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. 2018 Jan;232(1):80-104.
doi: 10.1111/joa.12719. Epub 2017 Nov 8.

The Evolution of the Manus of Early Theropod Dinosaurs Is Characterized by High Inter- And Intraspecific Variation

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The Evolution of the Manus of Early Theropod Dinosaurs Is Characterized by High Inter- And Intraspecific Variation

Daniel E Barta et al. J Anat. .
Free PMC article

Abstract

The origin of the avian hand, with its reduced and fused carpals and digits, from the five-fingered hands and complex wrists of early dinosaurs represents one of the major transformations of manus morphology among tetrapods. Much attention has been directed to the later part of this transition, from four- to three-fingered taxa. However, earlier anatomical changes may have influenced these later modifications, possibly paving the way for a later frameshift in digit identities. We investigate the five- to four-fingered transition among early dinosaurs, along with changes in carpus morphology. New three-dimensional reconstructions from computed tomography data of the manus of the Triassic and Early Jurassic theropod dinosaurs Coelophysis bauri and Megapnosaurus rhodesiensis are described and compared intra- and interspecifically. Several novel findings emerge from these reconstructions and comparisons, including the first evidence of an ossified centrale and a free intermedium in some C. bauri specimens, as well as confirmation of the presence of a vestigial fifth metacarpal in this taxon. Additionally, a specimen of C. bauri and an unnamed coelophysoid from the Upper Triassic Hayden Quarry, New Mexico, are to our knowledge the only theropods (other than alvarezsaurs and birds) in which all of the distal carpals are completely fused together into a single unit. Several differences between the manus of C. bauri and M. rhodesiensis are also identified. We review the evolution of the archosauromorph manus more broadly in light of these new data, and caution against incorporating carpal characters in phylogenetic analyses of fine-scale relationships of Archosauromorpha, in light of the high degree of observed polymorphism in taxa for which large sample sizes are available, such as the theropod Coelophysis and the sauropodomorph Plateosaurus. We also find that the reduction of the carpus and ultimate loss of the fourth and fifth digits among early dinosaurs did not proceed in a neat, stepwise fashion, but was characterized by multiple losses and possible gains of carpals, metacarpals and phalanges. Taken together, the high degree of intra- and interspecific variability in the number and identities of carpals, and the state of reduction of the fourth and fifth digits suggest the presence of a 'zone of developmental variability' in early dinosaur manus evolution, from which novel avian-like morphologies eventually emerged and became channelized among later theropod clades.

Keywords: Coelophysis bauri; Megapnosaurus rhodesiensis; archosaur; birds; carpus; dinosaur; manus; theropod.

