AMNH NOVITATES Tuesday Jan 09 2001 03:54 PM 2000 novi 00174 Mp 1 Allen Press x DTPro System File # 01TQ PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3312, 24 pp., 6 ®gures January 30, 2001 The Intermuscular System of Acanthomorph Fishes: a Commentary G. DAVID JOHNSON1 AND COLIN PATTERSON2 We got some `splainin' to do. Desi Arnaz, as Ricky Ricardo in ``I Love Lucy'' You can't have your cake and eat it too. Ancient Proverb ABSTRACT This paper is a response to Gemballa and Britz (1998), who presented a new interpretation of the intermuscular bones of acanthomorphs, homologizing them with the epicentrals of lower teleosts. We argue that their identi®cation of epineural ligaments above the intermuscular bones in many acanthomorphs is mistaken; the structures in question are fanlike arrays of collagen ®bers, not true intermuscular ligaments. We show also that undisputed epineural intermusculars penetrate or enter the horizontal septum in lower acanthomorphs (Velifer, Polymixia, beryci- forms), and reiterate arguments for regarding the single series of intermusculars in most acan- thomorphs as epineurals, secondarily displaced into the horizontal septum. 1 Curator, Division of Fishes, National Museum of Natural History, Smithsonian Institution, Washington DC 20560. 2 Honorary Research Fellow, FRS, deceased 1998, Department of Palaeontology, The Natural History Museum, London. Copyright q American Museum of Natural History 2001 ISSN 0003-0082 / Price $4.00 AMNH NOVITATES Tuesday Jan 09 2001 03:53 PM 2000 novi 00174 Mp 2 Allen Press x DTPro System File # 01TQ 2 AMERICAN MUSEUM NOVITATES NO. 3312 INTRODUCTION eton and thus act as tendons''; (3) when in- termuscular bones develop within the struc- Gemballa and Britz (1998; hereafter re- tures, thereby justifying the term ligament for ferred to as ``G & B'') have published an the proximal part joining bone to axial skel- account of the intermuscular bones and lig- eton, the bone appears relatively late in on- aments in acanthomorphs. Their interpreta- togeny, ``and we see no necessity to apply a tion of those structures differs in several different term to the same structure at a later ways from ours (Johnson and Patterson, point in ontogeny.'' 1993; Patterson and Johnson, 1995; hereafter We regret and will oppose their decision referred to respectively as ``J & P'' and ``P to change the names, in part because the in- & J''). In brief, G & B's main conclusion is termuscular system is already dif®cult, and that the single series of intermuscular bones to change the entire terminology only three found in the vast majority of acanthomorphs years after the system was ®rst mapped (in are not epineurals displaced ventrally into the P & J) will not help communication. We also horizontal septum, as we proposed, but are question the propriety of using tendon for all epicentrals. [Incidentally, this was also the the structures that we called ligaments. The opinion of Richard Owen, who ®rst named technical distinction is ambiguous in a num- the three intermuscular series (1866), and be- ber of cases. The term tendon was used for lieved that the bones in Perca and Gadus, structures in the horizontal septum by Kish- the acanthomorphs in his sample, were epi- inouye (1923) and Kafuku (1950), who centrals. As we wrote (P & J, p. 2), ``we both named the anterior and posterior oblique ten- initially thought Owen right.''] G & B found dons (AOTs, POTs); Kafuku's AOTs are our that our conclusion resulted from inadequate epicentral ligaments (in lower teleosts) and technique, which led us to miss ligamentous epineural ligaments (in percomorphs). Ka- elements of the intermuscular system in fuku named these structures tendons because many of the acanthomorphs that we exam- they connect the lateralis super®cialis (red ined. The purpose of the present paper is to muscle) with the axial skeleton. Westneat et explain some of the differences between their al. (1993) gave a detailed account of the ar- observations and our own and the back- rangement of AOTs and POTs in the hori- ground to our conclusions; then to review the zontal septum of scombrids and their role in different interpretations of structure in some the transmission of forces to the axial skel- acanthomorph taxa; and ®nally to compare eton. Among other things, they found that the two arguments for homology. In the In- AOTs and POTs are joined distally into a troduction to our paper (P & J) we wrote that loop by a segment of the POT that they our observations of ligaments ``include an called ITL (intertendon length)Ð``This loop unknown quantity of subjectivity, and we is so robust that the ®sh can be lifted by it.'' will be glad to see all or any of them checked Each POT slides relative to succeeding AOTs by others.'' We did not anticipate that we as it passes posteriorly, but a signi®cant por- should