Middleton, KM
The morphological basis of hallucal orientation in extant birds
Journal of Morphology , 250 (1): 51-60 2001
The perching foot of living birds is commonly characterized by a reversed or opposable digit I (hallux). Primitively, the hallux of nonavian theropod dinosaurs was unreversed and lay parallel to digits II-IV. Among basal birds, a unique digital innovation evolved in which the hallux opposes digits II-IV. This digital configuration is critical for grasping and perching. I studied skeletons of modern birds with a range of hallucal designs, from unreversed (anteromedially directed) to fully reversed (posteriorly directed). Two primary correlates of hallucal orientation were revealed. First, the fossa into which metatarsal I articulates is oriented slightly more posteriorly on the tarsometatarsus, rotating the digit as a unit. Second, metatarsal I exhibits a distinctive torsion of its distal shaft relative to its proximal articulation with the tarsometatarsus, reorienting the distal condyles and phalanges of digit I. Herein, I present a method that facilitates the re-evaluation of hallucal orientation in fossil avians based on morphology alone. This method also avoids potential misinterpretations of hallucal orientation in fossil birds that could result from preserved appearance alone.
Middleton, KM; Gatesy, SM
Theropod forelimb design and evolution
Zoological Journal of the Linnean Society , 128 (2): 149-187 2000
We examined the relationship between forelimb design and function across the 230-million-year history of theropod evolution. Forelimb disparity was assessed by plotting the relative contributions of the three main limb elements on a ternary diagram. Theropods were divided into five functional groups: predatory, reduced, flying, wing-propelled diving, and flightless. Forelimbs which maintained their primitive function, predation, are similarly proportioned, but non-avian theropods with highly reduced forelimbs have relatively longer humeri. Despite the dramatically different forces imparted by the evolution of flight, forelimb proportions of basal birds are only slightly different from those of their non-avian relatives. An increase in disparity accompanied the subsequent radiation of birds. Each transition to flightlessness has been accompanied by an increase in relative humeral length, which results from relatively short distal limb elements. We introduce theoretical predictions based on five biomechanical and developmental factors that may have influenced the evolution of theropod limb proportions.
Gatesy, SM; Middleton, KM; Jenkins, FA; Shubin, NH
Three-dimensional preservation of foot movements in Triassic theropod dinosaurs
Nature, 399 (6732): 141-144 1999
Dinosaur footprints have been used extensively as biostratigraphic markers, environmental indicators, measures of faunal diversity and evidence of group behaviour. Trackways have also been used to estimate locomotor posture, gait and speed, but most prints, being shallow impressions of a foot's plantar surface, provide little evidence of the details of limb excursion. Here we describe Late Triassic trackways from East Greenland, made by theropods walking on substrates of different consistency and sinking to variable depths, that preserve three-dimensional records of foot movement. Triassic theropod prints share many features with those of ground-dwelling birds, but also demonstrate significant functional differences in position of the hallux (digit I), foot posture and hindlimb excursion.
Gatesy, SM; Middleton, KM
Bipedalism, flight, and the evolution of theropod locomotor diversity
Journal of Vertebrate Paleontology , 17 (2): 308-329 1997
The evolution of theropod flight has been characterized as a shift from one to three locomotor modules. Basal theropods, which were terrestrial bipeds, had a single locomotor module composed of the hind limb and tail. In birds, aerial locomotion was acquired with the origination of the wing module and a decoupling of the hind limb and tail into separate pelvic and caudal modules. This increase in modularity is thought to have granted birds more locomotor "options" than non-avian theropods. More specifically, an aerial locomotor system could have eased constraints on the hind limb and allowed specialization for habitats and lifestyles unavailable to non-birds. If so, bird hind limbs should be more disparate than those of non-avian theropods. We addressed this hypothesis by visualizing one aspect of limb design, the proportions of the three main segments, using ternary diagrams. Our results show that avian hind limb proportions are much more disparate than those of non-avian theropods. This broad range of limb design correlates with a radiation in locomotor diversity founded on three locomotor modules. We propose that birds have reached regions of proportion morphospace that are off limits to bipeds with only one locomotor module. In comparison, the limbs of non-avian theropods are conservatively proportioned. Despite great variation in body size, theropods other than birds do not exhibit specializations for locomotion other than terrestrial bipedalism. Although ether aspects of size and shape need to be analyzed, the relationship between modular flexibility and morphological disparity appears to play an important role in theropod locomotor evolution.
