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A second lamprey from the Lower Carboniferous (Namurian) of Bear Gulch, Montana (U.S.A.)
A SECOND
LAMPREY
FROM THE LOWER
OF BEAR GULCH,
CARBONIFEROUS
MONTANA
(NAMURIAN)
(U.S.A.)
by RICHARD LUND * & PHILIPPE JANVIER **
ABSTRACT
RI~SUMI~
A second fossil lamprey is described from the Lower Carboniferous (Namurian) locality of Bear Gulch (Montana, U.S.A.), which had already yielded the type specimen of Hardistiella montanensis, the oldest and, probably, the most primitive of all known lampreys. This new specimen, which may possibly belong to the latter species, is badly preserved, but clearly shows the impressions of four to six branchial pouches which are relatively small and closely set, like those of the other Carboniferous lamprey Mayomyzon pieckoensis. They differ thus from extant lampreys, in which the branchial apparatus extends relatively far behing the eyeballs. This concentration of the branchial apparatus in early lampreys is regarded here as a primitive condition, which is also met with in many anaspids, as evidenced from their closely-set external branchial openings. The presence of an impression which recalls the loop of the trabecles in larval extant lampreys suggests that this specimen was a larval individual.
Une seconde lamproie fossile est d6crite dans le gisement carbonif~re inf6rieur (Namurien) de Bear Gulch (Montana, U.S.A.), qui avait d6j~t livr6 le sp6cimen type de Hardistiella montanensis, la plus ancienne, et probablement la plus primitive, de toutes les lamproies connues. Ce nouveau sp6cimen, qui ne peut ~tre rapport6 qu'avec doute ~ cette esp~ce, est relativement mal conserv6, mais montre nettement la trace de quatre ~ six poches branchiales &roitement acco16es, comme celles de Mayomyzon pieckoensis, l'autre lamproie fossile du Carbonif~re moyen. Elles diff~rent en cela des lamproies actuelles chez qui l'appareil branchial s'6tend relativement loin en arri~re de l'oeil. Cette concentration de l'appareil branchial chez les lamproies carbonif&es est ici consid6r6e comme un caract~re primitif pour le groupe, que l'on retrouve 6galement chez les Anaspides, autant que l'on puisse en juger par leurs orifices branchiaux externes 6troitement contigus. La pr6sence, sur ce nouveau sp6cimen, de traces 6voquant les trab6cules sugg~re aussi qu'il pourrait s'agir d'un individu larvaire.
KEY-WORDS : VERTEBRATA, PETROMYZONTIDA, LOWER CARBONIFEROUS, U.S.A., ANATOMY, BRANCHIAL APPARATUS, LARVAL STAGE. MOTS-CL]~S : VERTEBRATA, PETROMYZONTIDA, CARBONIFI~RE INF]~RIEUR, U.S.A., APPAREIL BRANCHIAL, STADE LARVAIRE.
INTRODUCTION A few years ago, the now famous Lower Carboniferous fish locality at Bear Gulch (Montana, U.S.A.) yielded a relatively well preserved lamprey, Hardistiellct montanensis JANVIER & LUND (1983). In being
slightly older (Namurian) than the previously described fossil lamprey Mayomyzon pieckoensis BARDARCK & ZANGERL (1968, 1971) from the Pennsylvanian of Illinois, Hardistiella is now the earliest
* Department of Biology, Adelphi University, Garden City, Long Island, New York 11530, U.S.A. ** LA 12 du CNRS, Institut de Pal6ontologie, 8 rue Buffon, 75005 Paris, France.
Geobios, n ° 19, fasc. 5
p. 647-652, 2 fig., 1 pl
Lyon, octobre 1986
-
-
lamprey hitherto recorded. It has been regarded as the most primitive member of the Petromyzontida, since it seems to have retained a true anal fin. It differs also from all other known lampreys in having relatively large otoliths (Janvier & L u n d 1983, fig. 3). A second lamprey has been discovered in Bear Gulch during the summer 1985, and despite its
648
-
-
comparatively poor state of preservation, it provides some complementary data to the knowledge of the structure of these early representatives of the Petromyzontida. This specimen, described herein belongs to the collection of Carnegie Museum of Natural History (C.M.N.H.), n ° 4105.
