Research Article |
Corresponding author: Juan Fernando Vélez-García ( jfvelezg@ut.edu.co ) Academic editor: Clara Stefen
© 2023 Juan Fernando Vélez-García, Diego Alejandro Carrión Blanco, Gabriela Moreno Gómez, Stephanie San Martín Cañas.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Vélez-García JF, Carrión Blanco DA, Moreno Gómez G, San Martín Cañas S (2023) Descriptive study of the intrinsic muscles of the shoulder and brachium in kinkajou (Potos flavus) and an evolutionary analysis within the suborder Caniformia. Vertebrate Zoology 73: 957-980. https://doi.org/10.3897/vz.73.e102645
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Abstract
The kinkajou (Potos flavus) is a carnivoran of the suborder Caniformia and the family Procyonidae, inhabiting regions throughout Central and South America. Potos flavus has arboreal preferences and exhibits unique anatomical adaptations that facilitate movement within trees. Its pelvic limbs enable hindfoot reversal, while its thoracic limbs possess remarkable prehensile capabilities. Previous anatomical studies in Potos flavus have presented discrepancies in the description of the intrinsic shoulder and brachial muscles. Therefore, this study aims to provide a comprehensive anatomical description of these muscles in five specimens. The findings are compared with descriptions reported for other caniforms. The application of the Density-Based Spatial Clustering of Application with Noise (DBSCAN) algorithm aids in identifying relationships among caniforms based on the presence or absence of specific muscles. Our analysis reveals several key differences, including the presence of a biceps brachii with two capita (longum and breve), two coracobrachiales muscles (longus and brevis), a tensor fasciae antebrachii with two distinct parts (cranialis and caudalis), and an anconeus medialis. The caput breve of the biceps brachii and coracobrachialis longus muscles are absent in some individuals, with prevalence rates of 10% and 20%, respectively. One specimen exhibited an accessory caput laterale of the m. triceps brachii bilaterally. The comparative analysis suggests that the shoulder and brachial muscles of Potos flavus share more similarities with those of Ailurus fulgens and ursids of the genera Ursus and Tremarctos. These findings suggest the retention of muscles that may have been present in the common ancestor of the infraorder Arctoidea.
Anatomy, Arctoidea, attachments, Carnivora, myology, Procyonidae
The kinkajou (Potos flavus) is a carnivoran species belonging to the family Procyonidae and suborder Caniformia (
While previous research has highlighted the importance of the anatomical adaptations of the antebrachial muscles of P. flavus for grasping tree branches and food (
On the other hand, intrinsic scapular and brachial muscles extend from the scapula to the brachial and antebrachial bones (
Gross anatomical dissections in the scapular, humeral joint, and brachial regions were conducted on five P. flavus specimens, which were previously employed in a study of the extrinsic thoracic limb muscles (
Specimen | Sex | Age | Limb | ID |
PfS1 | M | J | R | M1R |
L | M1L | |||
PfS2 | M | A | R | M2R |
L | M2L | |||
PfS3 | F | A | R | F3L |
L | F3R | |||
PfS4 | F | A | R | F4L |
L | F4R | |||
PfS5 | F | A | R | F5L |
L | F5R | |||
A, adult; F, female; I, infant; ID, Identification of the limb; J, juvenile; L, left; M, male; R, right. |
The comparative analysis of the intrinsic shoulder and brachial muscles of caniforms can be framed as a classification problem of categorical variables or a clustering problem. Clustering represents an unsupervised machine learning technique utilized to identify and segregate natural groups based on the inherited properties within a dataset (
Reviewed species within the suborder Caniformia. N = number of dissected specimens.
