FULL COMMUNICATION COMPARATIVE ANATOMY An unusual elbow muscle in the red howler monkey: does it deserve invention of a new name musculus contrahens cubiti? Mellin Novikova and Alexander Kuznetsov Faculty of Biology, Lomonosov Moscow State University, Leninskie gory, 1, build. 12, Moscow, 119234, Russian Federation; [email protected] Address correspondence and requests for materials to Alexander Kuznetsov Abstract During dissection of a juvenile specimen of howler monkey (Alouatta seniculus), we discovered a supernumerary muscle. This muscle originates from the lateral epicondyle of the humerus, distal to the m. brahioradialis and beneath the m. exten- sor carpi radialis longus, and runs deeply to insert on the proximal part of the radius adjacent to the m. supinator. To determine homology of the unusual muscle, we compared it with the known abnormal extra muscles in the group of preaxial forearm extensors in humans and other primates, as well as other mammals. The only similar muscle is the so-called m. brachioradialis accessorius, which is very rarely found in humans medial to the n. radialis r. superficialis. Both in howlers and humans, its unique topological interre- lations with the n. radialis suggest that this muscle is fundamentally different from all surrounding forearm extensors including the proper m. brachioradialis. At the same time, its innervation by the n. radialis confirms that it is a true extensor, contrary to the reptilian m. tractor radii. The general problem of identifying homology of anomalies and novelties is considered. As the enigmatic muscle departs from rules of myological architecture of the tetrapod forelimb, we failed to establish its general homology and, instead, suggest naming it as m. contrahens cubiti. This means that the muscle acts as the elbow flexor although it intimately belongs to extensors. Keywords: general homology problem, anomaly, forearm extensor muscles, bra- chioradialis, primates, Alouatta Citation: Novikova, M. and Kuznetsov, A. 2017. An unusual elbow muscle in the red howler monkey: does it deserve invention of a new name musculus contrahens cubiti? Bio. Comm. 62(1): 38–47. doi: 10.21638/11701/spbu03.2017.105 Introduction Author’s information: Mellin Novikova, Research engineer, orcid.org/0000-0001-8446-124X, In this paper, a very unusual forearm muscle will be considered, which was found Researcher ID: C-7679-2017; Alexander Kuznetsov, Ph.D., Ass. Prof., orcid.org/0000-0002-9928-6955, in one specimen of red howler monkey (Alouatta seniculus) in the course of Researcher ID: C-7645-2017, Scopus Author extensive research on the forearm musculature of the New World monkeys. In ID: 7401696738 mammals, extensor musculature of the forearm and hand originating from the Manuscript Editor: Yegor Malashichev, Saint Petersburg State University, Russia lateral epicondyle of the humerus typically includes: one central muscle — the Received: October 07, 2016; m. extensor digitorum communis; two postaxial muscles — the m. extensor digi- Revised: January 30, 2017; torum lateralis and m. extensor carpi ulnaris; and four preaxial muscles — the Accepted: February 16, 2017; m. supinator, m. brachioradialis (termed in the old literature m. supinator bre- Copyright: © 2017 Novikova and Kuznetsov. This is vis and m. supinator longus, respectively), m. extensor carpi radialis longus, and an open-access article distributed under the terms of m. extensor carpi radialis brevis. In our specimen of the howler monkey, the fifth the License Agreement with Saint Petersburg State University, which permits to the authors unrestricted preaxial forearm extensor was present bilaterally. distribution and self-archiving free of charge. Anatomical abnormalities rarely occur in the wild, especially rare are new Funding: The work was supported by the Russian formations, and even more rarely they are captured by anatomists. Because of Foundation for Basic Research (grant no. 15-04- 05049-a). The funders had no role in study design, data scarcity, the theoretical basis for analysis of natural anatomical anomalies is data collection and analysis, decision to publish, or poorly developed, and to properly present the morphological significance of our preparation of the manuscript. finding, we have to formulate the principles ourselves, according to fundamen- Competing interests: The authors have declared that no competing interests exist. tals of comparative anatomy. Among few useful publications on the topic is “The BIOLOGICAL COMMUNICATIONS, vol. 62, issue 1, January–March, 2017 | doi: 10.21638/11701/spbu03.2017.105 39 logic of monsters…” by Pere Alberch (1989), the founder block of the vertebrate body and (ii) the archetype de- of Evo-Devo. But its dashing title is rather misleading picting the normal composition (linear series) of these for understanding abnormalities, because it seemingly elementary building blocks. The linking notions are presumes that monsters bear some logic of their own, (i) the general homology, (ii) special homology, and which is independent of the logic of