals were assumed to have been killed and buried by hot, airborne volcanic debris and ash in a single event; this has yet to be confirmed. Field and laboratory evidence for the sedimentological context of the fossils from the Lujiatun Unit is described in detail, and used to assess whether the fossil remains correspond to a single depositional event and whether this event was the direct result of volcanic activity. Fossils of the Lujiatun Unit occur in several horizons of volcaniclastic sediments that represent multiple depositional events. Petrological analysis shows that the fossil-bearing sediments were remobilised and deposited by water. The Lujiatun dinosaurs and other fossils were therefore not killed by a single airborne volcanic ash, but in multiple flood events with a high load of volcaniclastic debris.

New Discoveries at From the Jehol Biota Lagerstatte

The Jehol Biota, an Early Cretaceous terrestrial Lagerstätte: new discoveries and implications

Author:

Zhou

Abstract:

The study of the Early Cretaceous terrestrial Jehol Biota, which provides a rare window for reconstruction of a Lower Cretaceous terrestrial ecosystem, is reviewed with a focus on some of the latest progress. A newly proposed definition of the biota based on paleoecology and taphonomy is accepted. Although the Jehol fossils are mainly preserved in two types of sedimentary rocks, there are various types of preservation with a complex mechanism that remains to be understood. New discoveries of significant taxa from the Jehol Biota, with an updated introduction of its diversity, confirm that the Jehol Biota represents one of the most diversified biotas of the Mesozoic. The evolutionary significance of major biological groups (e.g. dinosaurs, birds, mammals, pterosaurs, insects, and plants) is discussed mainly in the light of recent discoveries, and some of the most remarkable aspects of the biota are highlighted. The global and local geological, paleogeographic, and paleoenvironmental background of the Jehol Biota have contributed to the unique composition, evolution, and preservation of the biota, demonstrating widespread faunal exchanges between Asia and other continents caused by the presence of the Eurasia–North American continental mass and its link to South America, and confirming northeastern China as the origin and diversification center for a variety of Cretaceous biological groups. Although some progress has been made on the reconstruction of the paleotemperature at the time of the Jehol Biota, much more work is needed to confirm a possible link between the remarkable diversity of the biota and the cold intervals during the Early Cretaceous. Finally, future directions for the study of the Jehol Biota are proposed that highlight the great potential of more comprehensive and multidisciplinary studies to further our understanding of the biological and geological implications of the Jehol Lagerstätte.

Gansus zheni: a new Ornithuromorph Bird From Aptian Cretaceous Jehol Biota

An advanced, new long-legged bird from the Early Cretaceous of the Jehol Group (northeastern China): insights into the temporal divergence of modern birds

Authors:

Liu et al

Abstract:

We describe a new ornithuromorph bird species, Gansus zheni from the Lower Cretaceous lacustrine deposits of the Jiufotang Formation (Jehol Group), Liaoning Province, China. A cladistic analysis resolves Gansus zheni as the sister taxon of the roughly contemporaneous Gansus yumenensis (Xiagou Formation, Gansu Province), and together as the most immediate outgroup to Ornithurae. Gansus zheni is the most advanced bird known today for the Jehol Biota. Its discovery provides the best-documented case of inter-basinal correlations (Jehol and Changma basins of Liaoning and Gansu provinces, respectively) using low-taxonomic clades of fossil birds. The existence of close relatives of Ornithurae in deposits formed at about 120 million years ago helps to mitigate the long-standing controversy between molecular and paleontological evidence for the temporal divergence of modern birds (Neornithes).

