| Nipponoolithus | |
|---|---|
Egg fossil classification | |
| Oofamily: | incertae sedis |
| Oogenus: | †Nipponoolithus Tanaka et al. 2016 |
| Oospecies | |
Nipponoolithus is anoogenus offossil egg native toJapan. It is one of the smallest known dinosaur eggs, and was probably laid by some kind of non-avianmaniraptor.[1]
Nipponoolithus remains are known from theSasayama Group in Kamitaki, southeasternTamba, Hyogo, Japan. The site is dated to the lowerAlbian.[1]
Though fossil dinosaur skeletons are rare in Japan, they have been well-documented since1978.[1] The first Japanese fossil eggs were discovered in2003: Yoichi Azuma documented numerous eggs of theRatite morphotype in theKitadani Formation,[2] and Ren Hirayamaet al. documented dinosaur and turtle eggshells in theKuwajima Formation.[3] However, these eggs were never described in detail.[1] In2006, the dinosaur-rich Kamitaki locality in the Sasayama Group was discovered. In2016, a team of paleontologists from Japan and Canada collected numerous fossilized eggs at Kamitaki, including the specimens which they would refer to a new oogenus and oospecies:Nipponoolithus ramosus.[1]
Nipponoolithus ramosus is known only from a handful of isolated eggshell fragments ranging from 0.36 to 0.53 mm in thickness, just barely larger than a chicken egg.[1][4] It is estimated, based on the eggshell thickness, thatNipponoolithus eggs weighed about 100 grams (3.5 oz), making it among the smallest fossil dinosaur eggs ever discovered.[1]
Like most non-aviandinosaur eggs, it has two layers:[5] the outer continuous layer, and the inner mammillary layer.[1] The continuous layer is two to four times thicker than the mammillary layer.Nipponoolithus's outer surface is ornamented with low, branched ridges, similar to ornamentation observed inMacroelongatoolithus,Montanoolithus,Paraelongatoolithus,Reticuloolithus,Spheruprismatoolithus, and the eggs ofDeinonychus.[1] The crystals in the mammillary layer areacicular or wedge-like.[1]
N. rumosus was most likely laid by some kind of non-avianmaniraptor because of its similarities in microstructure and ornamentation tooviraptorosaur eggs and the eggs ofDeinonychus. Other very small theropod eggs (ranging in size from 27 grams (0.95 oz) to 135 grams (4.8 oz)), includingElongatoolithus,Prismatoolithus (and other indeterminateprismatoolithids), as well as ornithopod eggs assigned toSpheroolithus, are also known from the Kamitaki site. These eggs, along with skeletal remains, show that the parents ofNipponoolithus coexisted with numerous other small theropods, as well as an assemblage ofankylosaurs,titanosaurs,hadrosauroids,tyrannosaurs, andtherizinosaurs. The parent ofNipponoolithus probably weighed roughly 15 kilograms (33 lb), comparable to the size of some contemporary small theropods from theJehol biota in China.[1]
Nipponoolithus has not been place into any described oofamily. It contains a single oospecies:N. ramosus.[1]