Field of Science

Embryonic Skeletal Morphology of the Jurassic Sauropodomorph Massospondylus

A few weeks ago I mentioned an upcoming article on sauropodomorph embryos from the Lower Jurassic of South Africa.  That article is now published in the Journal of Vertebrate Paleontology. This find represents the oldest occurrence of dinosaur embryonic material, but it also represents one of the oldest records of eggshell in the fossil record.  As far as I am aware (and someone please correct me if I am wrong) the only occurence of fossil eggs in the Triassic were associated with hatchlings of the sauropodomorph Mussaurus from the Upper Triassic El Tranquilo Formation of Argentina (Bonaparte and Vince, 1979). No calcified eggs are known from the Permian or earlier (Hirsch, 1979). Having worked in Cretaceous units where fossil eggshell is ubiquitous I'm always been curious about the lack of fossil eggshell in the Triassic. Years back I suggested that maybe fully calcified eggs hadn't developed yet, but was told by someone purportedly knowledgeable in fossil eggs that they were present, I just didn't know what look for.  I'm not really advocating my initial suggestion, but I'm not buying the latter explanation either.  Anyone else have any thoughts?

Reisz, R. R. , Evans, D. C. , Sues, H.-D. and D. Scott. 2010. Embryonic skeletal anatomy of the sauropodomorph dinosaur Massospondylus from the Lower Jurassic of South Africa. Journal of Vertebrate Paleontology 30:1653-1665. DOI:10.1080/02724634.2010.521604

Abstract - Two embryonic skeletons preserved inside thin-shelled eggs of a partially preserved clutch from the Upper Elliot Formation (Lower Jurassic) of South Africa have been attributed to the sauropodomorph dinosaur Massospondylus carinatus. A virtually complete skeleton is exposed in right lateral view, with the slightly telescoped skull and several cervical vertebrae extending beyond the eggshell. A second, partial skeleton has a skull preserved in dorsal view. The embryos have proportionately very large skulls, with the broad skull table formed by wide parietals and frontals. The wide posterolateral wing of the frontal separates the postorbital from contact with the parietal. The embryos have short rather than elongated cervical vertebrae, with tall rather than low neural arches. The large forelimbs are only slightly shorter than the hind limbs, which suggests an obligatory quadrupedal posture for the hatchlings. This pattern may represent an ontogenetic constraint related to the large size of the head and horizontally oriented neck. Similarities between the embryonic and post-hatchling specimens include the slenderness of the lower jaw and slight ventral curvature of the symphyseal portion of the dentary, the large supraorbital process of the prefrontal, and the tall antorbital and infratemporal fenestrae. There are 10 cervical, 14 dorsal, and three sacral vertebrae. The large distal claw-bearing phalanx of manual digit 1 is longer than any other phalangeal element of either manus or pes. The embryos of Massospondylus carinatus represent the oldest dinosaurian embryos known to date.

5 comments:

  1. Theropods and ornithiscians both have hard-shelled eggs, too. While there are ultrastructural differences, hard shelled eggs are almost certainly the primitive condition for dinosaurs. That being said, nondinosaurs have leathery eggs primitively, with only geckos and a few crocs and turtles bucking the trend. So in the Cretaceous, basically all of the terrestrial animals that laid eggs were laying hard-shelled ones- and lots of them! While in the Triassic, most terrestrial things (re: crurotarsans) were laying leathery eggs.

    So, while hard shelled eggs almost certainly had evolved before the Norian it isn't surprising to me that there are no shell fragments.

    ReplyDelete
  2. Excellent! Thanks for the information.

    ReplyDelete
  3. The "eggs" reported by Bonaparte and Vince are tiny compared to the hatchlings of Mussaurus, and I am not convinced that they really belong to this dinosaur. There are additional references to eggshell, but, to the best of my knowledge, this material has never been documented. The absence of any pre-Jurassic eggshell to date is very odd.

    ReplyDelete

Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="http://www.fieldofscience.com/">FoS</a> = FoS