Environmental Biology of Fishes 32: 183-192, 1991. 0 1991 Kluwer Academic Publishers. Printed in the Netherlands.
CT scan reconstruction of the palate region of Latimeria chalumnae Hans-Peter Schultze Museum of Natural History and Department of Systematics & Ecology, The University of Kansas, Lawrence, KS 66045-2454, U.S.A. Received 2.8.1989
Accepted 2.8.1990
Key words: Sarcopterygii, Actinistia, Coelacanth, Morphology, CT scan, Relationships Synopsis
The palate of Latimeria chalumnae is described based mainly on three-dimensional CT scan reconstruction. It is compared with that of other osteichthyans. The palate of L. chalumnae compares best with that of rhipidistians; it is more advanced than that of actinopterygians in having fewer bones. This tendency toward bone reduction in the palate is even more pronounced in dipnoans. The interpretation of features of the Early Devonian genus Diabolepis determines if authors consider dipnoans or actinistians more closely related to tetrapods. Both groups are only distant relatives of tetrapods.
Introduction
The osteology of Latimeria chalumnae Smith 1939 has been described by Smith (1940) and Millot & Anthony (1958). Skull roof, cheek region and lower jaw have been described quite accurately (see compilation by Schultze & Cloutier 1991), whereas the palate has been presented only incompletely by Smith (1940, fig. 10). Smith (1940) figured the toothed part of the dermal bones, and Millot & Anthony (1958, fig. 2) presented the whole parasphenoid in contact with the endocranium but omitted other dermal bones (figured in medial view, Millot & Anthony 1958, pl. 38) except premaxillae and vomers. Schultze (1987, fig. 7B) compiled a composite palate from figures in Millot & Anthony (1958, fig. 2; pl. 2, 31,36, 38). The lack of a satisfactory presentation of the palate can be understood immediately if one realizes that the bones of the medial side of the palatoquadrate and the parasphenoid are not exposed on the palate except for their denticulated areas. The subcephalic muscles (Millot & Anthony 1958, pl.
37) and a pavement of ossifications (Fig. 1; ‘calcaires pavant’ of Millot & Anthony 1958, pl. 1, 14-l&25) lie below them so that Smith (1940) even figures a ‘suprapterygoid denticulate area’. Here I will describe the composition of the palate as perceived by CT scan (Computed Tomography) sections and a three-dimensional reconstruction made from them. Palatal morphology of L. chalumnae will then be compared with that of other osteichthyans.
Material and methods
The description of the palate of Latimeria chalumnae is based on specimen VIMS 8118, a female of 1452mm standard length deposited in the ichthyological collection of the Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point. The Computed Tomography (CT scan or popular CAT scan) was done on the frozen specimen with a SiemensDR-GH scanner in the radiological unit of the Riverside Hospital in
184 Newport News, Virginia on January 3, 1988. The 3D-reconstruction from tapes with the CT scan sections were assembled and computed on a CEMAX-1000 in the Department of Radiology of Johns Hopkins Hospital in Baltimore, Maryland, on August 22 and 23, 1988. The techniques are described in detail by Schultze & Cloutier (1991).
