Internal Morphology of Osteoderms of Extinct Armadillos and Its Relationship with Environmental Conditions

Abstract

The most complete and continuous fossil record of armadillos is composed mostly by isolated osteoderms, frequently found in paleontological and archaeological sites that bear continental South American mammals. Their external morphology has been used to define several species. In the last decade, many authors have focused on the internal structure of vertebrate osteoderms using histological and paleohistological studies. These studies allowed identification of useful features in systematic and phylogenetic contexts. In armadillos, osteoderms are constituted by compact bone tissue (primary and secondary osteons, and concentric layers or lamellae) that delimits cavities, which could contain different soft tissues (adipose tissue, hair follicles, bone marrow, and sweat and sebaceous glands). Traditional paleohistological techniques have allowed the recognition of homologous cavities to those found in osteoderms of current species and from comparison deduce which kind of tissue could had occupied them. We have recently utilized 3D reconstructions in osteoderms of extant species of armadillos to analyze the micromorphology, disposition, and the relationship of different cavities and understand them in depth. Here, we present the results of the application of paleohistology and microtomography in osteoderms of representatives of diferent taxa of extinct Dasypodidae (Astegotheriini, Stegotheriini, BUtaetini,^Euphractini, Eutatini), which allowed us to compare homologous structures based on their three-dimensional reconstruction. The results, added to the previous external morphology studies, have allowed us to define morphological patterns (consistent within each linage). The variation of the volume and extension of cavities associated with different tissues could be strongly associated with changes in the climate and environmental conditions of the species distribution areas.

Osteoderm microstructure of Riostegotherium yanei, the oldest Xenarthra

“…No growth marks were observed in both specimens in longitudinal or transverse sections, with optic microscopic, but thick and rather loose bundles of Sharpey fibers, crossing each other at various acute angles, were seen in transversal sections (Figure. the combining micro-Ct analysis and histological sections, to study the internal anatomy of buckler osteoderms of the extant armadillos Chaetophractus villosus (Desmarest, 1804) and Dasypus hybridus (Desmarest, 1804) (Krmpotic et al 2015, Ciancio et al 2017, provide the data for the association proposed here between the internal cavities and some of the aforementioned internal structures. however, hill (2006) argued that osteoderms of modern armadillos are physically associated with a variety of soft tissues, and in some cases, two or more different tissue types may cause similar osteological features.…”

“…Different publications have called these deep circular depressions as hair foramina, neurovascular foramina, or simply foramina. A histological section of the extant D. novemcinctus (the closest extant relative of Riostegotherium yanei, according to Ciancio et al 2017) revealed that the cavity beneath each of these deep and large circular depressions at the external surface also contains a complete hair follicle, radically smaller than its external aperture and embedded entirely within the osteoderm (hill 2006, Krmpotic et al 2012). holmes and Simpson (1931) stated that in all pilose armadillos, hair grows from oblique pits in the hinder margin of the osteoderm, through areas of soft skin in between each one.…”

“…In LMh 06, the cavities almost disappear beside the central keel near the distal border. Following Ciancio et al (2017), these cavities are regarded as small bone marrow cavities.…”

“…Studies about xenarthran osteoderm microstructure and its applications have increased in recent years, mainly in phylogenetic relationships and reconstruction of soft tissues in fossil osteoderms (e.g. hill 2006, Wolf 2007, Wolf et al 2012, Chávez-Aponte et al 2008, Krmpotic et al 2009, Ciancio et al 2017). For about 10 years, all data came from histological thin sections, but the last two publications employed also, for the first time, x-ray microtomography and 3D reconstructions of the internal microstructure.…”

Proximate causes of variation in dermal armour: insights from armadillo lizards

“…This is further complicated by accumulating bodies of evidence supporting the idea that defensive morphologies are not exclusively the outcome of predatorprey interactions, but instead have a multifunctional nature shaped by functional tradeoffs (Rivera and Stayton 2011, Magwene and Socha 2013, Broeckhoven et al 2017a. For example, body armour might play an important role during thermoregulation (Endo et al 2009, Krmpotic et al 2015, Broeckhoven et al 2017a, Ciancio et al 2017, Clarac et al 2017). In addition, besides being a product of natural selection, intrasexual aggression might contribute significantly to variation in the expression of defensive traits (Broeckhoven et al 2017b, English 2018.…”