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Conodont Anatomy, Chordate Phylogeny and Vertebrate Classification

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Conodont Anatomy, Chordate Phylogeny and Vertebrate Classification Conodont anatomy, chordate phylogeny and vertebrate classification PHILIP C.J. DONOGHUE, MARK A. PURNELL AND RICHARD J. ALDRIDGE Donoghue, P.C.J., Purnell, M.A. & Aldridge, R.J. 1998 09 15: Conodont anatomy, chordate LETHAIA phylogeny and vertebrate classification. Lethaia, Vol. 31, pp. 21 1-219. Oslo. ISSN 0024-1164. Interpretations of conodont anatomy and affinity continue to generate controversy. Fossilized soft-tissue evidence indicates that conodonts possessed eyes, extrinsic eye muscles, a noto- chord, myomeres, a differentiated tail with fin radials, possible otic capsules and possible branchial structures. Indirect evidence suggests a differentiated brain and cartilaginous head skeleton. The multi-component phosphatic tissue complexes of the conodont feeding appara- tus cannot be compared to the amorphous apatite of extant agnathan otoliths. By limiting cla- distic analysis to a restricted selection of these characters the hypothesis that conodonts are a sister group of the clade comprising extant hagfish, lampreys and gnathostomes can be sup- ported. However, exhaustive analysis of a more complete character-set strongly supports the hypothesis that conodonts are more derived than hagfish. From a taxonomic perspective, these two hypotheses have no effect on how conodonts should be classified. Whether they are a stem group (the former hypothesis) or part of the crown group (the latter), conodonts are clearly part of the total group Vertebrata (=Craniata). OChordates, craniates, Vertebrates, Agnatha, conodonts, cladistic analysis, phylogeny, anatomy, palaeobiology. Philip C.J. Donoghue [[email protected]], School of Earth Sciences, University of Bir- mingham, Edgbaston, Birmingham B15 ZTT,UK; Mark A. Purnell [[email protected]] &Richard I. Aldridge [[email protected]],Department of Geology, University ofleicester, University Road, Leices- terLE1 7RH, UK; 29th December, 1997; revised 16th June, 1998. Conodonts are provocative fossils. Their discovery by by investigations of element histology (Sansom et al. Pander in 1856 initiated a debate regarding conodont 1992, 1994) and supplemented by further soft-tissue evi- biology and affinities that has sometimes been heated and dence published by Gabbott et al. (1995). Pridmore et al. that continues to this day (see Aldridge 1987; Aldridge et (1997) have taken issue with this conclusion and have al. 1993; Janvier 1995; Aldridge & Purnell 1996). The argued that the evidence of the preserved anatomy of the recent paper by Pridmore et al. (1997) demonstrates that fossil conodonts places them outside the Craniata consensus remains elusive, and raises points that demand (=Vertebrata in the usage of many authors). Their paper a response. considers three separate but related aspects of the debate: A number of aspects of the debate are, however, no conodont anatomy, chordate inter-relationships and the longer controversial. The discovery of conodont fossils classification of vertebrates. Here we set out to reconsider preserving remains of myomeres and a notochord (Briggs conodonts in the light of the points raised by Pridmore et et al. 1983; Aldridge et al. 1986, 1993; Briggs & Kear 1994; al. (1997). Gabbott et al. 1995) has provided widely accepted evi- dence that the affinities of the conodonts (used here=euconodonts sensu Bengtson 1976) lie among the chordates (Pridmore et al. 1997 and references therein). Conodont anatomy Controversy is now focused primarily on details of cono- dont biology, such as soft-tissue anatomy, element histol- Interpretation of the anatomy of fossil specimens is often ogy and the functional morphology of the feeding appa- difficult, especially where there are no undisputed close ratus, particularly centering on the emotive question modern relatives. The vagaries of preservation invariably ‘were conodonts vertebrates?’ result in the loss of some information, and the features The case for a vertebrate affinity for conodonts was that survive decomposition become transformed during most fully advanced by Aldridge et al. (1993), supported compaction and mineralization. Hence, absence of evi- 212 P.C.J. Donoghue, M.A. Purnell e+ R.J. Aldridge LETHAIA 3 1 ( 1998) dence should not be taken as evidence of absence. Inter- then compare with the myomeres of ammocoetes, which pretation necessarily relies on comparisons with struc- are cryptically W-shaped with the outer limbs of the W tures in extant organisms, but it is important to avoid acutely deflected anteriorly as they approach the dorsal circularity: the choice of a modern comparator controls and ventral margins of the body. However, there is cur- the interpretation of anatomical characters, which, if then rently no evidence, either direct or indirect, to suggest used in a phylogenetic analysis, will inevitably link the fos- that conodont myomeres were anything other than V- sil and modern forms. To minimize these dangers, the shaped. Interestingly, it has been shown that the angle of most rigorous approach is to use a few unequivocal char- inflection of chordate myomeres is related to function acters to set a broad phylogenetic context within which to (Van Der Stelt 1968). The angle of inflection in the cono- interpret other features of the fossils. The best placement donts (80"-90") is similar to values measured in teleost within the selected clade can then be determined by an larvae (Van Raamsdonk et a/. 