Description The palate of Latimeria chalumnae is composed of premaxillae (‘rostra1 dental plates’of Smith 1940), vomers (‘postrostral dental plates’ of Smith 1940)) two pairs of dermopalatines (‘prevomer’ and ‘autopalatine’ of Smith 1940; ‘predermo-palatin’ and ‘dermo-palatin’ of Millot & Anthony 19.58), elongated ectopterygoids, broad entopterygoids, an unpaired median parasphenoid and two unpaired arcual plates (‘piece preoccipitale souschordale anterieure, posterieure’ of Millot & Anthony 1958). Premaxillae: As in all actinistians the premaxillae are the only bones of the outer dental arcade; maxillae present in actinopterygians and other crossopterygians are missing. The premaxillae form a pair of bones at the anterior margin of the palate (Millot & Anthony 1958, fig. 2); the threedimensional reconstruction of the palate (Fig. 2) indicates the same for specimen VIMS 8118. Smith (1940) figured four pairs of separate dental plates; these plates were referred to by other authors as fragmented premaxillae. One pair to many dental plates represents variation of the premaxillae within the species. The teeth increase in size from the symphysis posterolaterally, but they remain small in comparison to the fang-like teeth on the inner dental arcade. The inner dental arcade is formed by one pair of vomers, two pairs of dermopalatines (one pair less than actinopterygians) and one pair of ectopterygoids; each carrying enlarged fang-like teeth, but small in comparison to other crossopterygians. The inner dental arcade appears to be a continuous arcade with fang-like teeth except for the vomers on the three-dimensional reconstruction (Fig. 2). Vomers: The vomers form triangular plates which meet each other in the midline. Laterally,
they articulate with the anterior dermopalatines, but they have no common articulation with premaxillae or parasphenoid. Dermopalatines: Two long oval shaped dermopalatines follow the vomer posterolaterally. Labially they carry small teeth and lingually fang-like teeth which appear prominently on the three-dimensional reconstruction (Fig. 2). The anterior dermopalatine articulates with an anterolateral flange of the parasphenoid; the posterior dermopalatine articulates with the anterior dermopalatine and the posteriorly following ectopterygoid. Ectopterygoid: An elongated bone (five times longer than the posterior dermopalatine) follows the posterior dermopalatine and attaches posteriorly to the lateral margin of the entopterygoid. Large and small teeth are intermixed on the ectopterygoid. Paired entopterygoids and an unpaired median parasphenoid occupy most of the area of the palate. Two unpaired median arcual plates follow the parasphenoid. Entopterygoid: The entopterygoid attaches to the medial side of the palatoquadrate (Fig. 1) so that it forms the connection between the ventrally positioned ectopterygoid and the dorsal and median stem of the parasphenoid. Larger teeth compared to those on the main toothed area sit on the margin of the entopterygoid where it attaches to the ectopterygoid. An edentulous triangular portion reaches anteriorly to the posterior dermopalatine. The entopterygoid does not reach the parasphenoid. Parasphenoid: The parasphenoid underlies the anterior unit of the endocranium, the ethmosphenoid; it reaches from the ventral fissure of the intracranial joint to the anterior autopalatines. The parasphenoid has a narrow shaft or stem, which ends in two posterolateral swallow-tail shaped extensions. The stem diminishes in width from its posterior end anteriorly until about the middle of its length. From there on, the lateral margins flare out anteriolaterally to reach the anterior dermopalatine. The parasphenoid is five times as wide in the anterior triangular portion than at the narrowest part of the shaft. The triangular anterior portion carries an elevated toothed area in the form of an
185
Fig. 1. Lafimeriu
chalumnae,
specimen VIMS 8118. CT scan section (C44, scan 1811191)through posteriororbital region. c.p=
pavement of ossifications.
elongated oval. This oval extends at its widest extent to the width of the narrowest part of the shaft. The teeth along the margin of the toothed area are larger than the ones in the middle. Arcual plates: Two arcual plates follow the parasphenoid at about the same level in the mid line (Fig. 2) below the notochord in the otico-occipital region. The anterior one is one-quarter wider and longer than the second one. The posterolateral swallow-tail shaped extensions of the parasphenoid underlie the anterior arcual plate. The palatal bones are covered deeply by soft tissue (Fig. 1) except for the toothed areas. The superficial portion of the skin is packed with ossifications (‘calcaires pavant’ of Millot & Anthony 1958; ‘small tooth plates’of Nelson 1969, p. 486, pl. 83, fig. 2). Smith (1940, p. 71) spoke of ‘dentate area of skin’ and figured (Smith 1940, fig. 10) a ‘suprapterygoid denticulate area’ between entopterygoid and toothed area of parasphenoid. This
pavement appears as a complete bony cover of the palate in the CT scan sections (Fig. 1). Therefore, this pavement had to be cut away with the lower jaw in each CT scan section before the palatal bones could be reconstructed (Fig. 2). The toothed areas of dermal bones together with the pavement of ossification form a continuous biting surface of the palate despite large gaps between palatal bones.
Comparison Gardiner (1984, fig. 75,76) and Schultze (1987, fig. 7) have compared the palate of osteichthyans including tetrapods. These authors differ in the interpretation of bones anterior and lateral to the entopterygoids in the dipnoan Griphognathus and of the pterygoid bones in actinopterygians. Gardin-
186
chalumnae, specimen VIMS 8118. Three-dimensional reconstruction of palate from CT scan sections.
