Mesomyzon mengae Chang, Zhang & Miao, 2006

Wu, Feixiang, Chang, Mee-Mann & Janvier, Philippe, 2021, A new look at the Cretaceous Lamprey Mesomyzon Chang, Zhang & Miao, 2006 from the Jehol Biota, Geodiversitas 43 (23), pp. 1293-1307 : 1294-1300

Mesomyzon mengae Chang, Zhang & Miao, 2006

( Figs 2-4 )

HOLOTYPE. — IVPP ( Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China) collection number V14719 , a nearly complete adult specimen, without preservation of the tail ( Chang et al. 2006: fig. 1a). REFERRED MATERIAL. — IVPP V14718 A, B (a metamorphosed individual), V14981 A, B, V15027 A, B, V15450.3, V15448 A, B, V15449 . HORIZON AND LOCALITY. — Yixian Formation, Lower Cretaceous (c. 125 Ma) ( Zhou 2014; Zhou & Wang 2017); Ningcheng, Inner Mongolia and Yongyin County, Hebei Province, China .

AMENDED DIAGNOSIS. — A fossil lamprey with elongated and eelshaped body, body length 12 times the maximum body depth, 4 times the head length; first gill pouch posteroventral to otic capsule; prebranchial region long, twice the maximum head height and 1.3- 1.8 times the branchial length; oral papillae large and triangular (conical); oral fimbriae numerous, maximum number presumably no less than 80; circumoral teeth series incomplete and lacking the posterior section; supraoral lamina large and bicuspid; two dorsal fins widely separated, anterior dorsal fin long and commencing above 4 th gill pouch; ribbon-like preanal skin fold long and extending to anterior branchial region; anal fin fold present; 70-80 preanal muscular segments; preanal myomeres Z-shaped and caudal ones W-shaped; tail hypocercal.

DESCRIPTION

General appearance

As described byChang et al. (2006), Mesomyzon has an eel-like body and lacks paired fins. The adult individuals described here are larger than the holotype ( Table 1 ). They have a welldeveloped oral disc and a long prebranchial region. The new materials here show that the anterior dorsal fin is very long and the ventral midline of the trunk is fringed with a ribbonlike preanal skin fold. The tail is hypocercal ( Fig. 2 ).

Head

The cranial cartilages are preserved in imprints in some specimens. Here we follow Hardisty & Potter (1981) and De Iluiis & Pulerà (2011) for the terminology of the cranial cartilages referred below. The main parts of the anterior and posterior dorsal cartilages, and the anterior lateral cartilage can be delineated ( Figs 3 A-D; 4A-C). The anterior lateral cartilage is situated ventrolaterally to the front part of the anterior dorsal cartilage and is tapering in the ventral part. The anterior dorsal cartilage is wider in the hind part than in the front, whose lateral margin is flared backward into a posterolateral angle. The posterior limit of the anterior dorsal cartilage terminates at the level very close to the external narial opening. The posterior dorsal cartilage is wider than the anterior dorsal cartilage and overlaps the latter posteriorly as in living lampreys ( Fig. 1B ) ( Hardisty & Potter 1981: fig. 4; Marinelli & Strenger, 1954: (figs 24, 51a). There is a joint occasionally preserved in the fossils along the dorsal edge of the snout region (some distance in front of the eyes) ( Figs 3B, C ; 4A ) which might indicate the differentiation of the anterior dorsal and posterior cartilages.

To the rear of the posterior dorsal cartilage there is a notable dark imprint which is nearly vertically arranged immediately in front of the eye ( Figs 3A, B ; 4A ). This may represent the anterior pillar of the subocular arch ( Figs 3B ; 4A ) to form the anterior part of the orbital cavity as in living lampreys ( Johnels 1948: fig. 57; Hardisty & Potter 1981: fig. 4; Janvier 1993: fig. 4.4).