Iteravis huchzermeyeri: An Aptian Cretaceous Ornithuromorph Bird From Jehol Biota

A new species from an ornithuromorph (Aves: Ornithothoraces) dominated locality of the Jehol Biota

Authors:

Zhou

Abstract:

We report on a new species of ornithuromorph bird, Iteravis huchzermeyeri gen. et sp. nov., from the previously unreported Sihedang locality of the Lower Cretaceous Yixian Formation, the oldest ornithuromorph bearing deposit in the world. Unlike most other Cretaceous localities, specimens from this new quarry are largely referable to Ornithuromorpha, similar to the Lower Cretaceous Aptian Xiagou Formation in Gansu Province. Also similar to the Xiagou avifauna, the fauna at Sihedang is largely dominated by a single taxon (described here). Differences in faunal dominance may suggest the Sihedang records a unique ecological habitat. This may also explain the dominance of Gansus in the younger Xiagou Formation locality and suggests that previous hypotheses regarding the shift in dominance between Enantiornithes and Ornithuromorpha need to be reassessed in terms of potential ecological biases due to limited sampling. Furthermore, the recognition of an ornithuromorph dominated locality in the Sihedang significantly weakens the signal of such an inferred trend. Compared to most Jehol birds, the new specimen is relatively better preserved in three dimensions revealing morphological details of the skeleton, as well as preserves feather impressions including a rectricial morphology previously unknown among Mesozoic birds.

How the Early Cretaceous Jehol Lagerstatte Fossilized

A MODEL FOR ORGANIC FOSSILIZATION OF THE EARLY CRETACEOUS JEHOL LAGERSTÄTTE BASED ON THE TAPHONOMY OF “EPHEMEROPSIS TRISETALIS”

Authors:

Pan et al

Abstract:

The taphonomic pathways of “Ephemeropsis trisetalis” nymphs (mayfly larvae) were systematically investigated based on fossils of different preservational types, collected during three high–stratigraphic-resolution (mm to cm) excavations in the Lower Cretaceous Yixian Formation in the Sihetun area of western Liaoning, China. All fossils studied are fully articulated either in three or two dimensions, which indicates that decay was terminated at a stage before the exoskeleton became disarticulated. We conclude that the Jehol organic skeletons represent at least two general types of preservation produced by pyritization and collapse/compression, respectively. The two-dimensional compressions show no evidence for authigenic minerals, but the three-dimensionally preserved fossils are wholly or partially pyritized. Our study also indicates that aluminosilicate clay and pyrite mineralization are closely associated with fossil “Ephemeropsis trisetalis” nymphs, suggesting that both clay and pyrite played important roles in lacustrine fossil preservations, as in some marine fossil Lagerstätten. We propose a general model for organic tissue fossilization in the Jehol Lagerstätte based on study of taphonomy of “Ephemeropsis trisetalis” nymphs.

A Large Enantiornithine Bird from Hauterivian Cretaceous China

A large enantiornithine bird from the Lower Cretaceous of China and its implication for lung ventilation

Authors:

Zhang et al

Abstract:

The Enantiornithes were the most taxonomically diverse bird group in the Mesozoic. Most of the known taxa are from Lower Cretaceous deposits of the Jehol Group in north-eastern China. A new specimen from the Jiufotang Formation in Jianchang, Liaoning Province, is described here; being a subadult individual at the time of death it had reached a relatively large size. The presence of uncinate processes, bicapitate and forked vertebral ribs, sternal ribs that were all of similar length, as well as the location of parapophyses and diapophyses on the thoracic vertebrae, may imply a rigid and volume-constant lung, and less efficient lung ventilation in enantiornithines.

Eopengornis martini: a new Enantiornithine From China's Hauterivian Cretaceous Jehol Biota

Insights into the evolution of rachis dominated tail feathers from a new basal enantiornithine (Aves: Ornithothoraces)

Authors:

Wang et al

Abstract:

We report on a new enantiornithine Eopengornis martini gen. et sp. nov. from the lowest horizon of the Jehol Biota in Hebei, China; dated at 130.7 Mya, this is the second oldest avian bearing fossil deposit in the world, recording the First Appearance Datum of Enantiornithes. The new specimen, only the second enantiornithine and third bird reported from this horizon, preserves numerous synapomorphies with the largest Lower Cretaceous enantiornithine Pengornis houi from the Jiufotang Formation dated at 120 Mya. Together, they form a new avian lineage that lasted over 10 Myr, which is longer than any known clade of Lower Cretaceous enantiornithine. Eopengornis reveals new information about basal enantiornithine morphology such as the presence of a metatarsal V, helping to clarify the early evolution of these dominant Cretaceous avians. Furthermore, Eopengornis preserves a previously unrecognized tail morphology: a pair of elongate fully pennaceous rachis dominated feathers. This discovery confirms hypotheses proposing that the rachis dominated racket-plumes in basal birds represent modified pennaceous feathers. We suggest that the ornamental racket-plumes in enantiornithines and Confuciusornis evolved independently from the basal pygostylian condition, which we infer was a tail formed of normal flight feathers.

