Sunday, April 3, 2016

[PaleoOrnithology • 2014] Eopengornis martini • Insights into the Evolution of Rachis Dominated Tail Feathers from A New Basal Enantiornithine (Aves: Ornithothoraces)

 Eopengornis martini 
Wang, O'Connor, Zheng, Wang, Hu & Zhou, 2014  

We report on a new enantiornithine Eopengornis martini gen. et sp. nov. from the lowest horizon of the Jehol Biota in Hebei, China; dated at 130.7 Mya, this is the second oldest avian bearing fossil deposit in the world, recording the First Appearance Datum of Enantiornithes. The new specimen, only the second enantiornithine and third bird reported from this horizon, preserves numerous synapomorphies with the largest Lower Cretaceous enantiornithine Pengornis houi from the Jiufotang Formation dated at 120 Mya. Together, they form a new avian lineage that lasted over 10 Myr, which is longer than any known clade of Lower Cretaceous enantiornithine. Eopengornis reveals new information about basal enantiornithine morphology such as the presence of a metatarsal V, helping to clarify the early evolution of these dominant Cretaceous avians. Furthermore, Eopengornis preserves a previously unrecognized tail morphology: a pair of elongate fully pennaceous rachis dominated feathers. This discovery confirms hypotheses proposing that the rachis dominated racket-plumes in basal birds represent modified pennaceous feathers. We suggest that the ornamental racket-plumes in enantiornithines and Confuciusornis evolved independently from the basal pygostylian condition, which we infer was a tail formed of normal flight feathers. 

Figure 1. Eopengornis martini gen. et sp. nov. STM24-1.
A, main slab, scale bar = 2 cm. B, close up of the caudal plumage, scale bar = 2 cm. C, close up of the right rectrix, scale bar = 5 mm.

Systematic palaeontology

Aves Linnaeus 1758
Ornithothoraces Chiappe 1995

Enantiornithes Walker 1981
Pengornithidae fam. nov
Phylogenetic definition: The most recent common ancestor of Eopengornis martini and P. houi and all its descendants.

Type genus: Pengornis IVPP V15336 Zhou, Clarke & Zhang, 2008.

Included genera: Eopengornis gen. nov. (present study).

Stratigraphic distribution: Huajiying and Jiufotang Formations, 120–130.7 Mya (Zhou, 2006; Jin et al., 2008; Pan et al., 2013).

Geographical distribution: Fengning, northern Hebei Province, and Lingyuan and Dapingfang, Chaoyang, north-eastern Liaoning Province, China.

Diagnosis: Medium to large enantiornithine birds (minor metacarpal projecting distally farther than major metacarpal; straight scapula; dorsolaterally excavated furcula with rami defining a V; metatarsal IV reduced) with the unique combination of the following characters: upper and lower jaws with numerous small teeth (over ten in the maxilla alone); scapular acromion process hooked; sternum with single pair of trabeculae, intermediate trabeculae absent; xiphial region of sternum defines wide V (xiphoid process absent); short pygostyle with rounded distal margin; cranial surface of proximal humerus flat; ulna 15% longer than the humerus; femur almost as long as the tibiotarsus; fibula almost reaching the proximal tarsals; metatarsal I > 30% the length of metatarsal II; first phalanx of hallux longest pedal phalanx; and pedal claws heterogeneous.

Eopengornis martini gen. et sp. nov. 

Holotype: STM24-1, an almost complete and fully articulated subadult individual preserved in a slab (primarily ventral view; see Supporting Information, Fig. S1) and counterslab (primarily dorsal view; Fig. 1), preserving remiges, rectrices, and contour feathers as an outline of the body.

Locality and horizon: Near Luozigou Village, Fengning, Hebei, China. Lower Cretaceous ‘Protopteryx horizon’ (130.7 Mya), the first sedimentary member of the Huajiying Formation (previously also called Dabeigou Formation or Dadianzi Formation) (Jin et al., 2008; Pan et al., 2013).

Etymology: The genus name refers to the older age of the new specimen relative to known specimens of Pengornis; the species name is in honour of the late Larry Martin for his numerous contributions to the field of palaeornithology.

Diagnosis: A pengornithid enantiornithine bird with the following unique characters: numerous, small, occlusally tapered and slightly recurved teeth in the upper and lower jaws; accessory foramina piercing nasal absent; fibula ends in a rounded expansion; and elongate hallux with metatarsal I and the proximal phalanx each almost half the length of metatarsal II.

Xiaoli Wang, Jingmai K. O'Connor, Xiaoting Zheng, Min Wang, Han Hu and Zhonghe Zhou. 2014. 
Insights into the Evolution of Rachis Dominated Tail Feathers from A New Basal Enantiornithine (Aves: Ornithothoraces). Biological Journal of the Linnean Society. [Special Issue: Celebrating Dinosaur Island] 113(3); 805–819. DOI: 10.1111/bij.12370