DESCRIPTION
Like all the Bear Gulch specimens this lamprey is preserved as a slightly tarry impression in a calcareous flagstone (pl. 1, figs. 1, 2). It has been photographed dry (pl. 1, figs. la, 2a) and immersed in water (pl. 1, figs. lb, 2b) in order to reveal various details of the head. Contrary to the holotype of Hardistiella montanensis JANVIER & LUND 1983 (fig. 1), which had been pressed laterally during fossilization and, thus, shows quite accurately the actual body shape of the living animal [this applies also to the holotype of Mayomyzon pieckoensis (BARDACK & Z A R G E R L , 1968, fig. 1)], the new Bear Gulch specimen seems to have received a rather dorsolateral thrust during fossilization, hence its comparatively higher body shape. The tail and median fins are almost invisible, but the head structures are somewhat better preserved than in the first specimen. Whether or not this specimen also belongs to the species H. montanensis is difficult to say, however, the fact that it comes from exactly the same locality may support this attribution. The head shows an enlarged anterior lobe, which may correspond to the <>or the oral hood. It contains a few tarry dots (te., fig. 1B), which may be traces of some tectal cartilages. Behind this lobe, there is a dumbell-shaped black spot (e., fig. 1), which is almost certainly the trace of the two eyeballs. Dorsally to it, there is a lozenge-shaped impression (?tr., fig. 1B), which may be either a trace of the braincase or, more likely, that of the trabecles only. In fact, a comparison with the skeleton of an extant larval lamprey shows a striking similarity between this impression and the loop which is formed by the trabecles and the trabecular commissure. Moreover, this impression is produced anteriorly into a small bifurcated process (o1., fig. 1B), which may be the trace of the olfactory capsule, as also suggested by a comparison to an extant larval lamprey (pl. 1, fig. 3). If this interpretation is correct, we have to assume that this specimen is a larval individual, a condition which would be in agreement with its very small size.
A
-'~" I B
~ ~ " -.~:, '" ~ " I
~
?c. __ ~
?tr.
"~
•
d.t.
"
ol. ~
t(.
?b.6 71o.5 b.4 b.3 b2 b.1
Fig. 1 - -
Petromyzontida gen. et sp. indet., Lower Carboniferous, Namurian, Bear Gulch, M o n t a n a (U.S.A.). Cameralucida drawing of specimen n ° C M N H n ° 4505, head region. A counterpart o f B. Scale : 1 m m . b.1-4, impressions of branchial pouches ; ?b.5-6, possible impressions o f branchial p o u c h e s ; ?c., possible trace of the notochord ; d.t., trace of the digestive tract ; e., trace of the eyeballs ; o1., trace of the olfactory capsule ; te., trace of possibly tectal cartilages of the snout ; ? tr., possible traces of the trabecles. Carbonif~re inf6rieur, Namurien, Bear Gulch, M o n t a n a (U.S.A.). Dessin/t la chambre claire de la t&e du sp6cim e n n ° C M N H n ° 4105. A est la contre-empreinte de B. ]~chelle : 1 m m . b.l-4, traces de poches branchiales ; ?b.5-6, traces possibles de poches branchiales ; ?c., trace possible de la chorde ; d.t., trace du tube digestif ; e., trace des yeux ; o1., trace de la capsule olfactive ; te., trace possible des cartilages tectaux du m u s e a u x ; ?tr., trace possible des trab6cules.
-- 649 - -
The most interesting detail displayed by this specimen is the branchial apparatus. Contrary to the holotype of H. montanensis, which showed only a few branching impressions (Janvier &Lund 1983, fig. 2A, <>), interpreted as traces of the branchial arches, this specimen shows distinctly at least four dark impressions (bl-4, fig. 1B), situated posteroventrally to the eye trace, and which are certainly the traces of individual branchial pouches. A careful examination of the specimen in immersion reveals the faint trace of two more branchial pouch impressions (? b.56, fig. 1B). One may thus assume that there are possibly six branchial pouches.