Jeholornis curvipes: a new Cursorial, Nonperching, and Seed-eating Bird From Lower Cretaceous Jehol Biota

A new long-tailed basal bird from the Lower Cretaceous of north-eastern China

Authors:

Lefèvre et al

Abstract:

A new basal Avialae, Jeholornis curvipes sp. nov., from the Yixian Formation (Lower Cretaceous) of Liaoning Province (north-eastern China) is described. A revision of long-tailed birds from China and a phylogenetic analysis of basal Avialae suggest that Jeholornithiformes were paraphyletic, with Jixiangornis orientalis being the sister-taxon of pygostylia. The phylogenetic analysis also recovered that the tail reduction is a unique event in the evolution of birds. Jeholornis species were cursorial, nonperching, and seed-eating birds.

The PaleoEcology of the Lower Cretaceous Jehol Biota

The Early Cretaceous terrestrial ecosystems of the Jehol Biota based on food-web and energy-flow models

Authors:

Matsukawa et al

Abstract:

The ancient terrestrial ecosystems of the Lower Cretaceous Yixian Formation and the Jiufotang Formation, consecutive components of the Jehol Group in Northeast China were reconstructed using an energy-flow and food-web model. This model can be used to quantitatively estimate population densities for ancient terrestrial vertebrates based on food webs, net primary productivity, and three categories of energy-transfer efficiency. The results indicate that densities reached 866 individuals km−2 and 4122 individuals km−2 in two ecosystems, respectively. The main component of the vertebrate fauna of the Yixian Formation consisted of large herbivorous dinosaurs, while much smaller avians dominated the Jiufotang fauna. The model also indicates a temporal transition in the dinosaur fauna from the Yixian fauna to the Jiufotang fauna in which theropods decreased and ceratopsids became more abundant. We then compared these estimates of biodiversity with the Early Cretaceous Choyr fauna of Mongolia, and Tetori fauna of Japan using Simpson's diversity indices. Those indices, based on biomass, indicate that the biodiversities of the Jehol fauna lay between those of the Choyr and Tetori faunas. This range in biodiversity seems attributable to fundamental differences in vegetation and the environment.

Changyuraptor yangi: a new ‘Four-winged’ Microraptorine From Barremian Cretaceous China's Jehol Biota

A new raptorial dinosaur with exceptionally long feathering provides insights into dromaeosaurid flight performance

Authors:

Han et al

Abstract:

Microraptorines are a group of predatory dromaeosaurid theropod dinosaurs with aerodynamic capacity. These close relatives of birds are essential for testing hypotheses explaining the origin and early evolution of avian flight. Here we describe a new ‘four-winged’ microraptorine, Changyuraptor yangi, from the Early Cretaceous Jehol Biota of China. With tail feathers that are nearly 30 cm long, roughly 30% the length of the skeleton, the new fossil possesses the longest known feathers for any non-avian dinosaur. Furthermore, it is the largest theropod with long, pennaceous feathers attached to the lower hind limbs (that is, ‘hindwings’). The lengthy feathered tail of the new fossil provides insight into the flight performance of microraptorines and how they may have maintained aerial competency at larger body sizes. We demonstrate how the low-aspect-ratio tail of the new fossil would have acted as a pitch control structure reducing descent speed and thus playing a key role in landing.

