Pterosaurs and the origin of feathers

Reconstructed T. imperator skeleton, National Museum of Brazil. From Wikimedia Commons

Feathers were once considered to be unique avialan structures linked to birds evolutionary success. Primitive theropods, such as Sinosauropteryx and the tyrannosaurs Dilong and Yutyrannus, and some plant-eating ornithischian dinosaurs, such as Tianyulong, and Kulindadromeus are known from their spectacularly preserved fossils covered in simple, hair-like filaments called ‘protofeathers’. Other integumentary filaments, termed pycnofibres, has been reported in several pterosaur specimens. The discovery of integumentary structures in other pterosaurs, such as Pterorhynchus wellnhoferi (a rhamphorhynchoid pterosaur), and other exquisitely preserved specimens from China, suggest that all Avemetatarsalia (the wide clade that includes dinosaurs, pterosaurs and close relatives) were ancestrally feathered.

A new specimen of an adult Tupandactylus imperator, a tapejarid pterosaur from north-eastern Brazil, preserves extensive soft tissues which provides more evidence that pterosaurs had feathers. The fossil, originally poached from an undetermined outcrop of the Early Cretaceous Crato Formation, was in privated hands for an unknown period of time and later deposited at the Royal Belgian Institute of Natural Sciences (RBINS). The fossil was repatriated to Brazil early this year.

Details of the cranial crest of MCT.R.1884 and the scanning electron microscope images of melanosomes (g-i). Scale bars, 50 mm (a); 5 mm (b); 2 mm (c); 250 μm (d–f); 2 μm (g–i). From Cincotta et al., 2022.

The new specimen (MCT.R.1884) comprises the posterior portion of the cranium and the remains of a soft tissue cranial crest preserved on five separate slabs. Two types of fibrous integumentary structures were present. The monofilaments (approximately 30 mm long and 60–90 μm wide) resemble those present in the anurognathid Jeholopterus ningchengensis and the ornithischian dinosaur Tianyulong. The most striking feature is the presence of fossil melanosomes with diverse morphologies that supports the hypothesis that the branched integumentary structures in pterosaurs are feathers.

Melanosomes are granules of the pigment melanin. The diverse shape of the melanosomes recovered from the skin fibres in the crest, monofilaments and branched feathers resembles that in the skin of extant birds and mammals. This is an indication that pterosaurs had the genetic machinery to control the colors of their feathers.

References:

Cincotta, A., Nicolaï, M., Campos, H.B.N. et al. Pterosaur melanosomes support signalling functions for early feathers. Nature (2022). https://doi.org/10.1038/s41586-022-04622-3

Reimagining Amargasaurus

Amargasaurus cazaui. MACN

Dicraeosauridae is a family of mid-sized sauropod dinosaurs characterized by a distinctive vertebral column with paired, long, neural spines. Argentinian dicraeosarids include Amargasaurus cazaui, Pilmatueia faundezi and Bajadasaurus pronuspinax. The group was first described in 1914 by Werner Janensch with the discovery of the nearly complete skeletons of Dicraeosaurus in the expeditions to the upper Jurassic beds of Tendaguru, Tanzania. The discovery of Amargasaurus cazaui in 1991, from the Early Cretaceous beds of La Amarga Formation of Northern Patagonia, renewed the discussion on the peculiar vertebral anatomy of these sauropod dinosaurs.

The hyperelongated hemispinous processes of dicraeosarids were interpreted by some authors as a support structure for a thermoregulatory sail, a padded crest as a display and/or clattering structure, a dorsal hump, or as internal cores of dorsal horn. A new study lead by Ignacio Cerda tested these hypotheses using internal microanatomy and bone microstructure from the holotype of Amargasaurus, and a fragmentary dicreaosaurid specimen (MOZ-Pv 6126-1, consisting of an almost complete anterior dorsal vertebra) also from the La Amarga Formation (Barremian–Aptian, Lower Cretaceous).

Skeletal silhouette of Amargasaurus cazaui. From Cerda et al., 2022.

Despite that the organic components of mineralised tissues decay after death, the inorganic components of bone preserve the spatial orientation of organic components such as osteocyte lacunae, vascular canals, and collagen fibres. Armand de Ricqlès, in the 1960s and 1970s, observed that paleohistological features could be correlated with growth rates and thus could indirectly shed light on the thermal physiology of extinct organisms. Previous paleohistological studies in dicraeosarids revealed particular histological features regarding the vascularization pattern and cortical resorption. 

Bone histology of hyperelongate hemispinous processes of Amargasaurus cazaui. From Cerda et al., 2022

The hemispinous processes from Amargasaurus and MOZ-Pv 6126-1 essentially consist of compact bone tissue. The study lead by Ignacio Cerda also found that secondary remodelling is profuse not only in the perimedullary region but also in the outer cortex. The histological features analized comprise a highly vascularized fibrolamellar bone interrupted with CGMs (cyclical growth marks), presence of obliquely oriented Sharpey’s fibres, and abundant secondary osteons irregularly distributed within the cortex. 

The spatial distribution and orientation of the Sharpey’s fibres indicate the presence of an important system of interspinous ligaments, covered by the integumentary system, which resulted in the formation of a prominent cervical sail in this taxon. However, there is not anatomical or histological evidence that support the presence of a keratinized sheath. The new study also suggests that the cervical sail in Amargasaurus could be used as a display device, a term that includes ‘intraspecific agonistic, deterrent, or sexual display structures’. Unfortunately, the dicraeosaurid record is still too scarce to determine the existence of sexual dimorphism in this clade.

 

References:

Ignacio A. Cerda, Fernando E. Novas, José Luis Carballido, Leonardo Salgado (2022): Osteohistology of the hyperelongate hemispinous processes of Amargasaurus cazaui (Dinosauria: Sauropoda): Implications for soft tissue reconstruction and functional significance. In: Journal of Anatomy. DOI: 10.1111/joa.13659

Windholz, G. J., & Cerda, I. A. (2021). Paleohistology of two dicraeosaurid dinosaurs (Sauropoda; Diplodocoidea) from La Amarga Formation (Barremian–Aptian, Lower Cretaceous), Neuquén Basin, Argentina: Paleobiological implications. Cretaceous Research, 128, 104965. doi: 10.1016/j.cretres.2021.1049

Salgado, L. & Bonaparte, J. F. Un nuevo saurópodo Dicraeosauridae, Amargasaurus cazaui gen et sp. nov., de la Formación La Amarga, Neocomiano de la provincia del Neuquén, Argentina. Ameghiniana 28, 333–346 (1991).

Windholz, G. J., Baiano, M. A., Bellardini, F., & Garrido, A. (2020). New Dicraeosauridae (Sauropoda, Diplodocoidea) remains from the La Amarga Formation (Barremian–Aptian, Lower Cretaceous), Neuquén Basin, Patagonia, Argentina. Cretaceous Research, 104629. doi: 10.1016/j.cretres.2020.10462