A Primitive Trait in Two Breeds of Equus Caballus Revealed by Comparative Anatomy of the Distal Limb

Abstract

The 55-million-year history of equine phylogeny has been well-documented from the skeletal record; however, this is less true for the soft tissue structures that are now vestigial in modern horse. A recent study reported that two ligamentous structures resembling functional interosseous muscle II and IV were evident in Dutch Konik horses. The current study investigates this finding and compares it to members of the genus Equus to identify either a breed anomaly or functional primitive trait. Distal limbs (n = 574) were dissected from four species of Equus; E. caballus, E. asinus, E. przewalskii and E. quagga boehmi. E. caballus is represented by 18 breeds of horse, including the primitive Dutch Konik'. The interosseous muscle II and IV were evident in all four species, but only two breeds of E. caballus expressed this trait-the Dutch Konik and Bosnian Mountain Horse. These two breeds were the only close descendants of the extinct Equus ferus ferus (Tarpan) represented in this study. In conclusion, the interosseous muscle II and IV originated from the distal nodule of metacarpal II and IV, respectively, and inserted into the corresponding branches of interosseous muscle III proximal to the sesamoids. This suggests a functional role in medial and lateral joint stability and a primitive trait in modern equids.

Keywords: Atavism; Bosnian Mountain Horse; Donkey; Dutch Konik; III and IV; Przewalski’s horse; Zebra; interosseous muscle II.

Figure 1

The medial view of the metacarpophalangeal joint in the equine right foreleg. The metacarpal II and its distal nodule (beige) and rudimentary tendon of the interosseous muscle II (orange) are noted. The medial branch of the interosseus III muscle, its bifurcation into medial and lateral branches of the interosseus III muscle have been depicted (blue). Adapted from Schmaltz 1911 [17]. Mc.III–metacarpal III; Ph.I–phalanx I; 1. metacarpal II (beige); 2. distal nodule metacarpal IV (beige); 3. rudimentary tendon of the interosseous muscle II (orange); 4. Medial collateral ligament of the metacarpophalangeal joint; 5. anterior fascia of the metacarpophalangeal joint; 6. medial branch of the interosseus III muscle (blue); 7. medial aspect of the collateral sesamoidean ligament; 8. intersesamoidean ligament; 9. manica flexorium; 10. proximal aspect of the digital tendon sheath; 11. bifurcation of the branches of the interosseus III muscle; 12. deep digital flexor tendon (DDFT); 13. interosseus III muscle; 14. superficial digital flexor tendon (SDFT).

Figure 2

Left foreleg: the lateral view of the carpus, metacarpal III, metacarpophalangeal joint and proximal phalanx I with major soft tissue structures. Adapted from Ashdown and Done [20]. SDFT: flexor digitorum superficialis, DDFT: flexor digitorum profundus, ICL: profundus ligamentum accessorium or inferior check ligament, MC4: metacarpal IV, IM3: interosseous muscle III.

Figure 3

Resection of the distal foreleg revealing IM2 and IM4. (a). Left foreleg: the palmar view from the carpus to proximal Ph.I in a seven year old female E. caballus (Dutch Konik) from De Rug, The Netherlands, displaying the interosseous muscles IM2 and IM4, and resected IM3. (b). Left foreleg: the palmar view from the carpus to proximal Ph.I in a 11 year old male E. przewalskii (Przewalski’s horse) from Hortobagy National Park, Hungary, displaying interosseous muscles IM2 and IM4, and the resected IM3. (c). Right foreleg: the lateral palmar view from the carpus to proximal Ph.I in a 23 year old female E. quagga boehmi (Grant’s zebra) from Wildlands Adventure Zoo Emmen, The Netherlands, displaying interosseous muscle IM4 and the resected IM3 (IM2 was not present). MC2: metacarpal II, MC4: metacarpal IV, IM3: interosseous muscle III, IM4: interosseous muscle IV.

Figure 3

Resection of the distal foreleg revealing IM2 and IM4. (a). Left foreleg: the palmar view from the carpus to proximal Ph.I in a seven year old female E. caballus (Dutch Konik) from De Rug, The Netherlands, displaying the interosseous muscles IM2 and IM4, and resected IM3. (b). Left foreleg: the palmar view from the carpus to proximal Ph.I in a 11 year old male E. przewalskii (Przewalski’s horse) from Hortobagy National Park, Hungary, displaying interosseous muscles IM2 and IM4, and the resected IM3. (c). Right foreleg: the lateral palmar view from the carpus to proximal Ph.I in a 23 year old female E. quagga boehmi (Grant’s zebra) from Wildlands Adventure Zoo Emmen, The Netherlands, displaying interosseous muscle IM4 and the resected IM3 (IM2 was not present). MC2: metacarpal II, MC4: metacarpal IV, IM3: interosseous muscle III, IM4: interosseous muscle IV.