Armored with skin and bone: A combined histological and μCT‐study of the exceptional integument of the Antsingy leaf chameleon Brookesia perarmata (Angel, 1933)

Abstract

Madagascar's endemic ground‐dwelling leaf chameleons (Brookesiinae: Brookesia Gray, 1865 + Palleon Glaw, et al., Salamandra, 2013, 49, pp. 237–238) form the sister taxon to all other chameleons (i.e., the Chamaeleoninae). They possess a limited ability of color change, a rather dull coloration, and a nonprehensile tail assisting locomotion in the leaf litter on the forest floor. Most Brookesia species can readily be recognized by peculiar spiky dorsolateral projections (“Rückensäge”), which are caused by an aberrant vertebral structure and might function as body armor to prevent predation. In addition to a pronounced Rückensäge, the Antsingy leaf chameleon Brookesia perarmata (Angel, 1933) exhibits conspicuous, acuminate tubercle scales on the lateral flanks and extremities, thereby considerably enhancing the overall armored appearance. Such structures are exceptional within the Chamaeleonidae and despite an appreciable interest in the integument of chameleons in general, the morphology of these integumentary elements remains shrouded in mystery. Using various conventional and petrographic histological approaches combined with μCT‐imaging, we reveal that the tubercle scales consist of osseous, multicusped cores that are embedded within the dermis. Based on this, they consequently can be interpreted as osteoderms, which to the best of our knowledge is the first record of such for the entire Chamaeleonidae and only the second one for the entire clade Iguania. The combination of certain aspects of tissue composition (especially the presence of large, interconnected, and marrow‐filled cavities) together with the precise location within the dermis (being completely enveloped by the stratum superficiale), however, discriminate the osteoderms of B. perarmata from those known for all other lepidosaurs.

The veritable horns of a dilemma: Assessing homology of the parietal and squamosal asperities of Phrynosoma (Squamata: Iguania: Phrynosomatidae)

“…They do not exhibit the characteristics of osteoderms (de Buffrénil et al, 2016; Moss, 1969; Vickaryous & Sire, 2009; Vickaryous et al, 2015). Among the Iguania, the only instances of true osteoderms are found in the distantly related iguanid Amblyrhynchus cristatus and the chamaeleonid Brookesia perarmata (de Queiroz, 1987; Schucht et al, 2020; Williams et al, 2022). Norell (1989) considered the dermatocranial tuberosities of Phrynosoma to be bony outgrowths of the cranial bones capped by overlying scalation, and not secondary ossifications within these scales that fuse with the underlying bone during ontogeny.…”

Extreme miniaturization of a new amniote vertebrate and insights into the evolution of genital size in chameleons

“…35 mya) and between the different subgenera of Rhampholeon 9 . Most species of the large-bodied clade of Brookesia show very distinct rows of bony lateral projections along the vertebral column, which are based on a unique vertebral structure and might function as body armour to prevent predation 22 . These rows of projections are either absent, incomplete, or very poorly developed in the lineage of small-bodied species, which are also recognizable by their miniaturized adult size (22-51 mm versus 51-110 mm total length) and relatively larger hemipenis length (Fig.…”

The first description of dermal armour in snakes

“…Osteoderms have been reported in representatives of most major lepidosaur lineages 29 – 31 , e.g., in Sphenodontidae 32 , Gekkota 22 , 33 , 34 , Scincoidea 35 , Lacertoidea 36 , Anguimorpha 21 , 37 – 39 , and Iguania 40 sensu 41 . In some families, osteoderms are widespread, e.g., in cordylids, gerrhosaurids, scincids, and anguids 42 , 43 .…”

“…With extending knowledge about the presence of osteoderms in squamates, we can hypothesise about their function. As osteoderms are mostly located on the head and dorsal surface of the body 30 , 42 , the most widely accepted hypothesis about the function of dermal armour is that it serves for protection 22 , 40 . Nevertheless, osteoderms may contribute to a variety of other functions, such as locomotion 50 , thermoregulation 44 , 51 , and calcium storage 52 .…”