Fig. 2. Latirneria
er (1984) does not figure the palate of an actinistian. The palate of Latimeria (Fig. 3A) compares closely with that of rhipidistians (Fig. 3D: porolepiforms, Fig. 3E: osteolepiforms) with outer dental arcade (maxilla missing in actinistians), inner dental arcade (paired vomers, two pairs of dermopalatines but one pair in rhipidistians and tetrapods, paired ectopterygoids) and large entopterygoids between inner dental arcade and unpaired median parasphenoid. The parasphenoid lies below the ethmosphenoid and ends in front of the intracranial joint. The broad anterior plate reaching the anterior dermopalatines is a unique feature of the parasphenoid of L. chalumnae. The palate is distinct from that in actinopterygians (Fig. 3B) and dipnoans (Fig. 3C). The complete outer dental arcade with premaxilla and maxilla is present in actinopterygians, but missing in dipnoans. The inner dental arcade has a higher number of bones in primitive actinopterygians than in crossopterygians: one pair of vomers, three pairs of dermopalatines and one pair of ectopterygoids. I follow here the homologization of Gardiner (1984) who considered the ectopterygoid as the most posterior bone of the inner dental arcade whereas the bone surrounding the
medial border of the adductor fossa, and reaching to the quadrate, represents the entopterygoid (in contrast to Jollie 1986). The ectopterygoid of actinopterygians is short and toothed, of about the size of the dermopalatines, whereas it is elongated in crossopterygians. The entopterygoids of primitive sarcopterygians are large bones, occupying most of the palate, they reach from the adductor fossa anteromedially to or close to the midline. The same area is occupied by three bones in actinopterygians: posterior (= the ectopterygoid of Jollie 1986) and anterior entopterygoid (the pterygoid of Jollie 1986), and so-called dermometapterygoid. The division of this area into three ossifications is unique within osteichthyans. The parasphenoid has a position anterior to the ventral fissure (= ventral part of the intracranial joint of crossopterygians) as in crossopterygians. The palate of dipnoans (Fig. 3C) is quite different. Campbell & Barwick (1984) demonstrated the difficulties of homologizing any of the palatal bones with other osteichthyans; they denied homologization by using ingroup comparison (within dipnoans). If we accept Diabolepis as the sistergroup of dipnoans (Chang & Yu 1984, Maisey 1986), one can support some of Campbell & Barwick’s (1984) conclusions even by outgroup comparison. In Diabolepis (Fig. 4A), both nasal openings lie outside the outer dental arcade, outside premaxilla and ?maxilla, but ventral of the snout. Here Diabolepis is very similar to Youngolepis (Fig. 4B). A posterior continuation of the outer dental arcade in form of a maxilla has to be assumed for both forms because the area posterior to the posterior nasal opening and the premaxilla is identical build with overlapped area of lacrymal. Nevertheless the maxilla is not known in Diabolepis, Youngolepis or Powichthys. The premaxillae of Diabolepis are reaching onto the palate. The movement of the nasal openings onto the palate - a scenario (Schultze 1987) necessary to attain the pattern in dipnoans (Fig. 4B-E) - transfers the premaxillae onto the palate. It follows that the margin of the snout in dipnoans cannot be homologized with premaxillae and maxillae of other osteichthyans in contrast to Rosen et al. (1981) and Gardiner (1984). Campbell & Barwick (1984)
187
Pt
ac
B
“Dpl”
ac. p
E
F
Fig. 3. Comparison of the palate of A- Latimeria chalumnae with that of B- a primitive actinopterygian (Mimia roombsi), C- a dipnoan (Griphognathus whitei), D- a porolepiform (Glyptolepis groenlandica), E- an osteolepiform (Eusthenopteronfoordi) and F- a primitive tetrapod (Zchthyostega sp.). B-after Gardiner (1984, fig. 75A), C-after Miles (1977, fig. 6), D-after Jarvik (1972, fig. 3), E- after Jarvik (1980, fig. 124), F- after Jarvik (1980, fig. 171B). ac.p = arcual plate; ch= choana; Dpl= dermopalatine; ‘Dpl’ = so-called dermopalatine of dipnoans; Ecpt = ectopterygoid; Mpt = so-called dermometapterygoid; Mx = maxilla; na.a, na.p = anterior, posterior nasal opening; Pmx = premaxilla; Psp = parasphenoid; Pt = entopterygoids; s.Psp = swallow-tail shaped extension of parasphenoid; Vo = vomer, ‘Vo’ = so-called vomer of dipnoans.