Family | Species | N (sex) | Authors |
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Procyonidae | Potos flavus | 1 (unknown sex) |
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1 (unknown sex) |
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1 (unknown sex) |
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1 (unknown sex) |
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1 (unknown sex) |
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5 (2 males and 3 females) | Present study | ||
Bassaricyon alleni | 1 (unknown sex) |
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Procyon cancrivorus | 1 (unknown sex) |
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2 (males) |
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5 (unknown sex) |
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3 (two females and one unknown) |
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Procyon lotor | 2 (females) |
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1 (unknown sex) |
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1 (unknown sex) |
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2 (1 female and 1 male) |
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Nasua nasua | 1 (unknown sex) |
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3 (1 male and 2 females) |
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1 (unknown sex) |
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3 (males) |
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Nasua narica | 1 (unknown sex) |
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1 (unknown sex) |
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Bassariscus astutus | 1 (unknown sex) |
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Mustelidae | Eira barbara | 1 (unknown sex) |
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Galictis cuja | 3 (1 female and 1 male) |
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Taxidea taxus | 6 (unknown sex) |
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Martes americana | 25 (19 males and 6 females) |
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Martes martes | 1 (1 male) |
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3 (unknown sex) |
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Martes foina | 4 (unknown sex) |
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1 (unknown sex) |
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Martes caurina | 1 (1 male) |
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Pekania pennanti | 25 (7 males and 18 females) |
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4 (1 female and 3 males) |
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Meles meles | 1 (unknown sex) |
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2 (1 female and 1 male) |
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Lutra lutra | 1 (male) |
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1 (unknown sex) |
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Enhydra lutris | 1 (unknown sex) |
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Aonyx sp. | 1 (unknown sex) |
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Mephitidae | Mephitis mephitis occidentalis | 1 (male) |
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Spilogale gracilis phenax | 1 (female) |
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Ailuridae | Ailurus fulgens | 1 (male) |
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4 (3 females and 1 male) |
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1 (unknown sex) |
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Ursidae | Ursus americanus | 1 (male) |
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1 (male) |
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1 (unknown sex) |
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Ursus maritimus | 1 (female) |
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Ailuropoda melanoleuca | 2 (males) |
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Tremarctos ornatus | 1 (female) |
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Melursus ursinus | 3 (unknown sex) |
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Canidae | Canis lupus familiaris | Non specified |
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Canis lupus dingo | 1 (unknown sex) | Haughton (1866) | |
Canis latrans | 1 (female) 1 limb |
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Cuon alpinus | 1 (unknown sex) |
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Cerdocyon thous | 6 (1 males and 5 females) |
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Chrysocyon brachyurus | 3 (unknown sex) | Pereira et al. (2016) | |
Lycalopex gymnocercus | 22 (10 females and 12 males) |
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Lycaon pictus | 1 (male) |
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Vulpes vulpes | 5 (4 males and 1 female) |
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1 (unknown sex) |
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Urocyon cinereoargenteus | 4 (2 females and 2 males) |
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For each specimen, the seven categorical variables underwent an encoding process to assign an integer value class corresponding to the muscle variants. Teres minor (Tmin), coracobrachialis brevis (CBb), coracobrachialis longus (CBl) and anconeus medialis (AM) variables were encoded into two classes, denoted as absent (0) or present (1). “Absent” was indicated when the muscle was either not described or reported as vestigial or fused with other muscle (e.g., m. teres minor was considered absent when reported as fused with m. infraspinatus in P. flavus as described by
The variables triceps brachii (TB) and biceps brachii (BB) were numerically encoded into four classes based on the number of heads (capita). Therefore, values ranging from two (2) to five (5) were considered for each muscle. The reported caput angulare of the m. triceps brachii in mustelids was regarded as the caudal part of the m. tensor fasciae antebrachii (see the discussion section).
