First Evidence of Dinosaurian Secondary Cartilage in the Post-Hatching Skull of Hypacrosaurus stebingeri (Dinosauria, Ornithischia)

Bone and calcified cartilage can be fossilized and preserved for hundreds of millions of years. While primary cartilage is fairly well studied in extant and fossilized organisms, nothing is known about secondary cartilage in fossils. In extant birds, secondary cartilage arises after bone formation during embryonic life at articulations, sutures and muscular attachments in order to accommodate mechanical stress. Considering the phylogenetic inclusion of birds within the Dinosauria, we hypothesized a dinosaurian origin for this “avian” tissue. Therefore, histological thin sectioning was used to investigate secondary chondrogenesis in disarticulated craniofacial elements of several post-hatching specimens of the non-avian dinosaur Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). Secondary cartilage was found on three membrane bones directly involved with masticatory function: (1) as nodules on the dorso-caudal face of a surangular; and (2) on the bucco-caudal face of a maxilla; and (3) between teeth as islets in the alveolar processes of a dentary. Secondary chondrogenesis at these sites is consistent with the locations of secondary cartilage in extant birds and with the induction of the cartilage by different mechanical factors - stress generated by the articulation of the quadrate, stress of a ligamentous or muscular insertion, and stress of tooth formation. Thus, our study reveals the first evidence of “avian” secondary cartilage in a non-avian dinosaur. It pushes the origin of this “avian” tissue deep into dinosaurian ancestry, suggesting the creation of the more appropriate term “dinosaurian” secondary cartilage

The 2014-2017 outburst of the young star ASASSN-13db: A time-resolved picture of a very low-mass star between EXors and FUors

ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the lightcurve and the dynamical and physical properties of the accretion flow. The 2014-2017 outburst lasted for nearly 800 days. A 4.15d period in the lightcurve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blueshifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of $\sim$5800-6000 K in the accretion flow. Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (\.{M}=1-3$\times$10$^{-7}$M$_\odot$/yr), about 2 orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settled after the begining of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars. (Abridged)

A global analysis of C. elegans trans-splicing

Trans-splicing of one of two short leader RNAs, SL1 or SL2, occurs at the 5′ ends of pre-mRNAs of many C. elegans genes. We have exploited RNA-sequencing data from the modENCODE project to analyze the transcriptome of C. elegans for patterns of trans-splicing. Transcripts of ∼70% of genes are trans-spliced, similar to earlier estimates based on analysis of far fewer genes. The mRNAs of most trans-spliced genes are spliced to either SL1 or SL2, but most genes are not trans-spliced to both, indicating that SL1 and SL2 trans-splicing use different underlying mechanisms. SL2 trans-splicing occurs in order to separate the products of genes in operons genome wide. Shorter intercistronic distance is associated with greater use of SL2. Finally, increased use of SL1 trans-splicing to downstream operon genes can indicate the presence of an extra promoter in the intercistronic region, creating what has been termed a “hybrid” operon. Within hybrid operons the presence of the two promoters results in the use of the two SL classes: Transcription that originates at the promoter upstream of another gene creates a polycistronic pre-mRNA that receives SL2, whereas transcription that originates at the internal promoter creates transcripts that receive SL1. Overall, our data demonstrate that >17% of all C. elegans genes are in operons