188 reached the same conclusion by referring to the bone mosaic in the upper lip of the primitive dipnoan Uranolophus (Denison 1968) which cannot be homologized with premaxillae. Only a few bones occur between the upper lip and the entopterygoid of dipnoans and their homologization is difficult. We could expect to find premaxillae, possible maxillae, vomers, dermopalatines and ectopterygoids by comparison with Diabolepis and other osteichthyans. In contrast, we find a reduced and variable number of bones with changing relations to each other (Fig. 4C-G). The pairs of bones in front of the entopterygoids in Uranolophus (Fig. 4C), Dipnorhynchus (Fig. 4D), Speonesydrion, Griphognathus (Fig. 4E) and some other dipnoans may be homologized with the vomers of other osteichthyans using the criterion of position in front of entopterygoids and posteromedial to the posterior nasal opening; still one could not exclude the possibility that these are the premaxillae or premaxillae fused with the vomers, or dermopalatines (Miles 1977). But what are then the three bones in front of the entopterygoids in Chirodipterus (Fig. 4F), Holodipterus (Miles 1977, fig. 72) and other dipnoans? Most authors interpret the unpaired median bone as a vomer and the pair of bones as dermopalatines. Gnathorhiza, Ceratodus, Neoceratodus (Fig. 4G) and other post-Devonian dipnoans have a pair of very anterior bones behind the anterior nasal openings. Are these really vomers as they are always named? Griphognathus (Fig. 4E) has a variable number of bones anterolateral to its entopterygoids (Campbell & Barwick 1984, Schultze 1990). The ‘extra’ dermal bone (Rosen et al. 1981) may be the premaxilla shifted onto the palate; medial to it are the paired vomers and posterior to the latter are the dermopalatines. Entopterygoids and parasphenoid can be homologized with those of other osteichthyans. The parasphenoid is situated behind the ventral fissure in all dipnoans except the primitive genus Uranolophus (Denison 1968, Schultze 1991, in contrast to Campbell & Barwick 1989) where it reaches far anterior between the entopterygoids. Thus the upper jaw and palate of dipnoans deviates strongly from that of other osteichthyans and tetrapods (Fig. 3F). In conclusion, the palate of Latimeria compares
best with that of rhipidistians. The number of bones of the inner dental arcade and of the pterygoid complex is reduced in both groups over that in actinopterygians. This reduction may be an advanced feature (Rosen et al. 1981, Gardiner 1984), but polarity cannot be decided by outgroup comparison as such an outgroup is missing. A further complication is presented by the palate of dipnoans. The number of palatal bones anterior and lateral to the entopterygoids is further reduced, but it is not clear if that pattern was derived from a crossopterygian pattern (Fig. 5A) most closely related to porolepiforms (Maisey 1986, Chang 1990) or parallel to that of crossopterygians (Fig. 5B: Campbell & Barwick 1987, Schultze 1987, 1990). The position of the dipnoans within the sarcopterygians (including tetrapods) must be resolved in order to determine the relative position of actinistians to tetrapods. The closest relatives to tetrapods (Fig. 5) are the panderichthyids (Schultze 1987, 1990, Vorobyeva & Schultze 1990), the osteolepiforms being the next closest group (Holmes 1985, Maisey 1986, and others) followed by porolepiforms. The common ancestor of these groups lived in the early Lower Devonian (about 400 million years ago) or in the Late Silurian (as far back as 420 million years ago). About that time, we must also find the common ancestor of all sarcopterygians becausethere are no earlier records of osteichthyans. This means that the splitting between different sarcopterygian groups (Fig. 5) occurred in a short time period in comparison to the long duration of these groups. In comparing extant forms with each other we may choose to include or ignore a 400 million year history of each group, a temporal distance of at least 800 million years. The position of dipnoans as the closest relatives of tetrapods (Rosen et al. 1981, Gardiner 1984, Forey 1987) has been debated by many authors (Schultze 1981, 1987, 1990, Holmes, 1985, Maisey 1986, Panchen & Smithson 1987, Campbell & Barwick 1987, Chang 1990); these authors favor osteolepiforms, represented only as fossils, as the closest relatives. Still the dipnoans are the closest living relatives in some of these schemes(Fig. 5A) where the Early Devonian genus Diabolepis serves to link
189
“a a
na. p “&I” P,
Fig. 4. Anterior palate of A- Diabolepis speratus, B- Youngolepispraecursor, and of the dipnoans C- Uranolophus wyomingensis, DDipnorhynchus suessmilchi, E- Griphognathus whitei, F- Chirodipterus australis and G- Neoceratodus forsteri. A- after Chang & Yu
(1984, fig. 2B), B-after Chang (1982, fig. 10 + 7B), C- original, D- after Thomson & Campbell (1971, fig. 74 + 75), E- after Miles (1977, fig. 57), F- after Miles (1977, fig. 67), G- many sources. ‘Dpl’ = so-called dermopalatine of dipnoans; na.a, na.p = anterior, posterior nasal opening; Pms = premaxilla; Psp = parasphenoid; Pt = entopterygoid; Vo = vomer; ‘Vo’ = so-called vomer of dipnoans.