Finally, the tensor fasciae antebrachii (TFA) variable was encoded into five classes. “Absent” (0) was designated only when the author explicitly reported its complete absence (e.g.,
It is important to note that some specimens were excluded from the statistical analysis because only one of the two anatomical regions was studied by the authors, either the shoulder or the brachium (e.g.,
The m. deltoideus comprises two parts: a cranial part referred to as the acromial part (pars acromialis) and a caudal part known as the scapular part (pars scapularis). The acromial part originates via a tendon and fleshy fibers from the ventral margin of the hamatus process of the acromion, while the scapular part originates from an aponeurosis along the scapular spine (Figs
Lateral photographic views of the intrinsic shoulder and brachium muscles of Potos flavus. Superficial (a) and deep views (b) of a left thoracic limb. ACS, a. cervicalis superficialis; ACHCd, a. circumflexa humeri caudalis; ASb, a. subscapularis; ATD, a. thoracodorsalis; AL, m. anconeus lateralis; B, m. brachialis; ClB, m. cleidobrachialis; Da, m. deltoideus pars acromialis; Ds, m. deltoideus pars scapularis; IS, m. infraspinatus; LD, m. latissimus dorsi; SS, m. supraspinatus; TB, m. triceps brachii; TBLa, caput laterale; TBLo, caput longum; TFACd, m. tensor fasciae antebrachii pars caudalis; TMaj, m. teres major; TMin, m. teres minor. White bars: 10 mm.
Muscle maps of the intrinsic shoulder and brachial muscles in a left scapula of Potos flavus. (a) Lateral view, (b) caudal view, (c) medial view, (d) cranial view. AL, m. anconeus lateralis; B, m. brachialis; BB, m. biceps brachii; BBb, caput breve; BBl, caput longum; CBb, m. coracobrachialis brevis; CBl, m. coracobrachialis longus; Da, m. deltoideus pars acromialis; Ds, m. deltoideus pars scapularis; IS, m. infraspinatus; Sb, m. subscapularis, SS, m. supraspinatus; TBLo, m. triceps brachii caput longum; TMaj, m. teres major; TMin, m. teres minor.
Muscle maps of the intrinsic shoulder and brachial muscles in a left humerus of Potos flavus. (a) Cranial view, (b) lateral view, (c) caudal view, (d) medial view. AL, m. anconeus lateralis; AM, m. anconeus medialis; B, m. brachialis; CBb, m. coracobrachialis brevis; CBl, m. coracobrachialis longus; Da, m. deltoideus pars acromialis; Ds, m. deltoideus pars scapularis; IS, m. infraspinatus; LD, m. latissimus dorsi; Sb, m. subscapularis, SS, m. supraspinatus; TBLa, m. triceps brachii caput laterale; TBm, caput mediale; TBa, caput accessorium; TMaj, m. teres major; TMin, m. teres minor.
Lateral (a) and medial (b) photographic views of the intrinsic shoulder and brachial muscles with the arterial distribution in the left thoracic limbs of Potos flavus. Aa, a. axillaris; Ab, a. brachialis; Abi, a. brachialis superficiale; ACHCd, a. circumflexa humeri caudalis; ACHCr, a. circumflexa humeri cranialis; ACoR, a. collateralis radialis; ASb, a. subscapularis; ACoU, a. collateralis ulnaris; Apb, a. profunda brachii; AL, m. anconeus lateralis; AM, m. anconeus medialis; B, m. brachialis; BB, m. biceps brachii; BBb, caput breve; BBl, caput longum; CBb, m. coracobrachialis brevis; CBl, m. coracobrachialis longus; ClB, m. cleidobrachialis; LD, m. latissimus dorsi; Sb) m. subscapularis; SS, m. supraspinatus; TB, m. triceps brachii; TBa, caput accessorium; TBLa, caput laterale; TBLaa, caput laterale accessorium; TBLo, caput longum; TBm, caput mediale; TFACr, m. tensor fasciae antebrachii pars cranialis; TMaj, m. teres major; *, branches of the brachial artery to the m. biceps brachii. White bars: 10 mm.
