Abstract:The avian lung is highly specialized and is both functionally and morphologically distinct from that of their closest extant relatives, the crocodilians. It is highly partitioned, with a unidirectionally ventilated and immobilized gas‐exchanging lung, and functionally decoupled, compliant, poorly vascularized ventilatory air‐sacs. To understand the evolutionary history of the archosaurian respiratory system, it is essential to determine which anatomical characteristics are shared between birds and crocodilians… Show more
“…This recent research shows that some presumed ‘bird-like’ respiratory features, such as unidirectional air flow, are actually plesiomorphies characterizing much larger groups ( Schachner et al, 2014 ; Farmer and Sanders, 2010 ) and highlights the diversity of ways in which multiple anatomical systems interlink to effectively ventilate the lungs. New work on the pulmonary anatomy of the ostrich ( Struthio camelus ) has demonstrated that PSP relationships with the respiratory system in extant birds may not be as straightforward as previously thought ( Schachner et al, 2021 ), and reconstructions of dinosaur lungs that directly follow a standardized avian bauplan may need to be reconsidered. Additionally, primitive features like gastralia, simple sterna, and ‘propubic’ pelves impede attempts at completely superimposing the highly derived physiology of birds onto comparatively less-specialized clades like non-avian theropods and sauropods.…”
Ornithischian dinosaurs were ecologically prominent herbivores of the Mesozoic Era that achieved a global distribution by the onset of the Cretaceous. The ornithischian body plan is aberrant relative to other ornithodiran clades, and crucial details of their early evolution remain obscure. We present a new, fully articulated skeleton of the early branching ornithischian Heterodontosaurus tucki. Phase-contrast enhanced synchrotron data of this new specimen reveal a suite of novel postcranial features unknown in any other ornithischian, with implications for the early evolution of the group. These features include a large, anteriorly projecting sternum; bizarre, paddle-shaped sternal ribs; and a full gastral basket – the first recovered in Ornithischia. These unusual anatomical traits provide key information on the evolution of the ornithischian body plan and suggest functional shifts in the ventilatory apparatus occurred close to the base of the clade. We complement these anatomical data with a quantitative analysis of ornithischian pelvic architecture, which allows us to make a specific, stepwise hypothesis for their ventilatory evolution.
“…This recent research shows that some presumed ‘bird-like’ respiratory features, such as unidirectional air flow, are actually plesiomorphies characterizing much larger groups ( Schachner et al, 2014 ; Farmer and Sanders, 2010 ) and highlights the diversity of ways in which multiple anatomical systems interlink to effectively ventilate the lungs. New work on the pulmonary anatomy of the ostrich ( Struthio camelus ) has demonstrated that PSP relationships with the respiratory system in extant birds may not be as straightforward as previously thought ( Schachner et al, 2021 ), and reconstructions of dinosaur lungs that directly follow a standardized avian bauplan may need to be reconsidered. Additionally, primitive features like gastralia, simple sterna, and ‘propubic’ pelves impede attempts at completely superimposing the highly derived physiology of birds onto comparatively less-specialized clades like non-avian theropods and sauropods.…”
Ornithischian dinosaurs were ecologically prominent herbivores of the Mesozoic Era that achieved a global distribution by the onset of the Cretaceous. The ornithischian body plan is aberrant relative to other ornithodiran clades, and crucial details of their early evolution remain obscure. We present a new, fully articulated skeleton of the early branching ornithischian Heterodontosaurus tucki. Phase-contrast enhanced synchrotron data of this new specimen reveal a suite of novel postcranial features unknown in any other ornithischian, with implications for the early evolution of the group. These features include a large, anteriorly projecting sternum; bizarre, paddle-shaped sternal ribs; and a full gastral basket – the first recovered in Ornithischia. These unusual anatomical traits provide key information on the evolution of the ornithischian body plan and suggest functional shifts in the ventilatory apparatus occurred close to the base of the clade. We complement these anatomical data with a quantitative analysis of ornithischian pelvic architecture, which allows us to make a specific, stepwise hypothesis for their ventilatory evolution.