190
RECENT
DEVONIAN
SILURIAN
\
\
Y \
\ ‘.
\
‘\ \ \ ‘/ / \ / ’‘/ /I /
/
/
A
\\ \
\\ \\
\\
\
’ /I />
/
\ I \\ 7 ‘/’ /I
400
B
Fig. 5. Temporal position of actinistians in relation to tetrapods: A- dipnoans closer to tetrapods (after Maisey 1986); B- actinistians
closer to tetrapods (after Schultze 1987).
Table 1. Terminology of the bones of the palate.
Smith 1939
Millot & Anthony 1958
Schultze 1987khis paper
rostra1 dental plates (2 pairs) postrostral dental plate prevomer dental plate autopalatine dental plate ectopterygoid entopterygoid suprapterygoid denticulate area parasphenoid -
premaxillaire
premaxilla
vomer predermo-palatin dermo-palatin ectopterygoide entopterygoide ‘calcaires pavant’
vomer dermopalatine dermopalatine ectopterygoid entopterygoid pavement of ossifications
parasphenoide piece preoccipitale souschordale
parasphenoid arcual plates
191
primitive porolepiforms (youngolepiforms) and dipnoans (Maisey 1986). Diabolepis closely resembles youngolepiforms (Fig. 4B) in most features, but Chang & Yu (1984) cited features that indicate a closer relationship to dipnoans. These features are very doubtful, and Campbell & Barwick (1987) and Panchen & Smithson (1987) argued for a sistergroup relationship of Diabolepis to youngolepiforms or porolepiforms respectively and not to dipnoans. If that is true, actinistians can be considered as the closest living relatives of tetrapods (not in Panchen & Smithson’s 1987 scheme). Still there is the other possibility that dipnoans and actinistians together are the sistergroup of tetrapods (Northcutt 1987, Chang 1990, Forey et al. 1990). It appears that the early splitting of the sarcopterygians into different groups within a relatively short time span makes it difficult to resolve the sequence of events and thus the exact interrelationships of all sarcopterygian groups. We will see a continuing discussionover the relationships of sarcopterygians in the future.
Acknowledgements
The author is very thankful to J. Musick, Virginia Institute of Marine Science, for the invitation to take part in the investigation of two Latimeria chaZumnaespecimensdonated by the Explorers Club, New York, and for the arrangement of the radiological research. J. Daimler, Radiological Unit, Riverside Hospital, Newport News, Virginia, generously supported our request to make CT scan serial cross sections, and Christie White spent many hours CT scanning the fish. The three-dimensional reconstruction was made possible through a generous invitation by J. Zinreich, Department of Radiology, Johns Hopkins Hospital, Baltimore, Maryland. Cindy Quinn spent two days with the author working on the CEMAX 1000. Photographic prints were prepared by J. Chorn, and the drawings by Ann Musser, both from the Museum of Natural History, The University of Kansas, Lawrence. The manuscript was typed by J. Elder and J. Wiglesworth, Word Processing Center, Division of Biological Sciences,The University
of Kansas, Lawrence. The English was improved by J. Chorn, Museum of Natural History, The University of Kansas, Lawrence. I would like to express my thanks to all these people.
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