Reconstruction of oviraptorid clutches illuminates their unique nesting biology TZU-RUEI YANG, JASMINA WIEMANN, LI XU, YEN-NIEN CHENG, XIAO-CHUN WU, and P. MARTIN SANDER Yang, T.-R., Wiemann, J., Xu, L., Cheng, Y.-N. Wu, X.-C., and Sander, P.M. 2019. Reconstruction of oviraptorid clutches illuminates their unique nesting biology. Acta Palaeontologica Polonica 64 (3): 581–596. Oviraptorosaurs, a group of non-avian theropod dinosaurs from the Cretaceous of Asia and North America, left behind the most abundant and informative fossil evidence of dinosaur reproductive biology. Previous studies had suggested that oviraptorosaur reproductive biology represents an intermediate stage and exhibited unique modern avian traits. For instance, the adult-associated clutches were predominantly considered as evidence for brooding/thermoregulatory contact incubation (TCI) behaviors, whereas the hypotheses of laying or protection were neglected. Despite numerous oviraptorid egg clutches uncovered from China and Mongolia, their nest architecture and clutch arrangement were rarely investigated in detail. Here we present a comprehensive reconstruction of an oviraptorid clutch based on five new oviraptorid clutches from Jiangxi Province, China. A detailed examination of the new clutches reveals a partially-open oviraptorid nest that contains 3–4 rings of paired eggs (more than 15 pairs total) whose blunt end points toward the center devoid of eggs at an angle of 35–40°. Our detailed three-dimensional reconstruction indicates that the oviraptorid clutch has a unique architecture unknown from extant bird clutches, implying an apomorphic nesting mode. Such a unique nest architecture further contradicts the TCI hypothesis in oviraptorids, hindering sufficient heat transfer to the inner (lower) ring(s) of eggs. Moreover, the size of the new oviraptorid clutches (>30 eggs) is significantly larger than that of the adult-associated clutches (<22 eggs), raising the alternative hypothesis that the adult-associated clutches were uncom- pleted. This clue thus supports the hypothesis that the clutch-associated oviraptorid adults possibly represent females after an oviposition before a catastrophic sandstorm/flooding burial. Key words: Dinosauria, Oviraptor, clutches, nest, thermoregulatory contact incubation, Cretaceous, China. Tzu-Ruei Yang [[email protected]], Section Paleontology, Institute of Geosciences, Universität Bonn, Nussallee 8, 53115 Bonn, Germany; Department of Earth Sciences, National Cheng Kung University, 70101 Tainan, Taiwan; Division of Geology, National Museum of Natural Sciences, 40353 Taichung, Taiwan. Jasmina Wiemann [[email protected]], Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA. Li Xu [[email protected]], Henan Geological Museum, 450016 Zhengzhou, China. Yen-Nien Cheng [[email protected]], Department of Earth Sciences, National Cheng Kung University, 70101 Tain- an, Taiwan; Division of Geology, National Museum of Natural Sciences, 40353 Taichung, Taiwan. Xiao-Chun Wu [[email protected]], Canadian Museum of Nature, McLeod Street 240, Ottawa, Ontario, Canada. P. Martin Sander [[email protected]], Steinmann-Institut für Geologie, Mineralogie, Paläontologie, Uni- versität Bonn, Nussallee 8, 53115 Bonn, Germany; Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA. Received 27 April 2018, accepted 15 April 2019, available online 30 August 2019. Copyright © 2019 T.-R. Yang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. and an associated clutch (AMNH FARB 6508) from the Introduction and historical Late Cretaceous (Campanian) Djadokhta Formation of Bayn background Dzak (= The Flaming Cliffs = Shabarakh Usu), Mongolia (Osborn 1924). This was the first time that this kind of From a thief to a loving mom.—Oviraptor, which means adult- clutch association was reported. Osborn (1924) as- “egg thief”, was named by Osborn in 1924 based on the signed the associated clutch to protoceratopsians dinosaurs Oviraptor philoceratops specimen (AMNH FARB 6517) and hypothesized that oviraptors preyed on proceraptopsian Acta Palaeontol. Pol. 64 (3): 581–596, 2019 https://doi.org/10.4202/app.00497.2018 582 ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019 eggs. Seventy years later, Norell et al. (1994) described an Inner Mongolia, China, represent four different taxa of ovi- embryo-containing egg (IGM100/971) from the Djadokhta raptorosaurs (Table 1 and references therein). Formation of Ukhaa Tolgod, Mongolia, that exhibited striking morphological similarity to the putative protocer- Brooding and incubation: clarification of terms.