Behind this series of branchial pouches, there is an elongated black impression which is most probably that of the digestive tract (d.t., fig. 1), preserved in the same position as in the holotype of Mayomyzon pieckoensis (BARDACK & ZANGERL, 1968, fig. 1). In the posterior part of the specimen, there is a ventral dark area which may be due to the accumulation of organic matter in the posterior part of the digestive tract, in particular in the cloaca, as it is often observed in larval extant lampreys. No trace of the notochord is preserved, except perhaps a thin black line behind the head (?c., fig. 1B). Further, no traces are preserved of the large segmental structures labelled X in the holotype of H. montanensis (JANVIER & LUND, 1983).
COMPARISON AND DISCUSSION
The arrangement of the branchial pouches in this specimen shows a striking similarity to that in Mayomyzon pieckoensis, and differs markedly from that in extant lampreys. In fact, the branchial pouches in both carboniferous lampreys appear as very short, closely set, and gathered immediately behind the braincase (fig. 2B). This arrangement is particularly clear in a specimen of M. pieckoensis preserved in dorsal aspect (Bardack & Zangerl 1971, fig. 2). By contrast, in all extant lampreys, the branchial apparatus is much more elongated in shape, and extends over a length which is up to three times that of the preorbital region of the head (fig. 2A), whereas the length of the branchial apparatus in both Mayomyzon and the specimen described here is approximately equal to the preorbital part of the head. As noted earlier by Bardack & Zangerl (1971), this shorter branchial apparatus is virtually the only difference which can be noticed between Mayomyzon and extant lampreys. Whether it is derived or primitive is difficult to decide. However, there are some arguments in favor of the latter solution. First, a comparison with Silurian and Devonian jawless craniates, in particular the Anaspida, shows that most of these forms possessed numerous, but small and closely-set, branchial pouches. The closely-set external branchial openings of anaspids had been interpreted by Stensi6 (1964, fig. 28C) as indicating the presence of elongated external branchial duct gathered into a bundle, in the same way as in some myxinoids. It may also indicate that the branchial pouches were very short, high and closely set. If, as generally assumed on various
grounds (Janvier 1981 ; Janvier & Lund 1983), the Petromyzontida are <>among anaspids sensu lato or are the sister-group of the Anaspida (sensu stricto), the presence of such a concentrated branchial apparatus in the early representatives of the group may not be surprising.
A
B
Fig. 2 - -
Comparative representation of the branchial apparatus in the Recent (A) and Carboniferous (B) Petromyzontida. Outline of the branchial pouches. A, based on Marinelli & Strenger 1954 ; B, based on Bardack & Zangerl 1968. Comparaison des proportions de l'appareil branchial chez un Petromyzontida actuel (A) et carbonif6re (B). Contour des poches branchiales en gris6. A, d'apr~s Marinelli & Strenger 1954 ; B, d'apr~s Bardack & Zangerl 1968.
Secondly, Hardistiella is regarded as the primitive sister-group of Mayomyzon + extant lampreys
PLATE 1
Fig. 1 - - Petromyzontida gen, e t sp. indet., Lower Carboniferous, Namurlan, Bear Gulch, Montana. Specimen 4105, Carne. gie Museum of Natural History. Part (a) and counterpart (b) photographed dry. x 3. Carbonif~re inf6rieur, Namurian, Bear Gulch, Montana. Sp6cimen n ° 4105, Carnegie Museum of Natural History. Empreinte (a) et contre-empreinte (b), photographi6es A sec.
Fig. 2 - - The same specimen. Part (a) and counterpart (b) photographed in water, x 3. MSme sp6cimen. Empreinte (a) et contre-empreinte (b), photographi6es en immersion dans l'eau.
Fig. 3 - - Petromyzon marinus L., 100 mm larva, cleared and stained specimen, x 3. ol., olfactory capsule ; tr., trabecles. Larve de 100 mm 6claircie et color6e. ol., capsule olfactive ; tr., trab6cules.
