Tuesday, May 11, 2021

[PaleoOrnithology • 2021] A Juvenile Specimen of Archaeorhynchus Sheds New Light on the Ontogeny of Basal Euornithines

Archaeorhynchus spathula Zhou & Zhang, 2006

in Foth, Wang, ... et Yang, 2021. 
Illustration: Frederik Spindler facebook.com/FrederikSpindler7

The ontogenetic development of extant birds is characterized by rapid growth, bone fusion and an early onset of flight ability. In contrast, little is known about how these ontogenetic traits evolved in the bird stem lineage, and the available data pertains primarily to Enantiornithes. Here, we describe an almost complete skeleton of a juvenile euornithine bird (LNTU-WLMP-18) from the Early Cretaceous Jiufotang Formation (Aptian), which was discovered near Lamadong Town (Jianchang County, Liaoning, China). Despite its completeness, bone preservation is rather poor. Thus, to increase the contrast between bone tissue and matrix, we used cyan-red-based autofluorescence photography. The specimen is more or less articulated and exposed in ventral aspect. The jaws are edentulous, the coracoid bears a procoracoid process, and the ischium lacks a proximodorsal process. The pedal unguals are short and barely curved, indicating a ground-dwelling lifestyle. Feathers, including long primaries, are present as carbonized traces. Several characters indicate that LNTU-WLMP-18 is a juvenile: the bone surface has a coarsely striated texture and no fusion is evident between the carpals and metacarpals, between the tibia and the astragalus and calcaneum, or among the metatarsals. Although juvenile characters have the potential to impede accurate identification of the specimen, morphological comparisons and cladistic analysis identify LNTU-WLMP-18 as most likely referable to the basal euornithine Archaeorhynchus, which would make the specimen the first juvenile bird from the Jehol Group that could be assigned to a specific taxon. Based on its size and the incomplete ossification of the bone surface, LNTU-WLMP-18 represents the smallest and therefore youngest known individual of this genus. A statistical comparison of limb proportions shows that the forelimbs of LNTU-WLMP-18 are significantly shorter than the hindlimbs, while the forelimbs are longer than the hindlimbs in subadult and adult individuals. This is different from the situation in some Enantiornithes, in which the forelimbs exceed the length of the hindlimbs even in hatchlings. Similar to Enantiornithes, Archaeorhynchus probably exhibit an early onset of flight ability, as indicated by the extensive wing plumage in LNTU-WLMP-18. Finally, the lack of gastroliths in the visceral cavity might indicate a dietary shift in Archaeorhynchus during ontogeny. As a small-bodied, ground-dwelling, seed-eating bird with a precocial ontogeny, Archaeorhynchus filled an ecological niche that later allowed early crown birds to survive the K-Pg mass extinction.

Keywords: Euornithes, ontogeny, Early Cretaceous, China, bird evolution

FIGURE 2. Overview of the skeleton of LNTU-WLMP-18.
 (A) LNTU-WLMP-18 under normal light. (B) LNTU-WLMP-18 under cyan-red autofluorescence. (C) Explanatory drawing of LNTU-WLMP-18.

 Anatomical abbreviations: cv cervicals; do dorsals; gc gastric content; lco left coracoid; lfe left femur; lhu left humerus; lil left ilium; lis left ischium; lma left metacarpus; lmt left metatarsus; lpd left pedal digits; lpu left pubis; lra left radius; lre left remiges; lsc left scapula; lti left tibia; lul left ulna; rfe right femur; rhu right humerus; ril right ilium; ris right ischium; rpd right pedal digits; rpu right pubis; rra right radius; rre right remiges; rsc right scapula; rti right tibia; rul right ulna; sac sacrum; sk skull. Scale bar 10 mm.

 Christian Foth, Shiying Wang, Frederik Spindler, Youhai Lin and Rui Yang. 2021. A Juvenile Specimen of Archaeorhynchus Sheds New Light on the Ontogeny of Basal Euornithines. Front. Earth Sci.  9:604520. DOI: 10.3389/feart.2021.604520