The m. supraspinatus has a fleshy origin from the supraspinatus fossa and the cranial surface of the scapular spine (Figs
The m. infraspinatus originates via fleshy fibers from the infraspinatus fossa, the caudal surface of the scapular spine, and the origin aponeurosis of the m. teres minor (Figs
The m. teres minor originates via an aponeurosis from the two ventral thirds of the caudal scapular margin and inserts tendinously onto the teres minor tuberosity distal to the infraspinatus insertion (Figs
The m. subscapularis is multipennate with eight bellies originating from fleshy fibers within the subscapular fossa, along the caudal margin of the scapula, and intermuscular septum adjacent to the m. teres major (Figs
The m. teres major originates from the dorsal third of the caudal scapular margin, the lateral and medial surfaces of the caudal scapular angle, and the intermuscular septum shared with the m. subscapularis (Figs
There are two coracobrachiales muscles, namely m. coracobrachialis brevis and m. coracobrachialis longus, which originate from the coracoid process of the scapula via a common tendon shared with the caput breve of the m. biceps brachii (Figs
Medial deep photographic views of the intrinsic shoulder and brachial muscles of the left thoracic limbs of Potos flavus. (a) The caput accessorium of the m. triceps brachii was displaced caudally; (b) absence of m. coracobrachialis longus. AL, m. anconeus lateralis; AM, m. anconeus medialis; B, m. brachialis; BB, m. biceps brachii; BBb, caput breve; BBl, caput longum; CBb, m. coracobrachialis brevis; CBl, m. coracobrachialis longus; ClB, m. cleidobrachialis; Sb, m. subscapularis; TB, m. triceps brachii; TBa, caput accessorium; TBLo, caput longum; TBm, caput mediale. White bars: 10 mm.
Variants of the m. biceps brachii in Potos flavus. (a) Medial view of a right thoracic limb where the caput breve was fused to the caput longa of the m. biceps brachii; (b) medial view of a right thoracic limb with an accessory caput of the m. biceps brachii; (c) lateral deep view of a right thoracic limb. B, m. brachialis; Ba, caput accessorium of the m. brachialis (anatomical variant); BB, m. biceps brachii; BBa, caput accessorium; BBb, caput breve; BBl, caput longum; CBb, m. coracobrachialis brevis; CBl, m. coracobrachialis longus; Sb) m. subscapularis; TB, m. triceps brachii; TBa, caput accessorium; TBLo, caput longum; TBm, caput mediale. White bars: 10 mm.
The m. biceps brachii consists of two heads (capita), namely the caput longum and caput breve. The caput longum originates from the supraglenoid tubercle of the scapula via a tendon that passes inside the articular capsule of the shoulder. The caput breve originates from the coracoid process via a common tendon shared with the coracobrachiales longus and brevis muscles (Figs
Muscle maps of the intrinsic shoulder and brachial muscles in a left ulna (a-b) and a left radius (c-d) of Potos flavus. (a) lateral view, (b) medial view, (c) cranial view, (d) caudal view. AL, m. anconeus lateralis; AM, m. anconeus medialis; B, m. brachialis; BB, m. biceps brachii; TBLa, m. triceps brachii caput laterale; TBLo, caput longum; TBm, caput mediale; TBa, caput accessorium; TFA, m. tensor fasciae antebrachii.
The m. brachialis is a large muscle originating fleshy from the humeral neck, caudo-lateral surface of the humerus, sulcus of m. brachialis, and the medial aspect of the lateral supracondylar crest (Figs
Caudal photographic views of the intrinsic brachial muscles of the left brachia of Potos flavus. (a) Caudal view without the capita longum and laterale of the m. triceps brachii, (b) caudal view after medially displacing the caput accessorium of the m. triceps brachii, (c) caudal view after eliminating all capita of the m. triceps brachii; AL, m. anconeus lateralis; AM, m. anconeus medialis; B, m. brachialis; BB, m. biceps brachii; TB, m. triceps brachii; TBa, caput accessorium; TBm, caput mediale. White bars: 10 mm.