Geobios
P1.1
n ° 19, fasc. 5
R. L u n d & Ph. Janvier
la
lb
2a
2b
3 ol.
~i ¸ i~i I
tr.
- - 652 - -
(Janvier & Lund 1983). I f this specimen belongs to the genus Hardistiella, this would mean that both Carboniferous lampreys have this type of branchial apparatus and, thus, that it is a plesiomorphous condition for extant lampreys. The exact number of branchial pouches in these early lampreys is difficult to decide. Bardack & Zangerl (1968, 1971) claimed that Mayomyzon possessed seven branchial pouches like extant forms, but the discovery of new material (Bardack & Zangerl 1971) seems to support a lesser number, perhaps only five. In the bear Gulch specimen, there are surely four pouch traces, and possible one or two more.
I f this second specimen is a larval individual, as suggested by the possible traces of the trabecles, then this concentrated branchial apparatus was already present in the ammocoetes stage in Carboniferous times. In extant larval lampreys, the branchial apparatus is, as in adults, very elongated in shape (pl. 1, fig. 3). Moroever, the absence of the large postbranchial segmented structures seen in the holotype of H. montanensis (Janvier & Lund 1983, ~t X ~) m a y support the possibility that these were traces of gonads, as the quality Of preservation of this part of the two specimens in equal.
CONCLUSIONS
The discovery of this new fossil lamprey -possibly a larval individual- confirms that the branchial apparatus was considerably more concentrated and shorter in Carboniferous forms than in extant ones. Although
no fossil lamprey is known between the Carboniferous and Recent times, one m a y infer that the evolution of the group since that time is marked chiefly by an increase of the volume of the branchial apparatus.
REFERENCES
BARDACK D. & ZANGERL R. (1968) - First fossil lamprey : A record from the Pennsylvanian of Illinois. Science, Washington, 1962, 1265-1267. BARDACK D. & ZANGERL R. (1971) - Lampreys in the fossil record. In The Biology of Lampreys (M.W. HARDISTY & I.C. POTTER eds), Academic Press, London, 1, 67-84.
JANVIER P. & LUND R. (1983) - Hardistiella montanensis n. gen. et sp. (Petromyzontida) from the Lower Carboniferous of Montana, with remarks on the affinities of the Lampreys. J. Vert. Paleont., Norman, 2(4), 407-413. STENSIO E. (1964) - Les Cyclostomes fossiles ou Ostracodermes. In Trait6 de Pal6ontologie (J. PIVETEAU 6d.), Masson, Paris, 4(1), 96-382.
JANVIER P. (1981) - The phylogeny of the Craniata, with particular reference to the significance of fossil ~ agnathans )). J. Vert. Paleont., Norman, 1(2), 121-159.
M anuscrit d6finitif regu le 04.09.1986
LAMPREY
FROM THE LOWER
OF BEAR GULCH,
CARBONIFEROUS
MONTANA
(NAMURIAN)
(U.S.A.)
by RICHARD LUND * & PHILIPPE JANVIER **
ABSTRACT
RI~SUMI~
A second fossil lamprey is described from the Lower Carboniferous (Namurian) locality of Bear Gulch (Montana, U.S.A.), which had already yielded the type specimen of Hardistiella montanensis, the oldest and, probably, the most primitive of all known lampreys. This new specimen, which may possibly belong to the latter species, is badly preserved, but clearly shows the impressions of four to six branchial pouches which are relatively small and closely set, like those of the other Carboniferous lamprey Mayomyzon pieckoensis. They differ thus from extant lampreys, in which the branchial apparatus extends relatively far behing the eyeballs. This concentration of the branchial apparatus in early lampreys is regarded here as a primitive condition, which is also met with in many anaspids, as evidenced from their closely-set external branchial openings. The presence of an impression which recalls the loop of the trabecles in larval extant lampreys suggests that this specimen was a larval individual.
