The astragalar dorsal foramen and the astragalar flexor groove in Felidae and Coypus

doi:10.21203/rs.3.rs-4909088/v1

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The astragalar dorsal foramen and the astragalar flexor groove in Felidae and Coypus

https://doi.org/10.21203/rs.3.rs-4909088/v1

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The dorsal foramen and the flexor groove are traditionally considered traits of plantigrade and pentadactyl mammals, and therefore basal unspecialized taxa, and they are often cited as a features that would have limited plantar flexion of the foot. Blood vessels, nerves, ligaments and tendons have been proposed passing through the foramen. During the last two years I had the opportunity to dissect three roadkills: a Myocastor coypus, a Felis catus and a Leopardus geoffroyi. I found the distinct presence of the upper joint capsule border before the end of the trochlea in the three animals. The limit of the trochlea being marked by the posterior edges of the lateral and medial malleolar facets. The astragalar foramen was present in both felids. Through the dorsal foramen was a unidentified blood vessel. No nerves or ligaments were found passing through the foramen. The flexor digitorum longus muscle of L. geoffroyi had a sesamoid at the height of the tibioastragalar articulation. Although these are considered traits of plantigrade and pentadactyl mammals, they are common in Carnivora, particularly common in felids, being observed at least in the genera Felis, Leopardus, Panthera, Prionailurus, Puma, and Smilodon.

Eutheria. Astragalus

dorsal foramen. Felidae. Hystricomorpha

The astragalar flexor groove is a sliding surface for the ligaments of the flexor hallucis longus and the flexor digitorum longus muscles (lateral and medial deep digit flexors. NAV). The astragalar dorsal foramen is located in the flexor groove in the limit with the astragalar trochlea. In some animals, but rarely, it is confluent with the foramina of the sinus tarsi, taken then the name of astragalar canal (Szalay, 1966). The astragalar dorsal foramen is an anatomical feature found in various fossil and extant mammals. It is an important phylogenetic character in mammal studies incorporating the astragalus, typically being the first astragalar character (Cifelli, 1993; Bergqvist, 1996; O'Leary et al 2013; Muizon et al., 2015; Velazco et al, 2022). Both the dorsal foramen and the flexor groove are traditionally considered traits of plantigrade and pentadactyl mammals, often cited as limiting plantar flexion of the foot There is little evidence for either case as well as for what it is going through the foramen. Blood vessels, nerves, ligaments and tendons have been proposed passing through the foramen and it is cited often as a feature that would have limited plantar flexion of the foot (Ameghino, 1904; Schaeffer, 1947; Szalay, 1966; Wang, 1993; Shockey and Flynn; 2007). During the last two years I had the opportunity to dissect three roadkills: a coypu (Myocastor coypus), a domestic cat (Felis catus) and a Geoffroy's cat (Leopardus geoffroyi). The purpose of this short communication is to make available the results of those dissections about the astragalar dorsal foramen and the astragalar flexor groove.

Gross anatomical dissections of the ankle of three roadkills at variable grades of decomposition: a coypu (Myocastor coypus), a domestic cat (Felis catus) and Geoffroy's cat (Leopardus geoffroyi). All animals were found dead and their bodies treated with care.

The coypu (MLP-Mz 3114) was an adult female individual of about 3.8 kg. It was found during August 2022, in Paseo del Bosque, La Plata, Provincia de Buenos Aires, Argentina by another researcher (Dr. Cecilia Morgan) and myself and retrieve from the location by Museo de La Plata personnel (Lorente, Inaturalist 2024). It had several days dead at the moment of collection, but the cold winter weather had helped to preserve it.

The domestic cat was an adult female found recently dead by myself in a street in the same locality. I kept both skinned hindlimbs in a solution of ethyl alcohol 70%.

The Geoffroy's cat (MLP-Mz 3115) was a melanic adult male found by a team of researchers on the side of National Route 11 during November 2023 in General Lavalle, Provincia de Buenos Aires, Argentina and donated to Museo de La Plata (Acosta Albarracín M, iNaturalist 2024). It was probably less than a day dead when found. It was skinned by personnel from the museum and preserved in a freezer until dissection during December 2023. It was defreeze slowly in hope to less damage the specimen. During dissection, a colored polyvinyl acetate was injected by the popliteal artery for better recognition of its branches. Unfortunately, the blood vessels were punctured in several places and the technique was abandoned.

The coypu and L. geoffroyi were dissected directly in the Division of Mastozoology of Museo de La Plata. Their skeletons are now preserved at the museum.

Institutional abbreviations. MLP-Mz, Division of Mastozoology of Museo de La Plata, Argentina; NAV, Nomina Anatomica Veterinaria.

I dissected the coypu (M. coypus) first during august 2022, at the time I was interested in another feature of the tarsus, but the distinct presence of the upper joint capsule border before the end of the trochlea called to my attention. The same was observed in the two felids. The limit of the trochlea being marked by the posterior edges of the lateral and medial malleolar facets. The coypu had one talocalcaneum laterale ligament inserted in a oblique crest on the dorsal side of the astragalar neck; one posterior talofibularis ligament; one, very thick, posterior tibiotalaris; and two tibionavicularis ligaments (collateral ligaments).

