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, 109 (7), 2428-33

Oldest Known Dinosaurian Nesting Site and Reproductive Biology of the Early Jurassic Sauropodomorph Massospondylus

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Oldest Known Dinosaurian Nesting Site and Reproductive Biology of the Early Jurassic Sauropodomorph Massospondylus

Robert R Reisz et al. Proc Natl Acad Sci U S A.

Abstract

The extensive Early Jurassic continental strata of southern Africa have yielded an exceptional record of dinosaurs that includes scores of partial to complete skeletons of the sauropodomorph Massospondylus, ranging from embryos to large adults. In 1976 an incomplete egg clutch including in ovo embryos of this dinosaur, the oldest known example in the fossil record, was collected from a road-cut talus, but its exact provenance was uncertain. An excavation program at the site started in 2006 has yielded multiple in situ egg clutches, documenting the oldest known dinosaurian nesting site, predating other similar sites by more than 100 million years. The presence of numerous clutches of eggs, some of which contain embryonic remains, in at least four distinct horizons within a small area, provides the earliest known evidence of complex reproductive behavior including site fidelity and colonial nesting in a terrestrial vertebrate. Thus, fossil and sedimentological evidence from this nesting site provides empirical data on reproductive strategies in early dinosaurs. A temporally calibrated optimization of dinosaurian reproductive biology not only demonstrates the primary significance of the Massospondylus nesting site, but also provides additional insights into the initial stages of the evolutionary history of dinosaurs, including evidence that deposition of eggs in a tightly organized single layer in a nest evolved independently from brooding.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Geological section of Rooidraai, Golden Gate Highlands National Park, South Africa. Stratigraphic position of the nesting site within the uppermost (Early Jurassic) part of the Karoo Supergroup (A) and the upper Elliot Formation (B), respectively. Photograph (C) and illustration (D) of the cliff face showing the distribution of the clutches of eggs within the site, as well as additional depositional features. In total, 10 egg clutches (N1–N10) have been identified at Rooidraai, distributed across a 23-m-wide by 2-m-thick interval through the lower portion of the muddy siltstone unit. The egg clutches are found within at least four distinct levels in the 2-m-thick egg-bearing interval. The lowest clutch, N5, is found at the 2.6-m level, whereas the two clusters of eggs occur at the 3.05-m (N2 and N4) and the 3.35-m (N3 and N6) levels, respectively. Egg clutches N1, N7, and N8 are all distributed at approximately the same stratigraphic level, between 4.2 and 4.25 m. Two other clutches (N9 and N10) were recovered from the talus slope at the base of the nest-bearing cliff face; their source layer cannot be located precisely, but the clutches were clearly derived from the clutch-bearing interval.
Fig. 2.
Fig. 2.
Egg clutches recovered from Rooidrai. (A) Massospondylus (BP/1/5347a) egg clutch, showing the presence of two exposed skeletons; parts of 7 eggs (numbered) are preserved in this block, and fragments of 4 additional eggs are preserved in the counterpart block (BP/1/5347b, not shown). However, only 6 eggs are sufficiently complete to contain embryos. This clutch was collected as an isolated block in talus in 1976. Of the preserved near complete eggs, 5 contain embryonic remains, but only the remains in 2 eggs (N2 and N6) have been exposed through preparation and removal of most of the eggshell. (B) Part of the most completely prepared egg clutch that contains a total of 34 eggs (BP/1/6229). This clutch was preserved in situ in the cliff face. The matrix in the immediate area around the nest showed extensive bioturbation and lacked the fine laminations that normally characterize much of the nesting site, but there is no definite evidence of a nest beyond the organized nature of the egg clutches. (Scale bar, 5 cm.)
Fig. 3.
Fig. 3.
Footprints attributed to juvenile Massospondylus (BP/1/6923) from the nesting level at Rooidraai. (A) Pes print pair (BP/1/6923a). (B) Right manus print (BP/1/6923b). (C) Interpretive drawing of pes prints. (D) Interpretive drawing of manus print. (Scale bars, 10 mm.) cm, claw mark; lpI, first left pedal digit; lpII, second left pedal digit; lpIII, third left pedal digit; lpIV, fourth left pedal digit; pmI, phalangeal–metatarsal pad of the first pedal digit; pmII, phalangeal–metatarsal pad of the second pedal digit; pmIII+IV, coalesced phalangeal–metatarsal pad of the third and fourth pedal digits; rmI, first right manual digit; rmII, second right manual digit; rmIII, third right manual digit; rmIV, fourth right manual digit; rpI, first right pedal digit; rpII, second right pedal digit; rpIII, third right pedal digit; rpIV, fourth right pedal digit.
Fig. 4.
Fig. 4.
Time-calibrated cladogram of Dinosauria showing the known distribution of selected reproductive characters and the known distribution of fossil nests (thick bars). This illustration underscores the rarity of nest information in Dinosauria, and their temporal distribution. Although eggs of pterosaurs and crocodylomorphs have been recently reported, little useful nest information is available for these archosaurian outgroups to dinosaurs (24, 25) and is not included here. Square boxes to the Left of the major clade names indicate presence (solid box), absence (diagonally divided box), or unknown condition (empty box) for the following characters (listed above the boxes): 1, evidence of nesting-site fidelity; 2, evidence of gregarious nesting; 3, nests organization consisting of a single layer of tightly clustered eggs; 4, elongate egg shape (absence denotes spherical eggs); 5, evidence of brooding. Saurischian tree topology is from Zanno and Mackovicky (26) and Martínez et al. (27); ornithischian tree topology is from Butler et al. (28).
Fig. 5.
Fig. 5.
Plot of clutch mass versus body mass in archosaurs. Clutch volume versus adult body mass for extant archosaurs is divided into four taxon/care models (29). Color code for all graphs: green, crocodylian-maternal; black, bird-biparental; orange, bird-maternal; blue, bird-paternal. Red symbols represent nonavian dinosaurs: red diamonds, sauropods; red circles, hadrosaurs; red triangles, theropods other than birds; red star, Massospondylus, with range bars indicating minimum and maximum body mass estimates. Dinosaurs were not included in the regression models. The clutch size for Massospondylus precludes paternal care of offspring as suggested for some maniraptorans, but is consistent with the plesiomorphic condition of limited maternal care.

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