The m. triceps brachii consists of four heads (capita longum, mediale, accessorium and laterale). The caput longum originates via a tendon from the ventral half of the caudal scapular margin (medial to the teres minor aponeurosis) and infraglenoid tubercle (Figs
The caput mediale originates via fleshy fibers widely from the second and third proximal fifth of the caudomedial surface of the humerus until the caudal surface of the proximal extreme to the supracondylar foramen. The caput accessorium also originates via fleshy fibers from the humeral neck and the first proximal fifth of the caudal surface of the humerus, where it is fused to the caput mediale (Figs
The m. anconeus lateralis (m. anconeus or m. anconeus lateralis) originates via fleshy fibers from the distal half of the caudolateral surface of the humeral shaft, lateral supracondylar crest, and lateral epicondyle of the humerus. It inserts via fleshy fibers onto the joint capsule of the elbow and the lateral surface of the olecranon (Figs
The m. anconeus medialis originates via fleshy fibers from the cranioproximal margin of the supracondylar foramen and medial epicondyle of the humerus (Figs
The m. tensor fasciae antebrachii comprises two parts: a cranial part (pars cranialis) and a caudal part (pars caudalis). The caudal part originates from the latissimus dorsi and teres major muscles. The cranial part originates from the latissimus dorsi, pectoralis profundus, and cutaneus trunci muscles. The muscle forms an aponeurosis to insert onto the medial caudal margin of the olecranon and antebrachial fascia (Figs
Superficial medial photographic views of the intrinsic brachial muscles of a left thoracic limb of Potos flavus. (a) Medial view, (b) caudomedial view; BB, m. biceps brachii; BBb, caput breve; BBl, caput longum; CT, m. cutaneus trunci; LD, m. latissimus dorsi; TFACr, m. tensor fasciae antebrachii pars cranialis; TFACd, m. tensor fasciae antebrachii pars caudalis. White bars: 10 mm.
DBSCAN identified five groups as the optimal number of clusters of specimens based on the statistical analysis of the absence and presence of intrinsic shoulder and brachial muscles. The DBSCAN performance was considered good, with a silhouette coefficient of 0.65. It is worth noting that a minimum value of 0.5 is generally accepted as indicative of good clustering. Most P. flavus specimens (including those referenced in
The origin and insertion formerly reported for the supraspinatus and infraspinatus muscles in P. flavus were similar to the specimens of the present study but the origin of both muscles from the acromion (
In the present study, the m. teres minor was completely separated from the m. infraspinatus in all P. flavus specimens, consistent with reports in the same species (
While examining the m. subscapularis, it was found that its attachments do not differ among procyonids, corroborating previous descriptions (
The m. deltoideus of P. flavus closely resembled descriptions provided by other authors (
The m. teres major in P. flavus originates from the proximal third of the caudal margin of the scapula, consistent with previous descriptions (
The arrangement of the coracobrachialis brevis and longus muscles in P. flavus are consistent with previous descriptions (
Comparative terminology to the coracobrachialis, triceps, anconeus and tensor fasciae antebrachii muscles in caniforms.