Zhouornis hani: a new Lower Cretaceous Enantiornithine Bird

New information on the anatomy of the Chinese Early Cretaceous Bohaiornithidae (Aves: Enantiornithes) from a subadult specimen of Zhouornis hani

Authors:

Zhang et al

Abstract:

Enantiornithines are the most diverse avian clade in the Cretaceous. However, morphological specializations indicative of specific ecological roles are not well known for this clade. Here we report on an exquisitely well-preserved specimen from the Lower Cretaceous Jehol Group of northeastern China, which pedal morphology is suggestive of a unique ecological specialization within Enantiornithes. The morphology of the new specimen is largely indistinguishable from that of the holotype of the bohaiornithid enantiornithine Zhouornis hani, albeit the latter is somewhat larger. The new specimen provides important and previously unknown details of the skull of Zhouornis hani, which add to the limited knowledge about the cranial anatomy and evolution of enantiornithines. The information offered by the new specimen also augments our understanding of the postcranial morphology of bohaiornithid enantiornithines, a clade that has been only recently recognized. With the description of this specimen, Zhouornis hani becomes one of the most anatomically complete known enantiornithine species, which will facilitate future morphological studies.

Early Cretaceous Bird Yanornis was a Fish Eater

New Specimens of Yanornis Indicate a Piscivorous Diet and Modern Alimentary Canal

Authors:

Zheng et al

Abstract:

A crop adapted for an herbivorous diet of seeds has previously been documented in the Early Cretaceous birds Sapeornis and Hongshanornis. Here we report on several specimens of Yanornis that preserve a crop containing fish. One specimen preserves two whole fish in the oesophagus, indicating that Early Cretaceous birds shared trophic specializations with Neornithes for the increased energetic demands of flight – namely the storing of food for later consumption when the stomach is full. Whole fish also indicate that despite their presence, teeth were not used to orally process food, suggesting the hypertrophied dentition in this taxon were utilized in prey capture. The presence of macerated fish bones in the crop of other specimens indicates the highly efficient advanced muscular system of peristalsis responsible for moving ingested items between different segments of the alimentary canal was also in place. Despite the fact many features of the modern avian alimentary canal are inferred to compensate for the absence of teeth in birds (expandable oesophagus, grinding gizzard), the derived alimentary canal was apparently present in toothed Cretaceous birds. Although Yanornis was considered to have switched their diet from piscivorous to herbivorous, based on position and morphology we reinterpret the gastroliths reported in one specimen as sand impacted in the intestines, and reconstruct the taxon as primarily piscivorous. This is a novel interpretation for fossilized gastroliths, and the first documentation of this condition in the fossil record.

Chaoyangodens lii: a new Eutriconodont Mammal From Aptian Cretaceous Jehol Biota

A new eutriconodont mammal from the early Cretaceous Jehol Biota of Liaoning, China

Authors:

Hou et al

Abstract:

A new genus and species of eutriconodont mammal, Chaoyangodens lii gen. et sp. nov., from the Dawangzhangzi bed of Yixian Formation, Lingyuan, Liaoning, is reported. The new species has a tooth formula I5-C1-P1-M3/i4-c1-p1-m4, unique among eutriconodonts in having only one premolar in lower and upper jaws, respectively, and a distinctive diastema between the canine and the premolar. Its simple incisors and reduced premolars show a mosaic combination of primitive and derived features. This new taxon adds to the diverse group of Jehol eutriconodonts. Among the known species from at least three horizons, there seems no evolutionary trend in morphology that is recognizable, except for that larger species are all from the Lujiatun bed of the Yixian Fm. A thorough and systematic analysis involving all the Jehol eutriconodonts is needed to understand their phylogenetic relationships.

Cretaceous Chinese Jehol Biota Lived in a Cold Climate

Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates

Authors:

Amiot et al

Abstract:

Early Cretaceous vertebrate assemblages from East Asia and particularly the Jehol Biota of northeastern China flourished during a period of highly debated climatic history. While the unique characters of these continental faunas have been the subject of various speculations about their biogeographic history, little attention has been paid to their possible climatic causes. Here we address this question using the oxygen isotope composition of apatite phosphate (δGraphic) from various reptile remains recovered from China, Thailand, and Japan. δGraphic values indicate that cold terrestrial climates prevailed at least in this part of Asia during the Barremian—early Albian interval. Estimated mean air temperatures of about 10 ± 4 °C at midlatitudes (∼42 °N) correspond to present day cool temperate climatic conditions. Such low temperatures are in agreement with previous reports of cold marine temperatures during this part of the Early Cretaceous, as well as with the widespread occurrence of the temperate fossil wood genus Xenoxylon and the absence of thermophilic reptiles such as crocodilians in northeastern China. The unique character of the Jehol Biota is thus not only the result of its evolutionary and biogeographical history but is also due to rather cold local climatic conditions linked to the paleolatitudinal position of northeastern China and global icehouse climates that prevailed during this part of the Early Cretaceous.

