Anatomical and radiographic study of the scapula in juveniles and adults of Tamandua mexicana (Xenarthra: Myrmecophagidae)

Abstract

Tamandua mexicana is a mammal of the superorder Xenarthra geographically located between Mexico and Peru that remains insufficiently studied. These species are characterized by their movement, resting, and eating of termites and ants in arboreal and terrestrial environments, and therefore, their thoracic limbs have been adapted morphologically to move in these environments. Tamandua mexicana is a species constantly threatened by different causes, and their thoracic limbs can be affected in the scapular and shoulder regions as a result; therefore, specific anatomical knowledge of the scapula allows for better clinical procedures, surgical approaches, radiological diagnosis and muscular reconstructions in extinct species of anteaters. The main objective of this study was to report the anatomical and radiographic features of the scapula in juvenile and adult specimens of T. mexicana. Gross dissections and radiographic study were performed in six cadavers. The scapula had large differences compared with other mammals, such as two spines, a foramen for the suprascapular nerve, a long acromion, a cranial transverse scapular ligament and caudolateral fossa for the origin of the muscles teres major, tensor fasciae antebrachii and subscapular. In juvenile specimens, a scapular cartilage, an acromial cartilage, craniomedial and caudolateral coracoscapular ligaments forming the foramen for the suprascapular nerve, and progressive ossification of those ligaments and the cranial transverse scapular ligament were observed. These findings were corroborated by radiography; therefore, these characteristics change with age. The bony reliefs of the scapula of T. mexicana corroborate the specialization of the shoulder in this species.

Anatomical and radiographic study on the appendicular skeleton of the Tamandua mexicana

“…Also, a small portion of the subscapularis muscle and teres major muscle is inserted in the caudolateral fossa. The scapular head of the triceps muscle is attached to the accessory scapular and the serratus magnus muscle attaches to the backside of the caudolateral fossa when seen from a medial view (Taylor, 1978;Nyakatura, 2011;Vélez-García, et al 2019). The evolutionary development of the accessory scapular spine and caudolateral fossa is presumed to be a result of the need for greater force exertion for survival purposes such as looking for food or defending themselves, thus, the muscles that exert said force are the long head of the triceps brachii, teres minor and teres major muscles (Vélez-García et al, 2019).…”

“…The scapular head of the triceps muscle is attached to the accessory scapular and the serratus magnus muscle attaches to the backside of the caudolateral fossa when seen from a medial view (Taylor, 1978;Nyakatura, 2011;Vélez-García, et al 2019). The evolutionary development of the accessory scapular spine and caudolateral fossa is presumed to be a result of the need for greater force exertion for survival purposes such as looking for food or defending themselves, thus, the muscles that exert said force are the long head of the triceps brachii, teres minor and teres major muscles (Vélez-García et al, 2019). When compared to other mammals, the presence of the caudolateral fossa in T. mexicana and Myrmecophaga tridactyla (Sesoko et al, 2015) differs from the Choloepus hoffmanni species (McDonald et al, 2008) and domestic animals such as the C. familiaris since none of them have this scapular component, even though the C. hoffmanni also has a secondary scapular spine (Dyce et al, 2009;Liebich et al, 2020).…”

“…The presence of both accessory scapular spine and caudolateral fossa of the scapula may be a result of the need for a better muscle insertion to perform movements that require greater force and effort. Great examples of these actions are: climbing or destroying nests to collect termites (Taylor, 1978;Nyakatura, 2011;Rojano et al, 2014;Vélez-García, et al, 2019); Monteiro (2018) mentions that it has been reported that the shape of the scapula in a certain species is related to the function performed by the pelvic limb.…”

Evolutionary adaptations in the flexor digitorum profundus muscle in Tamandua mexicana (Xenarthra, Myrmecophagidae)

“…The forelimb in Tamandua is specialized for quadrupedal locomotion in trees and on the ground as well as for other tasks, such as securing food, defense (Montgomery, 1985; Navarrete & Ortega, 2011; Taylor, 1978; Vélez‐García, Torres‐Suárez, & Echeverry‐Bonilla, 2019), mating (Matlaga, 2006), swimming (Esser, Brown, & Liefting, 2010), and suspension from the mother as neonates and infants. This species has five digits on their hands, where the third digit is highly developed and the fifth digit is reduced and not visible externally (Cuarón, 2005; Machado‐Cruvinel et al, 2019; Polania‐Guzmán & Vélez‐García, 2019).…”

“…The forelimb muscles have special features. For instance, the teres major muscle is a highly developed muscle that facilitates limb retraction (Taylor, 1978; Vélez‐García et al, 2019). The biceps brachii muscle has two heads, where the long head forms a belly that is fused to the brachialis muscle, which is intended to increase the strength in elbow flexion and supination (Taylor, 1978).…”