Figures

Figure 1
Figure 1
Coelophysis bauri left manus. (A) AMNH FARB 30631. (B) 3D reconstruction in dorsal view. (C) 3D reconstruction in palmar view (B) and (C) © American Museum of Natural History.
Figure 2
Figure 2
Megapnosaurus rhodesiensis right manus. (A) QG 577. (B) 3D reconstruction in dorsal view. (C) 3D reconstruction in palmar view.
Figure 3
Figure 3
AMNH FARB 30631 Coelophysis bauri carpals. (A) Radiale. (B) Fused intermedium (?) and ulnare. (C) Centrale. (D) Fused distal carpals 1 and 2. (E) Fused distal carpals 3 and 4. All carpals are shown (respectively from top to bottom) in proximal, dorsal, medial, lateral, ventral/palmar, and distal views © American Museum of Natural History.
Figure 4
Figure 4
AMNH FARB 30631 Coelophysis bauri metacarpals (MCs). (A) MC I. (B) MC II. (C) MC III. (D) MC IV. (E) MC V. (F) Enlargement of (E) to show detail. All metacarpals are shown (from top of page to bottom) in proximal, dorsal, lateral, medial, ventral/palmar, and distal views © American Museum of Natural History.
Figure 5
Figure 5
QG 577 Megapnosaurus rhodesiensis metacarpals (MCs). (A) MC IV. (B) MC III. (C) MC II. (D) MC I. All metacarpals are shown (from top of page to bottom) in proximal, dorsal, lateral, medial, ventral/palmar, and distal views.
Figure 6
Figure 6
QG 573 Megapnosaurus rhodesiensis metacarpals (MCs). (A) Left MC IV. (B) Right MC III and IV. (C) Right MC II. (D) Right MC I. All metacarpals are shown (from top of page to bottom) in proximal, dorsal, lateral, medial, ventral/palmar, and distal views.
Figure 7
Figure 7
QG 577 Megapnosaurus rhodesiensis (A–D) and AMNH FARB 30631 Coelophysis bauri (E–H) manual phalanges. (A) M. rhodesiensis right phalanges I‐1 and I‐2. (B) M. rhodesiensis right phalanges II‐1, II‐2, and II‐3 (partial). (C) M. rhodesiensis right phalanges III‐1, III‐2, III‐3, and III‐4 (partial). (D) M. rhodesiensis right phalanx IV‐1. (E). C. bauri left phalanges I‐1 and I‐2. (F) C. bauri left phalanges II‐1, II‐2, and II‐3. (G) C. bauri left phalanges III‐1, III‐2, III‐3, and III‐4. (H) C. bauri left phalanx IV‐1. Each phalanx is shown (from top of page to bottom) in dorsal, ventral/palmar, lateral, and articular surface views (E)–(H) © American Museum of Natural History.
Figure 8
Figure 8
Variation among carpi. All except (F) are Coelophysis bauri, and all are shown in dorsal view except (C), which is shown in palmar view. (A) AMNH FARB 30631. Note the presence of a centrale and possible coossification of the intermedium and ulnare © American Museum of Natural History. (B) MCZ 4329. Note the lack of a centrale and the possible pisiform. (C) MCZ 4331. Note the presence of a free intermedium. (D) AMNH FARB 7243. Note the presence of a free intermedium. (E) NMMNH P‐42576. Note the fused row of distal carpals and a fifth metacarpal. (F) Unnamed Hayden Quarry coelophysoid GR 1033. Note the fused row of distal carpals. (G) AMNH FARB 7227. Note a possible free intermedium. Scale bar increment is 10 mm. (H) CM 81766. Note a possible coossified intermedium and ulnare, a possibly free third distal carpal. c, centrale; dc, distal carpal; fdcs, fused distal carpals; i, intermedium; mcI, metacarpal I; mcV, metacarpal V; p, pisiform; r, radius; re, radiale; u, ulna; ue, ulnare. Image credit for (B) and (C): Museum of Comparative Zoology, Harvard University, © President and Fellows of Harvard College.
Figure 9
Figure 9
Variation in Plateosaurus engelhardti carpi. (A) SMNS 91297 in dorsal view. Note the absence of an intermedium. (B) AMNH FARB 2595 (cast of AMNH FARB 6810) in dorsal view. Note the presence of an intermedium. Rectangle shows region in (D). (C) AMNH FARB 6810 in dorsal view. Compare with (B) and (D) and note the ‘absence’ of the intermedium, which may have been moved to a position over the fourth metacarpal prior to mounting of the specimen. The third distal carpal was also shifted slightly, and the dorsal notch in the first distal carpal was filled with plaster, giving the appearance of two separate elements in the final mounted skeleton. (D) Close‐up of AMNH FARB 2595 (= AMNH FARB 6810) carpus in dorso‐proximal view. dc, distal carpal; i, intermedium. Scale bar: (A,B) 50 mm.
Figure 10
Figure 10
Composite cladogram illustrating the approximate relationships of select amniote taxa discussed in the text. No preserved carpi are known for the taxa listed in faded gray text. The tree topology is based on those of Müller & Reisz (2006), Nesbitt (2011), Reisz et al. (2011), Pritchard et al. (2016), Nesbitt et al. (2015), and Ezcurra (2016).
Figure 11
Figure 11
Cladogram of early dinosaur left hands in dorsal view with phalangeal formulas and simplified diagrams of ossified carpals. Tree topology from Nesbitt & Ezcurra (2015). Illustrations by Nicole Wang, redrawn and modified from (L‐R) Santa Luca (1980), Sereno et al. (2012), this study, Sereno (1993), Burch (2014), Martinez et al. (2011), this study, Xu et al. (2009), Gilmore (1920), and Madsen (1976). R, radiale; I, intermedium; U, ulnare; C, centrale; P, pisiform; 1–5, distal carpals 1–5. Dashed lines around carpals in Coelophysis and Plateosaurus indicate the variable presence of those elements. Question marks after a carpal name indicate uncertainty in the identification of that element. Overlapping blocks indicate fused elements. Identifications of Tawa carpals follow Burch (2013). No ossified carpals are known for Ceratosaurus (Carrano & Choiniere, 2016). The number and identities of Dilophosaurus distal carpals remain uncertain (Welles, 1984).

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