Lishugou Formation is Contemporary of Cretaceous Jehol Biota Formations in China

40Ar/39Ar age constraints on Cretaceous fossil-bearing formations near the China-North Korea border

Authors:

Chang et al

Abstract:

Abundant Mesozoic terrestrial plant, fish and invertebrate fossils have been discovered from the Lishugou Formation and the underlying Xiaolin Formation in the eastern Liaoning province of NE China since the 1970s. These discoveries have great potential for substantially increasing our knowledge on the evolution of paleoecosystems and paleoenvironments in East Asia. However, previous geological and geochronological studies for these fossil-bearing formations are rare because the fossil outcrops are near the border between China and North Korea. Here we present robust 40Ar/39Ar ages of 99.9 ± 0.4 Ma for one sample from the Lishugou Formation and 109.1 ± 0.2 Ma and 122.4 ± 0.2 Ma for two samples from the upper and lower parts of the Xiaolin Formation from Dandong City, Liaoning, China. Our new age data indicate that the Lishugou and Xiaolin Formations were deposited contemporaneously with the Jiufotang Formation from the classic Jehol outcrops in western Liaoning. Therefore, our results provide an accurate age calibration for the fossils discovered from the Liaodong Peninsula and suggest that the hypothesis of the Jehol migration should be reappraised.

Did Jeholornis Have TWO Tails?

The early bird gets two tails? A 120-million-year-old bird sported a long tail and a second, unexpected tail frond, paleontologists suggest. The discovery points to a complicated evolutionary path for the tails we see in birds today.

The second-oldest known bird, Jeholornis, lived in what is today China, along with a trove of other feathered dinosaurs discovered in the region over the last decade. Fossils show that Jeholornis was turkey-size, had claws on its wing forelimbs, and possessed three small teeth in its lower jaw. It was also thought to sport only a long fan-feathered tail at its back end. Now, however, paleontologists are claiming discovery of a second tail frond adorning the bird.

"The 'two-tail' plumage of Jeholornis is unique," according to the study, which was led by Jingmai O’Connor of the Chinese Academy of Sciences in Beijing. The report of the discovery of the tail frond was published in the Proceedings of the National Academy of Sciences.

Of 11 Jeholornis fossils that retain evidence of ancient plumage, 6 have signs of this frond of 11 feathers, which would have jutted above the bird's back at a jaunty, upright angle in a "visually striking" manner, according to the study.

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A New Large Juvenile Enantiornithine Bird of the Cretaceous Jehol Biota of China

A large bird from the Early Cretaceous of China: new information on the skull of enantiornithines

Authors:

1. Zihui Zhang
2. Luis M. Chiappe
3. Gang Han
4. Anusuya Chinsamy

Affiliations:

blocked by lack of access...

Abstract:

We describe the anatomy and bone histology of an enantiornithine specimen from the Early Cretaceous Jehol Group of northeastern China. CNUVB-0903 is larger than most Early Cretaceous enantiornithine birds and also different from all other named taxa. Thus, we erect the new species Zhouornis hani for the new specimen. CNUVB-0903 preserves a suite of new morphologies for the clade. Noteworthy are those of the braincase and occipital region of the skull, which was previously poorly known for enantiornithines. The morphology and placement of the large basipterygoid processes and the well-developed basisphenoid recess—comparable in morphology to those of non-avian dinosaurs—highlights the evolutionary conservatism of the enantiornithine skull. The histological characterization of CNUVB-0903 indicates that it was not yet a full-grown individual at the time of death. This, combined with the comparatively large size of the skeleton, supports previous evidence indicating that early in their history, enantiornithines were able to achieve relatively large sizes.