Une seconde lamproie fossile est d6crite dans le gisement carbonif~re inf6rieur (Namurien) de Bear Gulch (Montana, U.S.A.), qui avait d6j~t livr6 le sp6cimen type de Hardistiella montanensis, la plus ancienne, et probablement la plus primitive, de toutes les lamproies connues. Ce nouveau sp6cimen, qui ne peut ~tre rapport6 qu'avec doute ~ cette esp~ce, est relativement mal conserv6, mais montre nettement la trace de quatre ~ six poches branchiales &roitement acco16es, comme celles de Mayomyzon pieckoensis, l'autre lamproie fossile du Carbonif~re moyen. Elles diff~rent en cela des lamproies actuelles chez qui l'appareil branchial s'6tend relativement loin en arri~re de l'oeil. Cette concentration de l'appareil branchial chez les lamproies carbonif&es est ici consid6r6e comme un caract~re primitif pour le groupe, que l'on retrouve 6galement chez les Anaspides, autant que l'on puisse en juger par leurs orifices branchiaux externes 6troitement contigus. La pr6sence, sur ce nouveau sp6cimen, de traces 6voquant les trab6cules sugg~re aussi qu'il pourrait s'agir d'un individu larvaire.
KEY-WORDS : VERTEBRATA, PETROMYZONTIDA, LOWER CARBONIFEROUS, U.S.A., ANATOMY, BRANCHIAL APPARATUS, LARVAL STAGE. MOTS-CL]~S : VERTEBRATA, PETROMYZONTIDA, CARBONIFI~RE INF]~RIEUR, U.S.A., APPAREIL BRANCHIAL, STADE LARVAIRE.
INTRODUCTION A few years ago, the now famous Lower Carboniferous fish locality at Bear Gulch (Montana, U.S.A.) yielded a relatively well preserved lamprey, Hardistiellct montanensis JANVIER & LUND (1983). In being
slightly older (Namurian) than the previously described fossil lamprey Mayomyzon pieckoensis BARDARCK & ZANGERL (1968, 1971) from the Pennsylvanian of Illinois, Hardistiella is now the earliest
* Department of Biology, Adelphi University, Garden City, Long Island, New York 11530, U.S.A. ** LA 12 du CNRS, Institut de Pal6ontologie, 8 rue Buffon, 75005 Paris, France.
Geobios, n ° 19, fasc. 5
p. 647-652, 2 fig., 1 pl
Lyon, octobre 1986
-
-
lamprey hitherto recorded. It has been regarded as the most primitive member of the Petromyzontida, since it seems to have retained a true anal fin. It differs also from all other known lampreys in having relatively large otoliths (Janvier & L u n d 1983, fig. 3). A second lamprey has been discovered in Bear Gulch during the summer 1985, and despite its
648
-
-
comparatively poor state of preservation, it provides some complementary data to the knowledge of the structure of these early representatives of the Petromyzontida. This specimen, described herein belongs to the collection of Carnegie Museum of Natural History (C.M.N.H.), n ° 4105.
DESCRIPTION
Like all the Bear Gulch specimens this lamprey is preserved as a slightly tarry impression in a calcareous flagstone (pl. 1, figs. 1, 2). It has been photographed dry (pl. 1, figs. la, 2a) and immersed in water (pl. 1, figs. lb, 2b) in order to reveal various details of the head. Contrary to the holotype of Hardistiella montanensis JANVIER & LUND 1983 (fig. 1), which had been pressed laterally during fossilization and, thus, shows quite accurately the actual body shape of the living animal [this applies also to the holotype of Mayomyzon pieckoensis (BARDACK & Z A R G E R L , 1968, fig. 1)], the new Bear Gulch specimen seems to have received a rather dorsolateral thrust during fossilization, hence its comparatively higher body shape. The tail and median fins are almost invisible, but the head structures are somewhat better preserved than in the first specimen. Whether or not this specimen also belongs to the species H. montanensis is difficult to say, however, the fact that it comes from exactly the same locality may support this attribution. The head shows an enlarged anterior lobe, which may correspond to the <
A
-'~" I B
~ ~ " -.~:, '" ~ " I
~
?c. __ ~
?tr.
"~
•
d.t.
"
ol. ~
t(.