“…3D surface models were segmented in Avizo 7.1 and 2020.3 (Thermo Fisher Scientific) following the methods established for extant archosaur lungs (Farmer & Sanders, 2010; Lawson et al, 2021; Sanders & Farmer, 2012; Schachner et al, 2013, 2017, 2021). For a detailed description of the segmentation methodology used, see Lawson et al (2021).…”
Section: Methodsmentioning
confidence: 99%
“…Quantitative measurements of the airways were collected from the DICOM editor and viewer OsiriX MD (http://www.osirix-viewer.com) and are included in Table S1. These measures have been established in previous work (Lawson et al, 2021; Schachner et al, 2021), and were based upon hypothesized homologous structures in other crocodilians (Farmer, 2015b; Sanders & Farmer, 2012; Schachner et al, 2013, 2021), as well as developmental work on reptiles and birds (Broman, 1939; Locy & Larsell, 1916a, 1916b). All measures were completed in the 3D MPR viewer of OsiriX.…”
Section: Methodsmentioning
confidence: 99%
“…Comparatively, the functional morphology of other crocodilian taxa has received considerably less attention with much of the research focusing instead on behavior, ecology, and systematics (Bittencourt et al, 2019; Campos et al, 2010; Campos et al, 2012). In terms of the respiratory morphology, to the best of our knowledge, only A. mississippiensis (Farmer & Sanders, 2010; Sanders & Farmer, 2012; Schachner et al, 2021) and C. niloticus (Perry, 1988; Schachner et al, 2013) have been reconstructed and described in detail in three dimensions making it difficult to understand which morphological characters relate to group wide trends within Crocodylia, and which are specific to a given crocodilian taxon. With respect to the respiratory system of caimans, Gans and Clark (Gans & Clark, 1976) conducted a seminal and thorough investigation of the ventilatory mechanics of C. crocodilus .…”
Section: Introductionmentioning
confidence: 99%
“…Through a series of descriptive models of lung architecture and pulmonary measurements, we present the detailed gross anatomy of the lungs of three hatchlings and two adults. We additionally provide a comparison between P. palpebrosus and A. mississippiensis across a limited growth series (Schachner et al, 2021) which demonstrates which aspects of the bronchial tree are constrained as the lungs increase in size. Given the related respiratory biology of both P. palpebrosus and A. mississippiensis , we predict gross lung architecture and airway size and shape to be similar across both species due to functional constraints associated with unidirectional airflow patterns and the hepatic–piston ventilation system.…”
We imaged the lungs of five Cuvier's dwarf caiman (Paleosuchus palpebrosus) via computed tomography (CT) and micro‐computed tomography (μCT) and compared these data to the lungs of the American alligator (Alligator mississippiensis). These data demonstrate anatomical commonalities between the lungs of P. palpebrosus and A. mississippiensis, and a few notable differences. The structural similarities are (a) a proximally narrow, distally widened, hook‐shaped primary bronchus; (b) a cervical ventral bronchus that branches of the primary bronchus and immediately makes a hairpin turn toward the apex of the lung; (c) a sequential series of dorsobronchi arising from the primary bronchus caudal to the cervical ventral bronchus; (d) intraspecifically highly variable medial sequence of secondary airways; (e) sac‐like laterobronchi; and (f) grossly dead‐ended caudal group bronchi in the caudal and ventral aspects of the lung. The primary differences between the two taxa are in the overall number of large bronchi (fewer in P. palpebrosus), and the number of branches that contribute to the cardiac regions. Imaging data of both a live and deceased specimen under varying states (postprandial, fasting, total lung capacity, open to atmosphere) indicate that the caudal margin and position of the lungs shift craniocaudally relative to the vertebral column. These imaging data suggest that the smooth thoracic ceiling may be correlated to visceral movement during ventilation, but this hypothesis warrants validation. These results provide the scaffolding for future comparisons between crocodilians, for generating preliminary reconstructions of the ancestral crocodilian bronchial tree, and establishing new hypotheses of bronchial homology across Archosauria.
Avian respiratory systems are comprised of rigid lungs connected to a hierarchically organized network of large, regional air sacs, and small diverticula that branch from them. Paramedullary diverticula are those that rest in contact with the spinal cord, and frequently invade the vertebral canal. Here, we review the historical study of these structures and provide the most diverse survey to date of paramedullary diverticula in Aves, consisting of observations from 29 taxa and 17 major clades. These extensions of the respiratory system are present in nearly all birds included in the study, with the exception of falconiforms, gaviiforms, podicipediforms, and piciforms. When present, they share connections most commonly with the intertransverse and supravertebral diverticula, but also sometimes with diverticula arising directly from the lungs and other small, more posterior diverticula. Additionally, we observed much greater morphological diversity of paramedullary airways than previously known. These diverticula may be present as one to four separate tubes (dorsal, lateral, or ventral to the spinal cord), or as a single large structure that partially or wholly encircles the spinal cord. Across taxa, paramedullary diverticula are largest and most frequently present in the cervical region, becoming smaller and increasingly absent moving posteriorly. Finally, we observe two osteological correlates of paramedullary diverticula (pneumatic foramina and pocked texturing inside the vertebral canal) that can be used to infer the presence of these structures in extinct taxa with similar respiratory systems.
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