Muscle name* | Homologous names used by other authors in Caniforms |
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M. coracobrachialis brevis* | M. coraco-brachialis brevis ( |
Short belly of the m. coracobrachialis ( |
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M. articularis humeri ( |
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M. coracobrachialis longus* | M. coraco-brachialis longus ( |
M. coracobrachialis accessorius ( |
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Main belly of the m. coracobrachialis ( |
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M. coracobrachialis ( |
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M. triceps brachii caput longum | M. anconeus longus ( First or scapular head of the triceps ( |
Scapular head of the triceps ( |
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Posterior division of the outer head ( |
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Anterior part or anterior head of the m. triceps longum ( |
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Medial head of the m. triceps longum ( |
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Triceps brachii – medial head ( |
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Anterior part of the caput longum of m. triceps brachii ( |
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M. triceps brachii caput longum accessorium* | Posterior part or posterior head of the caput longum of m. triceps brachii ( |
Lateral head of the m. triceps longum ( |
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Caput magnum of the m. triceps brachii ( |
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M. triceps brachii caput laterale | M. anconeus lateralis ( Second or lateral humeral portion ( |
Outer head of the triceps ( |
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M. triceps lateralis ( |
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Anterior division of the outer head ( |
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M. triceps brachii caput laterale accessorium* | Fourth head of the triceps ( |
M. triceps brachii caput mediale | M. anconeus medialis ( Third portion of the triceps ( |
internal head of the triceps ( |
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Intermediate head of the m. triceps medialis ( |
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Deep portion of the m. triceps brachii, which originates from the proximal part of the humerus ( |
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M. triceps medialis longus ( |
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M. triceps brachii caput accessorium | M. anconeus posterior ( |
Accessory muscular fibers of the second portion of m. triceps brachii ( |
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Long head of the m. triceps medialis ( |
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Deep portion of the m. triceps brachii, which originates from the under the lateral head ( |
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M. triceps medialis brevis or intermediate head of the caput mediale of the m. triceps brachii ( |
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M. tensor fasciae antebrachii | Latissimus dorsi head ( |
M. dorsi-epitrochlearis ( |
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Dorso-epitrochlear slip ( |
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Epitrochlearis ( |
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M. tensor fasciae antebrachii pars cranialis* | Inferior portion of the m. latissimus dorsi ( |
Part B of the m. epitrochlearis ( |
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Medial head of the m. epitrochlearis ( |
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Cranial portion of the m. tensor fasciae antebrachii ( |
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Epitrochlearis ( |
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Second part of the m. tensor fasciae antebrachii ( |
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M. tensor fasciae antebrachii pars caudalis* | “Large slip from dorsal portion of the panniculus” ( |
Fourth or latissimus dorsi head of the triceps ( |
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Second dorso-epitrochlear ( |
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Caput anguli of the m. triceps brachii ( |
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Part A of the m. epitrochlearis ( |
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Main mass of the m. epitrochlearis ( |
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Caudal portion of the m. tensor fasciae antebrachii ( |
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Triceps brachii – long head ( |
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M. triceps brachii caput angulare ( |
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First part of the m. tensor fasciae antebrachii ( |
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Caudal belly to the m. tensor fasciae antebrachii ( |
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M. anconeus lateralis* | M. anconeus externus ( |
M. anconeus medialis* | M. anconeus epitrochlearis ( |
M. anconeus internus ( |
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Second slip of the third portion of the triceps ( |
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“Part of the internal head which rose from the bridge of bone over the supracondylar foramen” ( |
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Medial portion of the m. triceps brachii ( |
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Short portion of the medial head of the m. triceps brachii ( |
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Triceps brachii caput mediale accessorium ( |
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M. triceps brachii caput accessorium ( |
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*Names not based on the NAV ( |
In this study, we found that the m. biceps brachii in P. flavus exhibited two heads, as that reported previously (
Our findings suggest variations regarding the origin of the m. brachialis, particularly in P. flavus. While previous authors (
We identified discrepancies in the nomenclature used in previous studies concerning the m. triceps brachii in P. flavus.
Concerning the m. tensor fasciae antebrachii, our study identified variations in its configuration among the examined caniforms. The muscle was found to be divided into two parts, with diverse origins for both the caudal and cranial parts. Previous descriptions did not describe this division and offered different configurations for its origins. Only in the case of P. lotor, the muscle was also reported as divided into two parts (
In non-procyonid caniforms, the m. tensor fasciae antebrachii has also been reported as consisting of two parts in the ursid A. melanoleuca (
In other studies of P. flavus, the m. anconeus medialis was referred to as anconeus epitrochlearis (
Tendency of presence and absence of the most variant muscles in the shoulder and brachial regions in caniform families.