?b.6 71o.5 b.4 b.3 b2 b.1
Fig. 1 - -
Petromyzontida gen. et sp. indet., Lower Carboniferous, Namurian, Bear Gulch, M o n t a n a (U.S.A.). Cameralucida drawing of specimen n ° C M N H n ° 4505, head region. A counterpart o f B. Scale : 1 m m . b.1-4, impressions of branchial pouches ; ?b.5-6, possible impressions o f branchial p o u c h e s ; ?c., possible trace of the notochord ; d.t., trace of the digestive tract ; e., trace of the eyeballs ; o1., trace of the olfactory capsule ; te., trace of possibly tectal cartilages of the snout ; ? tr., possible traces of the trabecles. Carbonif~re inf6rieur, Namurien, Bear Gulch, M o n t a n a (U.S.A.). Dessin/t la chambre claire de la t&e du sp6cim e n n ° C M N H n ° 4105. A est la contre-empreinte de B. ]~chelle : 1 m m . b.l-4, traces de poches branchiales ; ?b.5-6, traces possibles de poches branchiales ; ?c., trace possible de la chorde ; d.t., trace du tube digestif ; e., trace des yeux ; o1., trace de la capsule olfactive ; te., trace possible des cartilages tectaux du m u s e a u x ; ?tr., trace possible des trab6cules.
-- 649 - -
The most interesting detail displayed by this specimen is the branchial apparatus. Contrary to the holotype of H. montanensis, which showed only a few branching impressions (Janvier &Lund 1983, fig. 2A, <
Behind this series of branchial pouches, there is an elongated black impression which is most probably that of the digestive tract (d.t., fig. 1), preserved in the same position as in the holotype of Mayomyzon pieckoensis (BARDACK & ZANGERL, 1968, fig. 1). In the posterior part of the specimen, there is a ventral dark area which may be due to the accumulation of organic matter in the posterior part of the digestive tract, in particular in the cloaca, as it is often observed in larval extant lampreys. No trace of the notochord is preserved, except perhaps a thin black line behind the head (?c., fig. 1B). Further, no traces are preserved of the large segmental structures labelled X in the holotype of H. montanensis (JANVIER & LUND, 1983).
COMPARISON AND DISCUSSION
The arrangement of the branchial pouches in this specimen shows a striking similarity to that in Mayomyzon pieckoensis, and differs markedly from that in extant lampreys. In fact, the branchial pouches in both carboniferous lampreys appear as very short, closely set, and gathered immediately behind the braincase (fig. 2B). This arrangement is particularly clear in a specimen of M. pieckoensis preserved in dorsal aspect (Bardack & Zangerl 1971, fig. 2). By contrast, in all extant lampreys, the branchial apparatus is much more elongated in shape, and extends over a length which is up to three times that of the preorbital region of the head (fig. 2A), whereas the length of the branchial apparatus in both Mayomyzon and the specimen described here is approximately equal to the preorbital part of the head. As noted earlier by Bardack & Zangerl (1971), this shorter branchial apparatus is virtually the only difference which can be noticed between Mayomyzon and extant lampreys. Whether it is derived or primitive is difficult to decide. However, there are some arguments in favor of the latter solution. First, a comparison with Silurian and Devonian jawless craniates, in particular the Anaspida, shows that most of these forms possessed numerous, but small and closely-set, branchial pouches. The closely-set external branchial openings of anaspids had been interpreted by Stensi6 (1964, fig. 28C) as indicating the presence of elongated external branchial duct gathered into a bundle, in the same way as in some myxinoids. It may also indicate that the branchial pouches were very short, high and closely set. If, as generally assumed on various
grounds (Janvier 1981 ; Janvier & Lund 1983), the Petromyzontida are <
A
B
Fig. 2 - -
Comparative representation of the branchial apparatus in the Recent (A) and Carboniferous (B) Petromyzontida. Outline of the branchial pouches. A, based on Marinelli & Strenger 1954 ; B, based on Bardack & Zangerl 1968. Comparaison des proportions de l'appareil branchial chez un Petromyzontida actuel (A) et carbonif6re (B). Contour des poches branchiales en gris6. A, d'apr~s Marinelli & Strenger 1954 ; B, d'apr~s Bardack & Zangerl 1968.
