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| Avimaia schweitzerae
Bailleul, O’Connor, Zhang, Li, Wang, Lamanna, Zhu & Zhou, 2019
Reconstruction by Michael Rothman
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Abstract
Understanding non-crown dinosaur reproduction is hindered by a paucity of directly associated adults with reproductive traces. Here we describe a new enantiornithine, Avimaia schweitzerae gen. et sp. nov., from the Lower Cretaceous Xiagou Formation with an unlaid egg two-dimensionally preserved within the abdominothoracic cavity. Ground-sections reveal abnormal eggshell proportions, and multiple eggshell layers best interpreted as a multi-layered egg resulting from prolonged oviductal retention. Fragments of the shell membrane and cuticle are both preserved. SEM reveals that the cuticle consists of nanostructures resembling those found in neornithine eggs adapted for infection-prone environments, which are hypothesized to represent the ancestral avian condition. The femur preserves small amounts of probable medullary bone, a tissue found today only in reproductively active female birds. To our knowledge, no other occurrence of Mesozoic medullary bone is associated with indications of reproductive activity, such as a preserved egg, making our identification unique, and strongly supported.
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| Fig. 1 Photograph and line drawing of the holotype of Avimaia schweitzerae, IVPP V25371. a Photograph of the partial skeleton with feather impressions, and the crushed preserved egg between the pubes; b interpretive line drawing, with white arrows indicating the two fragments extracted for microscopic analysis with a super-imposed CT-scan revealing the egg and underlying elements of the right pelvis in dorsal (synsacrum) and medial (ilium) view.
Gray denotes bones (darker gray indicating poor preservation), blue denotes the egg, and dark gray denotes feather impressions.
cv caudal vertebra, d digit, dp dorsal process, f fibula, fc fibular crest, fe femur, if ilioischiadic foramen, il ilium, is ischium, l left, mt metatarsal, p pedal phalanx, pu pubis, py pygostyle, r right, ri rib, sy synsacrum, tb tibiotarsus, tm tarsometatarsus, tv thoracic vertebra.
Scale bar is 1 cm.
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| Photograph of the holotype of Avimaia schweitzerae.
(Image by Barbara Marrs)
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Systematic paleontology
Aves Linnaeus, 1758
Pygostylia Chiappe, 2002
Ornithothoraces Chiappe, 1995
Enantiornithes Walker, 1981
Avimaia schweitzerae gen. et sp. nov.
Etymology: The generic name Avi- (bird) maia (mother) refers to the fact the specimen is a female preserved with an egg in the body cavity. Schweitzerae is in honor of Mary Higby Schweitzer for her ground-breaking works on MB and for her role in establishing the field of molecular paleontology.
Holotype: IVPP V25371 (Institute of Vertebrate Paleontology and Paleoanthropology), an articulated partial skeleton with some feather traces, consisting of the caudal half of the axial column, the pelvis, and the hind limbs, mostly exposed ventrolaterally (Fig. 1).
Locality and horizon: Near Changma Village, Yumen City, Gansu Province, northwestern China; Lower Cretaceous (lower–middle Aptian) Xiagou Formation10.
Diagnosis: Small-bodied enantiornithine (robust, cranially forked pygostyle, distal condyles of tibiotarsus contacting medially, J-shaped metatarsal I, metatarsal IV mediolaterally reduced relative to metatarsals III and IV, metatarsal IV trochlea reduced to single condyle) with the following autapomorphies: pubis delicate and strongly curved so that the caudal margin is concave throughout; distal end of ischium dorsally curved.
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| Reconstruction of the Xiagou Formation with colonial nesting ground of Avimaia schweitzerae. The female individual dead in the water on the left (with an unlaid egg not visible inside its abdomen), represents the fossilized individual described here.
(Image by Michael Rothman)
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Alida M. Bailleul, Jingmai O’Connor, Shukang Zhang, Zhiheng Li, Qiang Wang, Matthew C. Lamanna, Xufeng Zhu and Zhonghe Zhou. 2019. An Early Cretaceous Enantiornithine (Aves) Preserving An Unlaid Egg and Probable Medullary Bone. Nature Communications. volume 10, 1275. DOI: 10.1038/s41467-019-09259-x
New Cretaceous Fossil Sheds Light on Avian Reproduction - english.cas.cn/newsroom/research_news/201903/t20190320_207048.shtml
New Cretaceous fossil sheds light on avian reproduction eurekalert.org/e/933b via @EurekAlert