Muscle | Canidae | Procyonidae | Mustelidae | Mephitidae | Ailuridae | Ursidae |
Teres minor | Present | Present | Variable | Absent or vestigial | Present | Present |
Coracobrachialis brevis | Present | Present | Variable | Absent | Present | Present |
Coracobrachialis longus | Present | Absent except P. flavus | Variable | Absent | Present | Present |
Biceps brachii caput breve | Absent | Absent except P. flavus | Absent | Absent | Variable | Present in Ursus and Tremarctus |
Absent in Ailuropoda and Melursus | ||||||
Triceps brachii caput longum accessorium | Absent | Absent | Absent | Absent | Absent | Present |
Tensor fasciae antebrachii pars cranialis | Present | Present | Variable | Present | Variable | Present |
Tensor fasciae antebrachii pars caudalis | Absent | Present except Nasua | Present | Present | Present | Present except Melursus |
Anconeus medialis | Absent | Present | Present | Present | Present | Absent or vestigial |
The shoulder movements play a pivotal role in the arboreal and prehensile habits of P. flavus, requiring high abilities and precise control to move among the tree branches. Although the extrinsic muscles contribute to shoulder support, the intrinsic muscles execute more precise shoulder joint movements. Further, the scapular muscles are responsible for the lateral and medial rotation of the humerus when the limb is free. These muscles also stabilize the shoulder joint when the limb is in contact with a substrate. These functions are not unique to P. flavus but are also observed in other caniforms (
The m. biceps brachii in P. flavus delivers similar functions to those of other caniforms, including elbow flexion, shoulder extension, and cranial and medial stabilization of the shoulder joint (
The presence of m. coracobrachialis longus and the caput breve of m. biceps brachii in arctoid species may be phylogenetically associated with a common ancestor with proficient arboreal and prehensile abilities. Among procyonids, both muscle bellies are preserved in P. flavus, whereas the muscle probably disappeared in the common ancestor of other procyonid genera. Notably, the genus Bassaricyon, which shares similar locomotor skills with P. flavus, is the only genus showing such abilities. Therefore, although the presence of both muscle bellies is not essential for arboreal and prehensile abilities, its absence may represent an intra- and interspecific disadvantage. Moreover, the presence of both muscles likely contribute to better anatomical adaptations in most P. flavus specimens. However, the B. alleni and P. flavus specimens lacking these bellies may exhibit reduced supination and elbow flexion forces compared to other specimens, implying decreased abilities. Thus, further myological studies in B. alleni are required, as only one specimen has been studied to date (
Elbow extension in caniforms is primarily performed by the caudal group of brachium muscles, including the m. triceps brachii with its four heads, m. tensor fasciae antebrachii, and m. anconeus lateralis (
The presence of m. anconeus medialis appears consistent in species within the infraorder Arctoidea, with the exception of ursids, where the muscle may appear in a vestigial shape or fused with the caput mediale of the m. triceps brachii (
The presence of the caput breve of m. biceps brachii, m. coracobrachialis longus and m. anconeus medialis is not exclusive to caniforms but extends to other mammalian taxa, including monotremes (
In carnivorans within the suborder Caniformia, the presence or absence of intrinsic shoulder and brachial muscles holds both phylogenetic and functional implications. Notably, canids form a distinct group independent of arctoids, indicating the divergence of these species with fewer muscle bellies to provide medial support to the shoulder and elbow. This divergence may be attributed to the fact that canids predominantly require shoulder and elbow movements in a sagittal plane (
Another cluster featuring fewer muscles is the fifth cluster, including mephitids, the mustelid T. taxus and mustelids of the subfamily Lutrinae. These species lack the coracobrachialis muscles and, in some cases, even the m. teres minor in mephitids and lutrines. Notably, one limb of Aonyx sp. is grouped in the fourth cluster due to the presence of m. coracobrachialis brevis. Therefore, if the tendency of lutrines had been taken into account, this limb would have been classified into the fifth cluster. This indicates that these species require less intrinsic muscular support for the shoulder. Thus, the coracobrachiales muscles may appear in a vestigial shape or fuse with another muscle. The absence of these muscles in mephitids could be a feature that emerged during their divergence from the common ancestor of ailurids and mustelids, while in lutrines, it might have occurred during their subsequent divergence within the family Mustelidae. The consistent presence of the m. teres minor in T. taxus indicates a phylogenetic connection with non-lutrine mustelids.