normally developed (iii) serial homology. The priority of terms “archetype” organisms. This presumption is incongruous, as mon- and “homology” was contested by Maclise (Rupke, sters are not united with each other directly, bypassing 1993), and the vertebrate archetype diagram was defi- the normal organisms. Undoubtedly, “monsters” and nitely developed by Owen on the base of similar dia- anomalies (i.e. partial monsters) are the sprouts of the grams published by Carl Carus 20 years earlier (Rupke, logic of norm. So, by studying anomalies, we study the 1993; Richards, 2016). Thus, the major novelty of Owen general logic of the morphological architecture, its de- himself was the demarcation of three sorts of homol- grees of freedom and constraints. ogy, of which the first and most important has been When an anomaly is identified, a name should be almost entirely lost by contemporary biology. It is the given, like in the case of normal organs. To avoid ex- general homology, which was Owen’s tool to associate, cess multiplication of terms, the anatomist investigates part by part, real organisms with the ideal archetype. As the literature in search of previous findings of the most soon as Darwinism kidnapped the archetype concept similar anomalies within the same species, or around and equalized it with the ancestral form, the necessity the taxonomic neighborhood. If no precedent is found, in the concept of general homology vanished, because a new appropriate term is invented. Since Joseph Ma- the ancestor, even if unknown to science, was definitely clise (1846) and Richard Owen (1846) invented the built from the same matter as its offsprings, and, hence, terms “archetype” and “homology”, in the anatomical the special homology is adequate to associate its body thesaurus, anatomists avoid using the same names for parts with theirs. In the process of substitution of the non-homologous structures. It is more difficult to iden- archetype with the ancestor, nobody noticed an arising tify homologies of abnormal structures than of normal absurdity: as an organism has an innumerable chain of ones. The problem is that, due to the rarity of abnormali- ancestors, it is obviously senseless to adopt every one of ties, it is usually technically impossible, to approve that them as a new archetype. But if we decide that not ev- two anomalies labeled with the same term bear the same ery ancestor deserves the rank of archetype, the ques- genetic basis. Does this mean that every case of anom- tion arises of how to distinguish those which deserve? aly with unknown genesis should be cautiously termed Owen did not face such an artificial problem, because he with a unique name? Or, per contra, anomalies do not did not expect that Nature bears numerous archetypes. deserve to be termed at all? Or, may be only heritable More than that, the less is the number of archetypes, to anomalies deserve proper names? Actually, there is often which the diversity of real bodies can be reduced, the no confidence in whether the anomaly is a heritable trait greater is, hence, the efficiency of comparative anatomy. or a non-heredity morphosis (sensu Schmalhausen, see The archetype in anatomy, like laws in physics, is the tri- Levit et al., 2006). For Richard Owen, when he began to umph of generalization. Owen repeatedly emphasized, introduce the concept of archetype and homology into that the general homology of organs of a real organism anatomical practice (Owen, 1848, 1849), these questions is established by naming those in terms of the archetype. were much easier to solve, than to modern biologists. Implicitly, we often follow the same procedure today. For This is the penalty for Darwin's suggestion to incorpo- instance, when comparing the hindlimb muscles of real rate Owen’s ideas into evolutionary theory by straight- tetrapods, we state the special homology of mammalian forward equalization of his archetype with an ancestor; m. obturator externus and avian m. obturator internus, since then, homologization is virtually prohibited with- while the general homologization with the myological out the knowledge of heredity and phylogenetic origin. archetype of the tetrapod hindlimb is performed when To loosen the fetters and bypass the difficulties, which we term both of these special muscles as the m. pubois- derive from equalization of archetype to ancestor, a tre- chiofemoralis externus. Only those real organisms belong mendous amount of shades of homology were noticed to particular archetype, which organs and structures can and termed (Pavlinov, 2012). In our case, this termi- be entirely named in terms of this archetype. If an or- nological activity fails to help make a step forward in gan is found, which does not fit there, this means that the simple applied question of how to treat the unusual its general homology cannot be identified and, hence, an howler’s muscle. It is thus more useful to rely on Owen. organism with this organ belongs to some other arche- The kernel of Owen's concept of homology (the type, may be not yet defined.