I collected the cat myself from the street in full knowledge that the dorsal foramen was common in felids (personal observation; Spaulding & Flynn, 2012). The astragalar foramen was present in both felids. It was difficult to identify the tissues going through or out of the foramen because of the state of the roadkills and the difficulty to access the astragalotibial capsule. Nevertheless, it was clear it was a blood vessel, although the relationship with larger vessels could not be established. The foramen was just after the articular capsular tissue, so it may also have some relationship with the fibrous membrane of the joint. In L. geoffroyi, while removing the articular capsule, the foramen started to bleed. In both cases, the tendons of the flexor hallucis longus and flexor digitorum longus muscles slided over the flexor groove with ease and they were unrelated to the dorsal foramen, well separated by the fascia. The flexor digitorum longus muscle of L. geoffroyi had a sesamoid at the height of the tibioastragalar articulation, slightly attached to the end of the flexor groove. No nerves passing through the foramen were observed.

The astragalar flexor groove serves as a sliding surface for the ligaments of the flexor hallucis longus and the flexor digitorum longus muscles (lateral and medial deep digit flexors. NAV). It is positioned beyond the posterior or proximal end of the tibial trochlea (lateral tibial facet in metatheria and basal mammals. Szalay, 1994) and it usually continues with the sustentaculum of the calcaneus bone. The astragalar dorsal foramen is located in the astragalar flexor groove at the edge of the astragalar trochlea. This foramen is near the mid trochlea, and not in the trochlear ridges where other foramina may be found. Both features, the dorsal foramen and the flexor groove are traditionally considered traits of plantigrade and pentadactyl mammals, and they are often cited as limiting plantar flexion of the foot (Ameghino, 1904; Schaeffer, 1947; Szalay, 1966; Wang, 1993; Shockey and Flynn; 2007). But, Potos flavus (Gebo and Rose, 1993) possesses both features and can achieve plantar flexion up to 180° during hindlimb inversion (Jenkins and McClear, 1984; Meldrum et al 1997).

Ameghino (1904) interpreted the astragalar canal as the passage for the tendon of the flexor hallucis longus and the posterior branch of the tibial artery. According to this author, the astragalar canal (and with it, the dorsal foramen) was related to the presence of a well-developed functional first digit and, most importantly, with the transition from plantigrady to digitigrady. As other coetaneus authors, Ameghino considered that the flexor hallucis longus was lost along the first digit, a view now considered erroneus, as the of pentadactyl animals is considered homologous to the lateral component of the deep digit flexor. Both in pentadactyl and non pentadactyl mammals, both flexors attach distally to the navicular bone, in various degrees (Miller et al, 1965; Kelifian and Sarrafian, 2011). Ameghino (1905) also reported the peroneal artery passing through the foramen in Meles meles.

Although these are considered traits of plantigrade and pentadactyl mammals, rodents in general are supposed to lack both (Szalay, 1985). Nonetheless, the flexor groove and the astragalar dorsal foramen are common in Carnivora, in plantigrade and pentadactyl carnivores (such as Potos and Lontra), but also in digitigrade and tetradactyl carnivores (eg. Chrysocion, Lycalopex). They are particularly common in felids, being usually observed at least in the genera Felis, Leopardus, Panthera, Prionailurus, Puma, and Smilodon.

M. coypus instead is a plantigrade pentadactyl semiaquatic hystricomorph rodent without any evidence of a dorsal foramen. Rodents are considered to lack the flexor groove and have an extended trochlea (Szalay, 1985). During dissections it was clear the flexor groove was present, the limit between it and the trochlea marked by the posterior end of the astragalar malleolar facets (both medial and lateral) and the same happened in the two felids. This is probably also true for other animals that seemingly have an extended trochlea (as fossil South American Litopterna. Cifelli, 1993).

The flexor groove of the astragalus may be present in mammals that seem to have an extended throclea, the limit marked by the posterior end of the astragalar malleolar facets (both medial and lateral).

The flexor groove and, particularly, the astragalar dorsal foramen are supposed to be characters of pentadactyl and plantigrade mammals, with plantar flexion of the foot. But, both are common in Carnivora, in plantigrade and pentadactyl carnivores, but also in digitigrade and tetradactyl carnivores being extremely common in the tetradactyl felidae. At least in felids, a blood vessel was found in the dorsal astragalar foramen.

Acknowledgements

Thanks Dr. Martín Acosta Albarracín and his team for the collection of Leopardus geoffroyi. To Eduardo Etcheverry for the pre and post preparation of Myocastor coypus and L. geoffroyi. To Drs Diego Verzi and Itatí Olivares for information and access to the specimens. Special thanks to Dr. Cecilia Clara Morgan for the finding and identification of M. coypus.

Statement of Animal Ethics, including the number of the relevant Ethics Committee's protocol, where appropriate.

Conflict of Interest Statement

No funding was received to assist with the preparation of this manuscript.

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The astragalar dorsal foramen and the astragalar flexor groove in Felidae and Coypus

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