Embryos of therizinosauroid theropods from the Upper Cretaceous of China: diagnosis and analysis of ossification patterns

Abstract

Embryos of therizinosauroid theropods from the Upper Cretaceous of China: diagnosis and analysis of ossification patterns. -Acta Zoologica (Stockholm) 89 : 231-251Exceptionally complete, in ovo dinosaur embryos from the Upper Cretaceous of China are analysed. Ossification patterns of these embryos suggest that they died during the final third of their development. The therizinosauroid identity of the embryos follows from: (1) an edentulous premaxilla with a sharp downturned edge; (2) dentary with a lateral shelf; (3) teeth with fan-shaped crowns, with a few marginal cusps; (4) humerus with a massive deltopectoral crest extending proximally, with a pointed proximomedial tuberosity; (5) ilium with an expanded and hooked preacetabular process; (6) strongly curved hypertrophied manual unguals tapering to sharp points. These embryos are closest to two Chinese therizinosauroids, Neimongosaurus yangi Zhang et al . 2001 and Erliansaurus bellamanus Xu et al . 2002. An elongated narial opening, reduced basipterygoid process, low cervical neural spines, a transversely narrow pubic apron, and a pubic foot expanded anteriorly are found in these embryos and are synapomorphies uniting the Therizinosauroidea and the Oviraptorosauria. Fusion of cervical and caudal neural arches and centra, complete ossification of thoracic ribs and ilium, possible co-ossification of tibia and fibula, fused pubes, complete metaand acropodial elements, together with small portions of unossified epiphyses of long bones suggest an advanced precociality of these embryos.

Histology and postural change during the growth of the ceratopsian dinosaur Psittacosaurus lujiatunensis

“…Even hatchlings of at least some therizinosaurian theropods may have been preferentially quadrupedal, given the existence of embryos whose forelimbs are nearly equal in length to their hindlimbs 25 , although in the absence of preserved hatchlings this inference must be considered uncertain. An ontogenetic shift from quadrupedality to bipedality has also been inferred in the ornithopod Dryosaurus, based on changes in the femoral cross-sectional geometry during growth 26 .…”

“…Such shifts have now been inferred to have taken place in some members of at least two of the three major dinosaurian clades, Sauropodomorpha and Ornithischia. There is even a possible example of such a shift within the third clade, Theropoda, based on the elongate forelimbs that have been reported in therizinosaurian embryos 25 . However, the humerus measures 55-65% of the length of the femur in some theropod embryos, including oviraptorid 27 and troodontid 28 specimens, in striking contrast to the near-equality in length between the humerus and femur seen in the therizinosaurians 25 .…”

Cranial anatomy ofErlikosaurus andrewsi(Dinosauria, Therizinosauria): new insights based on digital reconstruction

“…The floor of the narial chamber is deeply excavated and forms a prominent fossa, which is clearly separated from the incisive foramen. The presence of similarly developed fossae in a therizinosauroid embryo referred to Neimongosaurus or Erliansaurus (Kundrat et al, 2008), and the basal taxon Jianchangosaurus yixianensis (Pu et al, 2013) suggests an ontogenetically early development and more widespread distribution of this feature amongst Therizinosauria.…”

Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development

“…The estimated volume of the eggs based on similarly proportioned ellipsoid reptile eggs [(π/6000)LD 2 (91)] is 177.98 cc, making them the smallest eggs that can be definitively referred to a nonavian dinosaur yet discovered. [The next largest eggs containing identifiable dinosaur embryos are 9 × 7-cm therizinosaur eggs with an estimated volume of 230.79 cc (47).] For P. andrewsi, the estimated mass of the egg and embryo [vol.…”

“…However, this also predicts rapid values (68-71 d) because the regression line for crocodilians intercepts avian rates when extrapolated out to the grossly larger sauropod egg sizes.] Direct determination of dinosaurian incubation duration has been considered intractable (43,47). However, in crocodilian (48) and human (49) embryonic teeth, incremental lines of von Ebner that reflect diurnal pulses of mineralization during odontogenesis are present.…”