The imprints of the styliform cartilage is preserved posteroventral to the otic capsule ( Figs 3 A-D; 4A). It extends posteroventrally towards the pharynx as its counterpart in living forms ( Hardisty & Potter 1981: fig. 4). It is linked to the velar skeleton by small muscles in modern lampreys ( Janvier 1993). The structures of the naso-hypophysial complex is wellpreserved in IVPP V 15449 and V 15450.3, with the olfactory organ’s left side exposed in the former and the dorsal aspect in the latter ( Figs 3 C-F; 4B, F). It is enclosed within the imprints of the nasal sac, which is located in the midline and just anterodorsally to the eyes and likely independent from the cranial cartilages. The olfactory organ is preserved in a way that it partly overlaps the naso-hypophysial atrium in IVPP V 15449 ( Figs 3C, D ; 4B ). The preserved part of this organ bears eight folds in IVPP V 15449 (dorsolateral view) ( Figs 3C, D ; 4B ) and six parallel and evenly arranged folds in IVPP V 15450.3 (dorsal view) ( Figs 3E, F ; 4F ). The naso-hypophysial duct extends anterodorsally for a length that approximates that of the short axis of the eye to the naris, which is dorsally positioned without a depression surrounding itself as in living lampreys ( Janvier 1974). The hypophysial pouch (tube) ( Figs 3C, D ; 4B ), a blind-ending structure serving to pump water into and out of the naso-hypophysial atrium ( Janvier 1974), extends posteroventrally from the bottom of the olfactory organtowards the pharynx, and bends downward abruptly when it approaches the branchial apparatus. It must have extended more posteriorly, as its rear is covered by the gill structures in IVPP V 15449 . The region of the pineal organ cannot be clearly distinguished as it is blurred by a patch of dark matter above the olfactory organ.

Brain and cranial nerves

(IVPP V15450.3): The remnants of the brain are represented by a patch of coarse sediments in IVPP V 15450.3 ( Figs 3E, F ; 4F ). Based on the position of the roots of some cranial nerves and their arrangement in living lamprey ( Fig. 1C ), the major divisions of the brain can be roughly recognized. The paired olfactory nerves can be roughly discerned, which contact anteriorly with the hind part of the nasal sac. The posterior part of the telencephalon is rounded and terminated at the level of the posterior rim of the eyes. After exiting from the braincase, the trigeminus ophthalmicus profundus (V 1) extends above the eyes and branches off to innervate the annular muscles of the oral disc. The buccal ramus of the facial nerve (VII.buc) exits below the eyes and extends anteriorly to be convergent with the V 1 toward the oral disc. The recurrent rami of the facial nerve are well preserved in situ, and originally surround the otic capsules. The labyrinth membrane cannot be detected in our materials. The glossophyngeus (IX) and vagus (X) nerves are also arranged in a pattern that is almost identical to that in living lampreys ( Fig. 1C ) ( Marinelli & Strenger 1954: figs 51, 52).

Oral disc

The oral disc and other associated structures are detected based on the pattern in living lamprey ( Fig. 1D ). In larger individuals, the oral disc is well developed, with the anterior fieldwider

than the lateral and posterior ones. In IVPP V 15449 , where the oral disc is best preserved, the oral disc occupies 6.64% of the total body length. In the same specimen, the impressions of a dozen of oral fimbriae (right side) are preserved in the perimeter of the oral disc ( Fig. 3C, D ). They are small, rod-like, and so closely arranged that they form together a comb-like pattern. This morphology is reminiscent of the oral fimbriae in living lampreys ( Renaud 2011) and the ridges along the periphery of the oral disc of fossil lamprey Priscomyzon ( Miyashita et al. 2021) Based on the arrangement of these structures, and the proportion of the length with the fimbriae preserved, at least 80 oral fimbriae might be present around the oral disc in IVPP V 15449 .

The oral papillae are partly preserved in IVPP V 15027 and 15449 (larger individuals) ( Figs 3 A-D; 4C). They fringe the outer rim of the oral disc. The papillae are fairly large in relative to the oral disc size and triangular in shape (possibly conical as in living lampreys when the animals were alive, Renaud 2011: 5, fig. 20). They are connected to each other at the base, so that they must have entirely surrounded the oral disc. Judging from their size and arrangement, there could be no more than 30 oral papillae in the material at hand.