1977) but greater than analysis involving all the anatomical characters. In the angles in Branchiostorna (60°-70"). Measurements taken case of conodonts, the starting point is the presence of a from the figures of Briggs & Kear (1994) suggest that these notochord and myomeres, characters unique to the phy- angles do not change significantly during decay. lum Chordata. Pridmore et al. ( 1997) suggested that the relative dispo- sition of myomeres on either side of the trunk might be an important feature. In Branchiostoma, and possibly in The tail Myxine, these are staggered, whereas in the hagfish Eptatretus and in other vertebrates they are aligned. It is a The tail bears a clear caudal fin, within which fin radials functional requirement of the axial locomotory system of are observable (Briggs er al. 1983; Aldridge et al. 1986, chordates that the apices of V-shaped myomeres are 1993), comparable in their position, length, orientation aligned with the notochord or vertebral column (see Bone and number to the fin radials of vertebrates (Briggs & et al. 1995), but in the conodont fossils the axes of the Vs Kear 1994; Pridmore et al. 1997). There is currently no are displaced. There must, therefore, have been some rel- evidence to determine whether musculature was present ative dislocation of the myomeres and the notochord post at radial bases. The fin on one side of the animal is appar- mortem, and details of original myomere disposition ently divided into at least two lobes (Briggs et al. 1983). appear to be beyond the resolution of the known fossil material. Trunk musculature The trunk muscles of conodonts preserve the characteris- Paired structures in the head tic inflection of chordate myomeres (Aldridge et al. 1993; Not all chordates possess a head, but there is clearly a Gabbott etal. 1995),but as the fossils lack clear body mar- degree of anatomical elaboration at the anterior end of the gins it is more difficult to determine whether the myo- conodont body that warrants use of the term. This should meres were originally V- or W-shaped. The preserved not be taken as an a priori implication that conodonts portions are, however, clearly V-shaped. V-shaped myo- were craniates or vertebrates; we use the term merely as meres are preserved in the Carboniferous fossils Gilpich- shorthand for the collection of organs and structures at thys and Pipiscius (Bardack & Richardson 1977), a possi- the anterior of the animal. ble hagfish and a possible lamprey, respectively (Janvier Prominent among the soft-tissue features of the cono- 1981, 1996b), but as Pridmore et al. (1997) indicated, this dont head is a pair of lobate or doughnut-shaped struc- is unparalleled by any extant adult vertebrate. However, tures, which have been interpreted as the remnants of embryonic and hatchling hagfishes have V-shaped myo- sclerotic eye capsules (Aldridge & Theron 1993; Aldridge meres (Dean 1899), and they are widespread among larval era/. 1993; Gabbott et al, 1995; Purnell 1995a). Pridmore stages of gnathostomes, including teleosts and anurans. et a/. (1997) considered these structures to be more rea- As Pridmore et al. ( 1997) have pointed out, it is possible sonably interpreted as otic capsules, based on the size, that the apparent V-shape of the conodont myomeres shape and position of the lobes, on the relative preserva- reflects post mortem decay of originally more complex tion potential of agnathan eyes and otic capsules, and on structures. They considered, however, that the dorsal por- the interpretation of associated musculature. tions of the myomeres are complete and that only M- Relative size alone may not be a reliable indicator for shaped myomeres could be produced if additional chev- differentiating eyes from otic capsules in extant agnathans rons of ventral musculature were postulated. An alterna- (Pridmore et a/. 1997), although diagrams of dissections tive would be that the preserved portion represents the published by Marinelli & Strenger (1954) show that the axial inflection of the W, with the ventral limb lost and a eyes of the lamprey Lampetra fluviatilis are considerably shorter dorsal limb either unpreserved or concealed in the larger than the otic capsules (contra Pridmore et al. 1997, known lateral views of the trunk. The structures would Fig. 2). However, Pridmore et al. (1997) considered that LETHAIA 3 1 (1998) Conodonts, chordates and vertebrates 213 the paired lobes in conodonts are more similar in size to need to resort to ad hoc speculations regarding unpre- the otic capsules of fossil hagfish and lampreys than they served anterior extensions to the head. are to their eyes. The data here are limited to two taxa: the The original interpretation of the conodont lobes as fossil lamprey Mayomyzon and the fossil hagfish Myxi- sclerotic cartilages surrounding eyes (Aldridge & Theron nikela.
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