An integrative insight into the synsacral canal of fossil and extant Antarctic penguins

Abstract

The lumbosacral-canal system in birds most likely operates as a sense organ involved in the control of balanced walking and perching, but our knowledge of it is superficial. Penguins constitute interesting objects for the study of this system due to their upright walking, but only the Humboldt penguin, Spheniscus humboldti, and some incomplete fossil penguin synsacra have been studied in this respect. Here, we give an integrative comparative insight into the synsacral canal of extant Emperor penguin, Aptenodytes forsteri, Adelie penguin, Pygoscelis adeliae, and Eocene giant Anthropornis and/or Palaeeudyptes Antarctic penguins, using computed tomography imaging and associated data-extraction methodologies, complemented by analytical approaches ranging from geometric morphometrics to modularity, curvature, and wavelet analyses. We document that the variability in the number of synsacro-lumbar vertebrae is evolutionarily conserved, and all studied synsacra possess osteological correlates of the lumbosacral-canal system. We also found that Eocene and extant Antarctic penguins were separable on the basis of the main direction of the shape-related (size-independent) variability within said system, and A. forsteri was unique in the entire studied set in terms of the relative cranial shift of this compound structure. Moreover, we suggest that the evolutionary processes, shaping both the terrestrial posture and gait, were responsible, in extant penguins, for the increased simplicity and stability of the synsacral canal cross-sectional periodic patterns, as well as pave the way for the lumbosacral-canal system modularity characterized by reduced atomization/complexity.

The Cretaceous Neornithine record and new Vegaviidae specimens from the López de Bertodano Formation (Upper Maastrichthian) of Vega Island, Antarctic Peninsula

“…These canals are found in most neornithine groups (Jalgersma 1951) and, up to now, the oldest record of lumbosacral canals in a synsacrum was reported in a Maastrichthian Ornithurae FMNH PA 741 from Madagascar (O'Connor & Forster 2010). Thus, in the fossil Neornithes context, the specimen MV 7832-V exhibits the oldest record of the lumbosacral canals, since before they were only reported in early Sphenisciformes penguins from the Eocene of Seymour Island, Antarctic Peninsula (Jadwiszczak et al 2022). The canalis synsacri expands cranially, which likely corresponds to the lumbosacral intumescence (= bulla intumescentia lumbosacralis) (Fig.…”

“…4i). The functional hypothesis about such canals is that they are part of a secondary balance sensing system, working similarly to the semicircular channels of the inner ear, involved in the control of walking and perching (Stanchak et al 2020, Jadwiszczak et al 2022). These canals are found in most neornithine groups (Jalgersma 1951) and, up to now, the oldest record of lumbosacral canals in a synsacrum was reported in a Maastrichthian Ornithurae FMNH PA 741 from Madagascar (O'Connor & Forster 2010).…”

Understand comprehensive perspectives of evolution

“…Contrary to the vast majority of hybridization cases in frogs, they showed that even species with complex pre-mating behaviors are subjected to hybridization. The study of the lumbosacral system of Emperor and Adelie penguins and Eocene giant Antarctic penguins using nondestructive CT imaging and geometric morphometrics, modularity, curvature, and wavelet analyses documents that the variability in the number of synsacro-lumbar vertebrae is evolutionarily conserved and traceable at least to the Eocene epoch, but the main synsacral canal differs in presentday and fossil penguins in regard to the observed complexity of periodicity patterns (Jadwiszczak et al 2022). Van Linden et al (2022) quantified bite force and sagittal crest height across modern and extinct tapirids us-ing the dry-skull method.…”

“…The study of the lumbosacral system of Emperor and Adelie penguins and Eocene giant Antarctic penguins using nondestructive CT imaging and geometric morphometrics, modularity, curvature, and wavelet analyses documents that the variability in the number of synsacro‐lumbar vertebrae is evolutionarily conserved and traceable at least to the Eocene epoch, but the main synsacral canal differs in present‐day and fossil penguins in regard to the observed complexity of periodicity patterns (Jadwiszczak et al. 2022). Van Linden et al.…”