In the case of P. flavus, the intrinsic shoulder and brachial muscles consistently maintain a phylogenetic association within the infraorder Arctoidea, primarily with the ailurid A. fulgens and the ursids of the genera Ursus and Tremarctos (Cluster 3). This implies that these species have the highest level of muscular support for shoulder and elbow movements, including enhanced supination compared to other specimens and species. In the first and fourth clusters, support for elbow flexion and supination is diminished due to the absence of the caput breve of the m. biceps brachii. Within the first group, the enhanced medial support for the shoulder results from the presence of m. corabrachialis longus. Consequently, some P. flavus specimens have reduced intrinsic muscle support for the shoulder and elbow, similar to other procyonids, mustelids, and the ursids A. melanoleuca and M. ursinus. Nevertheless, as previously discussed, this may indicate functional disadvantages for specimens preserving all these muscles. Additionally, individual specimens may exhibit dominance of one thoracic limb, as observed in PfS4, where the right limb belongs to the first cluster, while the left limb is grouped within the third cluster.
In summary, P. flavus stands as the sole extant musteloid species potentially preserving the intrinsic shoulder and brachial muscles traced back to the common ancestor of arctoids. Furthermore, the presence of all these muscles in caniforms appears to provide both intra- and interspecific functional advantages. Consequently, P. flavus specimens equipped with all intrinsic shoulder and brachial muscles may exhibit greater strength and enhanced abilities in the shoulder and elbow when compared to other specimens or different species.
Despite our findings, it is important to acknowledge certain limitations in this study. Many authors did not provide detailed anatomical descriptions and often analyzed few specimens. Additionally, some characteristics were assigned based on family tendencies introducing potential biases into the statistical analysis. Therefore, to improve future investigations, it is imperative to conduct detailed anatomical studies including a broader range of arctoid species, using a larger sample size. Finally, although this study focused on a specific group of muscles in caniforms, future comparative analyses applying similar statistical methods can extend to diverse muscle groups and species.
We extend our deepest appreciation to Michael A. Kaminski (King Fahd University of Petroleum and Minerals, Editor-in-Chief, Micropaleontology) for his prompt and diligent review of our manuscript, which played a pivotal role in publishing our research. We are also grateful to the anonymous reviewers for their constructive feedback. Their meticulous assessment to maintain the high standards of this journal helped us to improve the quality of our research. This research was funded by the Research and Scientific Development Office (Oficina de investigaciones y desarrollo científico) of the Universidad del Tolima with the award number 160130517.
Figure S1
Data type: .jpg
Explanation note: Medial view of a right thoracic limb of Potos flavus. B, m. brachialis; BB, m. biceps brachii; BBa, accessory belly; BBb, caput breve; BBl, caput longum; Mc, musculocutaneous nerve.
Figure S2
Data type: .jpg
Explanation note: Branch of the radial nerve to the tensor fascia antebrachii in Procyon cancrivorus and Lontra longicaudis. Medial views of a right thoracic limb of P. cancrivorus (a, b); medial view of a left thoracic limb of L. longicaudis (c). R, radial nerve; R’) branch to the m. tensor fasciae antebrachii, TFA) m. tensor fasciae antebrachii; TFACd, pars caudalis. TFACr, pars cranialis.
Figure S3
Data type: .pdf
Explanation note: Complete figure 10 with all specimens.