Two New Alvarezsaurian Dinosaurs Unearthed in China

Tuesday, August 28, 2018

Xu et al report two new Early Cretaceous alvarezsaurian theropods representing transitional stages in alvarezsaurian evolution. The analyses indicate that the evolutionary transition from a typical theropod forelimb configuration to a highly specialized one was slow and occurred in a mosaic fashion during the Cretaceous period. Image credit: Xu et al, doi: 10.1016/j.cub.2018.07.057.

Paleontologists in China have found fossil fragments from two new dinosaur species — named Xiyunykus pengi and Bannykus wulatensis — that walked the Earth approximately 120 million years ago (Cretaceous period).

Xiyunykus pengi and Bannykus wulatensis are both alvarezsaurs, an enigmatic group of dinosaurs that share many characteristics with birds.

Their bodies are slender, with a bird-like skull and many small teeth instead of the usual large, sharp cutting teeth of their meat-eating relatives.

“Alvarezsaurs are weird animals. With their strong, clawed hands and weak jaws, they appear to be the dinosaurian analogue to today’s aardvarks and anteaters,” said Professor Jonah Choiniere, from the University of the Witwatersrand.

Xiyunykus pengi was discovered in 2005 in Xinjiang, north-western China.

Bannykus wulatensis was discovered a few years later, in 2009, in Inner Mongolia, north-central China.

Skeletal anatomy of Xiyunykus pengi: (A) skeletal silhouette showing preserved bones (in gray); (B) histological thin-section of the fibula; arrows denote growth lines used to age the specimen; (C) left frontal in dorsal view; (D-F) partial braincase in left lateral (D), ventral (E), and posterior (F) views; (G and H) right dentary (G) and right surangular (H) in lateral view; (I) left articular in dorsal view; (J) vertebrae in left lateral view, including a middle cervical (left), a middle dorsal (middle left), the last sacral (middle right), and a posterior caudal (right); (K) left scapula and coracoid in lateral view; (L) left humerus in anterior view; (M) right ulna in anterior view; (N) partial left metatarsal III in ventral view. Scale bars – 100 mm for (A) and 500 μm for (B). Abbreviations: absf – anterior border of supratemporal fossa; bo – basioccipital; bt – basal tuber; bsr – basisphenoid recess; cg – curved groove; cp – cultriform process; ct – coracoid tubercle; dlf – dorsolateral flange; dpt – dorsomedially projecting tab; ec – exoccipital; if – infracondylar fossa; it – internal tuberosity; ld – lateral depression; oc – occipital condyle; op – olecranon process; pf – pneumatic fossa; rc – radial condyle; sor – subotic recess; uc – ulnar condyle; vk – ventral keel; vr – ventral ridge. Image credit: Xu et al, doi: 10.1016/j.cub.2018.07.057.

“When we described the first well-known alvarezsaur, Mononykus, in 1993, we were amazed at the contrast between its mole-like arms and its roadrunner-like body, but there were few fossils connecting it back to other theropod groups,” said Professor James Clark, a paleontologist at the George Washington University.

“However, alvarezsaurs did not always look this way. Early members of the group had relatively long arms with strong-clawed hands and typical meat-eating teeth.”

“Over time, the alvarezsaurs evolved into dinosaurs with mole-like arms and a single claw.”

“This transition plays out in an incremental fashion over more than 50 million years. It could one day potentially serve as a classic example of macroevolution akin to the ‘horse series’ of North America,” said Dr. Xing Xu, a researcher at the Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences.

Skeletal anatomy of Bannykus wulatensis: (A) skeletal silhouette showing preserved bones (in gray); (B) histological thin-section of the fibula; (C) left frontal in dorsal view; (D) basioccipital in ventral view; (E) left surangular in lateral view; (F-H) vertebrae in left lateral view, including a middle cervical (F), a middle dorsal (G), and a middle caudal (H); (I) left scapula and coracoid in lateral view; (J and K) left humerus (J) and left ulna (K) in anterior view; (L) left manus in anterior view; (M) left ilium in lateral view; (N) right femur in posterior view; (O) right metatarsals in posterior view. Scale bars – 100 mm for (A) and 500 μm for (B). Abbreviations: absf – anterior border of supratemporal fossa; bt – basal tuber; ct – coracoid tubercle; ecc – ectepicondyle; enc – entepicondyle; fmcIII – facet for metacarpal III; lpp – laterally protruding parapophysis; mc II-III – metacarpals II-III; mdc – medial distal condyle; mt II-V – metatarsals II-V; pof – popliteal fossa; oc – occipital condyle; op – olecranon process; pf – pneumatic fossa; rc – radial condyle; uc – ulnar condyle; sc – surangular crest; sf – surangular foramen. Image credit: Xu et al, doi: 10.1016/j.cub.2018.07.057.

The discovery of Xiyunykus pengi and Bannykus wulatensis allowed the team to uncover an important shift in how the specialized features of the alvarezsaurs evolved.

“It can be hard to pin down the relationships of highly specialized animals,” Professor Choiniere said.

“But fossil species with transitional features, like Xiyunykus pengi and Bannykus wulatensis, are tremendously helpful because they link bizarre anatomical features to more typical ones.”

“The new fossils have long arms, and so show that alvarezsaurs evolved short arms only later in their evolutionary history, in species with small body sizes,” said Oxford University’s Professor Roger Benson.

“This is quite different to what happens in the classic example of tyrannosaurs, which have short arms and giant size.”

The findings appear in the journal Current Biology.

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Xing Xu et al. Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution. Current Biology, published online August 23, 2018; doi: 10.1016/j.cub.2018.07.057

Source: www.sci-news.com

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