Secondly, Hardistiella is regarded as the primitive sister-group of Mayomyzon + extant lampreys
PLATE 1
Fig. 1 - - Petromyzontida gen, e t sp. indet., Lower Carboniferous, Namurlan, Bear Gulch, Montana. Specimen 4105, Carne. gie Museum of Natural History. Part (a) and counterpart (b) photographed dry. x 3. Carbonif~re inf6rieur, Namurian, Bear Gulch, Montana. Sp6cimen n ° 4105, Carnegie Museum of Natural History. Empreinte (a) et contre-empreinte (b), photographi6es A sec.
Fig. 2 - - The same specimen. Part (a) and counterpart (b) photographed in water, x 3. MSme sp6cimen. Empreinte (a) et contre-empreinte (b), photographi6es en immersion dans l'eau.
Fig. 3 - - Petromyzon marinus L., 100 mm larva, cleared and stained specimen, x 3. ol., olfactory capsule ; tr., trabecles. Larve de 100 mm 6claircie et color6e. ol., capsule olfactive ; tr., trab6cules.
Geobios
P1.1
n ° 19, fasc. 5
R. L u n d & Ph. Janvier
la
lb
2a
2b
3 ol.
~i ¸ i~i I
tr.
- - 652 - -
(Janvier & Lund 1983). I f this specimen belongs to the genus Hardistiella, this would mean that both Carboniferous lampreys have this type of branchial apparatus and, thus, that it is a plesiomorphous condition for extant lampreys. The exact number of branchial pouches in these early lampreys is difficult to decide. Bardack & Zangerl (1968, 1971) claimed that Mayomyzon possessed seven branchial pouches like extant forms, but the discovery of new material (Bardack & Zangerl 1971) seems to support a lesser number, perhaps only five. In the bear Gulch specimen, there are surely four pouch traces, and possible one or two more.
I f this second specimen is a larval individual, as suggested by the possible traces of the trabecles, then this concentrated branchial apparatus was already present in the ammocoetes stage in Carboniferous times. In extant larval lampreys, the branchial apparatus is, as in adults, very elongated in shape (pl. 1, fig. 3). Moroever, the absence of the large postbranchial segmented structures seen in the holotype of H. montanensis (Janvier & Lund 1983, ~t X ~) m a y support the possibility that these were traces of gonads, as the quality Of preservation of this part of the two specimens in equal.
CONCLUSIONS
The discovery of this new fossil lamprey -possibly a larval individual- confirms that the branchial apparatus was considerably more concentrated and shorter in Carboniferous forms than in extant ones. Although
no fossil lamprey is known between the Carboniferous and Recent times, one m a y infer that the evolution of the group since that time is marked chiefly by an increase of the volume of the branchial apparatus.
REFERENCES
BARDACK D. & ZANGERL R. (1968) - First fossil lamprey : A record from the Pennsylvanian of Illinois. Science, Washington, 1962, 1265-1267. BARDACK D. & ZANGERL R. (1971) - Lampreys in the fossil record. In The Biology of Lampreys (M.W. HARDISTY & I.C. POTTER eds), Academic Press, London, 1, 67-84.
JANVIER P. & LUND R. (1983) - Hardistiella montanensis n. gen. et sp. (Petromyzontida) from the Lower Carboniferous of Montana, with remarks on the affinities of the Lampreys. J. Vert. Paleont., Norman, 2(4), 407-413. STENSIO E. (1964) - Les Cyclostomes fossiles ou Ostracodermes. In Trait6 de Pal6ontologie (J. PIVETEAU 6d.), Masson, Paris, 4(1), 96-382.
JANVIER P. (1981) - The phylogeny of the Craniata, with particular reference to the significance of fossil ~ agnathans )). J. Vert. Paleont., Norman, 1(2), 121-159.
M anuscrit d6finitif regu le 04.09.1986