so soon be required to check some of tion of its ®bers converge with and join those them ourselves. To anticipate our results, af- of the distal end of the anteriormost AOT of ter a lot of work and some initial doubts, we the loop (Westneat et al., 1993: 195). Here are convinced that the intermuscular bones of then, AOT and POT together form what is acanthomorphs are epineurals. technically a ligament, since it joins two bones (vertebrae). Also, although POTs gen- TERMINOLOGY erally insert in the lateralis super®cialis, the Following advice from M. W. Westneat, G anteriormost POTs may insert on intermus- & B elected to use the name ``tendon'' for cular bones in percomorphs (P & J), here the connective tissue bundles in myosepta again meeting the technical criterion of lig- that we (P & J) named ``ligaments.'' Their ament. reasons for the change of name were: (1) the The other series of structures that we (P & structures connect muscle to bone (axial J) named ligaments are the epineurals and skeleton); (2) the structures ``transfer mus- epipleurals. Members of these series have cular forces from myomeres to the axial skel- been best illustrated by Gemballa (1995). AMNH NOVITATES Tuesday Jan 09 2001 03:53 PM 2000 novi 00174 Mp 3 Allen Press x DTPro System File # 01TQ 2001 JOHNSON AND PATTERSON: ACANTHOMORPH INTERMUSCULARS 3 They attach to the axial skeleton proximally (which he called intramuscular) bones (spe- and lie in the myoseptum, where they fan out ci®cally epineurals), and also used the term distally into an array of small ®ber bundles ligament for unossi®ed intermuscular ele- that have no speci®c insertion in the body ments. Perhaps reinforcement and stabiliza- musculature. Some epaxial muscle ®bers at- tion do not exclude a transmission of force tach perpendicularly along the length of the function as well, but if the latter is the cri- intermusculars (be they ossi®ed or unossi- terion by which we are to distinguish tendon ®ed), as they do over the entire surface of from ligament, then structures such as the the myoseptum, but the muscle ®bers do not maxillo-mandibular ligament, semicircular converge or concentrate at the points of in- ligament, and others must also be renamed. sertion as is typically the case for tendons. G G & B's usage also entails some ambigu- & B stated that such ®bers ``transfer mus- ity. For example, in describing Velifer they cular forces from myomeres to the axial skel- called the ligaments attaching the epineural eton and thus act as tendons'' but this is to bones of V7±10 ``a short proximal tendinous oversimplify, and of course the same then sheath.'' We will therefore continue to use must be true of the entire myoseptum, as dis- our earlier terminology, and call the connec- cussed by Van Leeuwen (1999). Further- tive tissue bundles ligaments, not tendons. more, we are unable to reconcile G & B's conclusion (nor did they address it) about Abbreviations transmission of forces with the following statement from Gemballa (1995: iii), in BL Baudelot's ligament which the term ligament was used for the cf collagen ®ber ecb epicentral bone (G & B only) unossi®ed elements of all three series: ecc epicentral cartilage Epineural and epipleural ligaments are unlikely to ecl epicentral ligament play a role in transmission of muscular forces because enb epineural bone its collagen ®bres and white muscle ®bers include an ent epineural tendon (G & B only) obtuse angle. They resist the radial expansion of con- epb epipleural bone tracting muscle ®bres of the VAC's and, DAC's and lb lateral band (G & B only) thus may stiffen the trunk during undulatory loco- ns neural spine motion (bracing and bandaging ``Querverspannungs- V vertebra funktion''). Furthermore during sharp bendings these ligaments keep the relative positions of axial skeleton Institutional abbreviations are those of and integument to each other against the expanding musculature (holding device or guy, ``Haltefunk- Leviton et al. (1985). tion''). DIFFERENT METHODS; DIFFERENT In a more recent paper, Gemballa (1998: CRITERIA; DIFFERENT RESULTS? 30) hypothesized that ``Muscle arches con- sisting of white muscle ®bers are reinforced G & B used the following technique de- by epineural tendons and lateral bands that veloped by Gemballa during work for his may (emphasis ours) transmit forces to the doctoral thesis (1995): cleared-and-stained (c backbone.'' It would appear that the precise & s) specimens are transferred from glycerin function of unossi®ed epineural and epi- to ethanol; then individual myosepta are cut pleural elements remains unresolved. Epi- free, mounted on slides, and studied with po- neural and epipleural intermuscular bones, larized light. With this method, G & B are when present, are ossi®cations in the epineu- able to visualize the collagen ®bers in a myo- ral and epipleural ligaments.