—In mod- atopsian eggs of the 1924 clutch. However, the embryonic ern birds, the term “brooding” covers the two categories cranial remains showed features diagnostic of oviraptorids “pre hatching brooding” and “posthatching brooding” (Man- and not of Protoceratops, falsifying the hypothesis that the dal 2012). For dinosaurs, previous studies mainly discussed adult oviraptor AMNH FARB 6517 was preying on the egg “prehatching brooding” (Norell et al. 1995; Dong and Currie clutch AMNH FARB 6508 and questioning the hypothesis 1996; Clark et al. 1999; Fanti et al. 2012), while “posthatch- that oviraptors were egg thieves. Subsequently, Norell et al. ing brooding” was rarely discussed. Posthatching brooding (1995) reported another oviraptor adult-clutch association comprises protecting and taking care of the young after (IGM 100/979) from Ukhaa Tolgod and interpreted the fos- hatching, and also includes body heat transfer from the adult sil as evidence for bird-like “brooding” behavior. to the hatched offspring (Mandal 2012; see also Norell et al. In the 1990s, two more adult-clutch associations were dis- 2018). Prehatching brooding involves incubation behavior, covered and interpreted as representing brooding behavior as which means the adult is sitting on the eggs to keep them well: IVPP V9608 from the Djadokhta Formation of Bayan warm and to bring them to hatching, also known as thermo- Mandahu, Inner Mongolia, China (Dong and Currie 1996; regulatory contact incubation (TCI). TCI implies an open Longrich et al. 2010), and IGM 100/1004, again from Ukhaa nest because the body heat is transferred by contact between Tolgod (first reported by Clark et al. 1999 and re- described adult and eggs and generally involves heat generated by the by Norell et al. 2018). The specimens IGM 100/979 and IGM endothermic metabolism of the adult. In altricial birds, en- 100/1004 were used to erect the new taxon Citipati osmol- dothermy develops in the chicks only after hatching, which skae in 2001 (Clark et al. 2001). Subsequently, Fanti et al. is why they are thermally dependent on brooding adults for (2012) described yet another adult-clutch association consist- the first days after hatching (Dawson et al. 1976; Whittow ing of a clutch of 18 eggs from the Baruungoyot Formation and Tazawa 1991; Hohtola and Visser 1998). In superpreco- of the Nemegt Basin of southern Mongolia (MPC-D 107/15). cial and precocial birds, endothermy is gained even before The specimen lacks a clear clutch patterning due to poor hatching (Whittow and Tazawa 1991). Since altricial bird preservation but sports a partial skeleton in brooding po- embryos and hatchlings show a strong allometry of skull, sition on top of the clutch. Fanti et al. (2012) erected the new taxon Nemegtomaia barsboldi based on MPC-D 107/15. wing, and hindlimb proportions, freshly hatched chicks are Most recently, oviraptor adult-clutch associations were dis- rather immobile, which forces them to remain in their nest at covered in southern China. Two specimens (AGM 4990 and least for some time (Whittow and Tazawa 1991). More pre- an uncatalogued specimen) were excavated from the fluvial cocial birds have already well developed walking legs and deposits of the Nanxiong Formation in Jiangxi Province, and musculature and are thus not that limited in their locomo- the uncatalogued specimen was preliminarily described by tion (Whittow and Tazawa 1991). Precocial birds do not Bi and Xu (2017). need as much “post-hatching” parental care as altricial birds Thus, to date at least seven oviraptorid clutches topped do. Furthermore, adults usually “brood” their hatchlings by an adult individual have been discovered, including during the first days after hatching also to protect them from five from the sand-dune deposits in Mongolia and Inner predation (Dial 2003; McLennan et al. 2004). These obser- Mongolia, China, and two from the fluvial Nanxiong For- vations on modern birds contribute to possible explanations mation of southern China (Table 1 and references therein). for the common occurrences of oviraptorid adult-associated The ones from the sand-dune deposits in Mongolia and clutches (Table 1 and references therein). Table 1. Characteristics of previously reported oviraptorid adult-associated clutches. The question mark means number of rings is still uncertain. Taxonomic Number of Stratigraphic Specimen number Clutch size Locality information References assignment rings horizon Bayn Dzak (The Flamming Oviraptor Djadokhta AMNH 6508/AMNH 6517 15 2 Cliffs = Shabarakh Usu), Osborn 1924 philoceratops Formation Mongolia 15 (visible), Djadokhta IGM 100/979 (Big Mamma)