Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features

Abstract

Ankylosaurid ankylosaurs were quadrupedal, herbivorous dinosaurs with abundant dermal ossifications. They are best known for their distinctive tail club composed of stiff, interlocking vertebrae (the handle) and large, bulbous osteoderms (the knob), which may have been used as a weapon. However, tail clubs appear relatively late in the evolution of ankylosaurids, and seemed to have been present only in a derived clade of ankylosaurids during the last 20 million years of the Mesozoic Era. New evidence from mid Cretaceous fossils from China suggests that the evolution of the tail club occurred at least 40 million years earlier, and in a stepwise manner, with early ankylosaurids evolving handle-like vertebrae before the distal osteoderms enlarged and coossified to form a knob.

Keywords: Ankylosauria; Ankylosauridae; Cretaceous; Dinosauria.

Figure 1

Three hypotheses for the evolution of the ankylosaurid tail club. In the knob-first hypothesis, knob osteoderms that completely envelop the terminus of the tail should appear before handle vertebrae in the fossil record. In the handle-first hypothesis, handle vertebrae should appear in the fossil record before the knob osteoderms completely envelop the terminus of the tail. In the tandem hypothesis, the tail club handle and knob appear at about the same time in the fossil record. Lateral caudal osteoderm pattern modified after MPC 100/1305, cf. Pinacosaurus.

Figure 2

Caudal anatomy of ankylosaurs. (A) UALVP 47273, Dyoplosaurus acutosquameus tail club, in slightly oblique dorsal view, anterior is to the left. This represents the typical morphology of derived ankylosaurine tail clubs, with lengthened prezygapophyses interlocking with the neural spines of the adjacent vertebra, and large terminal osteoderms that envelop the tip of the tail (although the knob can be substantially wider in some specimens). (B) HGM 41HIII-002, Gobisaurus domoculus (= ‘Zhongyuansaurus’), tail club handle in left dorsolateral view, drawn from Xu et al. (2007). (C) HGM 41HIII-002, handle in right ventrolateral view; the deep groove along the bottom is the haemal canal. The terminal vertebra appears to be present: in X-ray images of the ankylosaurid tail club knob UALVP 16247 (X-ray in D, interpretive drawing in E; posterior is up) the terminal vertebra is a small nub compared with the long distal caudals of the handle. (F) MWC 5819, Mymoorapelta maysi, two distal caudal vertebrae in right lateral view, mirrored for comparative purposes (anterior is to the left), showing the typical distal caudal morphology for basal ankylosaurs and nodosaurids. The prezygapophyses overlap the preceding vertebra by about 25% the length of the centrum. (G) IVPP V12560, Liaoningosaurus paradoxus whole specimen in ventral view, anterior is to the left, box outlines area magnified in (H). (H) Distal caudal vertebrae of IVPP V12560, anterior is to the left, scale is in millimeters. The prezygapophyses overlap the preceding vertebra by at least 50% of the centrum length, similar to what is observed in ankylosaurid tail clubs. c, centrum; ha, haemal arch; hc, haemal canal; kn, knob; ns, neural spine; prz, prezygapophyses; poz, postzygapophyses.

Figure 3

Ancestral state reconstruction for the presence of elongated prezygapophyses (characteristic of the tail club handle), and enlarged knob osteoderms, showing proportional likelihoods, using the 50% majority rule phylogenetic tree from Arbour & Currie (in press).

Figure 4

Fifty percentage majority rule phylogenetic tree from Arbour & Currie (in press) showing acquisition of characters of the tail club in a stratigraphic context. Unmodified distal caudal vertebrae are found in basal ankylosaurs (Mymoorapelta maysi), nodosaurids (Sauropelta edwardsorum), and the basal ankylosaurid Gastonia burgei; in these taxa the prezygapophyses overlap no more than 25% of the preceding vertebra. Liaoningosaurus paradoxus and Gobisaurus domoculus have distal caudal vertebrae with prezygapophyses that overlap at least 50% of the preceding vertebra. Pinacosaurus grangeri and all more derived ankylosaurines have a complete tail club with handle vertebrae and knob osteoderms. Only an incomplete tail club handle is known for Talarurus plicatospineus.