Annular cartilage

This is the structure that supports the supraoral and infraoral laminae ( Renaud 2011). The remains of the annular cartilage are seen in some specimens, which are demonstrated either as a horseshoe-like impression (smaller individuals) or a nearly complete circle (larger individuals) ( Figs 3 A-F; 4A, C). When complete, its anterior part appears to be somehow broader than the posterior part. Interestingly, in the specimen (IVPP V 15450.3) with a horseshoe-like impression of the annular cartilage, no traces of the posterior part of the cartilage are discernable although the remainder of this cartilage (element) is very well-preserved.

“Teeth”

In all large adult individuals with the well-preserved impression of the oral disc and annular cartilage, some non-mineralized, presumably keratinous teeth are already in place, though being developed to a lesser extent than in extant lampreys ( Marinelli & Strenger 1954; Renaud 2011). Although the arrangement of the keratinous teeth varies among living lampreys, e.g., Geotria australis Gray, 1851 has few teeth in the posterior field of the oral disc, Lampetra fluviatilis (Linnaeus, 1758) lacks anterior and posterior circumoral teeth ( Potter & Hilliard 1987), as a whole they all have more teeth on the oral disc than Mesomyzon . In Mesomyzon , there are no teeth preserved on the anterior, lateral and posterior fields (see Hubbs & Potter 1971; Renaud 2011 and Fig. 1D for delineation) of the oral disc outside of the circumoral teeth. Given the good preservation of the circumoral teeth under the same taphonomic condition, teeth might be absent, or at least very weakly developed on those fields. The series of the circumoral teeth is incomplete, only the anterior and lateral rows are prominent. These triangular teeth are relatively loosely arranged and similar in size. The teeth on the

supraoral lamina are strong and bicuspid ( Figs 3A, B, E, F ; 4A, C ). Those on the infraoral lamina are occasionally seen, and are also roughly triangular in shape ( Figs 3A, B ; 4A, C ). Although their detailed structure of the transversal and longitudinal lingual laminae can be recognized, some dark matter preserved roughly at the anterior end of the piston cartilage might suggest their presence ( Figs 3 B-D; 4A).

Lingual (piston) cartilage

The piston cartilage is usually preserved as a slender longitudinal band extending along the ventral part of the pharynx from the mouth and posteriorly towards the branchial region ( Fig. 3 B-D). In IVPP V 15035 A ( Figs 3B ; 4A ), IVPP V 15449 ( Fig. 3C, D ), the anterior end of the piston cartilage, or apical cartilage, can be roughly recognized. It is continuous with the remaining part of this cartilage. This part is obviously darker in color than other part of the cartilage, possibly suggesting the fairly thick and complex structure (e.g., the apicalis cartilage) as in living lampreys ( Marinelli & Strenger 1954: figs 19-21; Janvier 1993: fig. 4.4).

Velar skeleton

Immediately in front of the branchial apparatus, there is a prominent dark impression with an irregular profile ( Figs 3B ; 4A ) which points to velar skeleton on the basis of the comparison to its homologue in extant lampreys (in Fig. 1A and Marinelli & Strenger 1954: figs 64, 65; Janvier 1993: fig. 4.4). More dorsally, there is a band-like impression extending anterodorsally toward the cranium, which is likely the imprinted styliform cartilage.This cartilage is linked to the velar skeleton by some muscles in modern lampreys ( Janvier 1993).

Branchial apparatus

The distance between the eye and the branchial apparatus roughly equals the interval of four branchial openings. Seven gill pouches are seen, slightly slanting posteriorly, and of which the foremost one is triangular and much smaller than the remaining ones ( Figs 2C ; 3 A-D). The branchial basket is partially preserved in IVPP V 15027 A and 15449, where the impressions of the ventral part of branchial basket is well exposed. In the anteriormost part, some relics of the extra hyal arch can be detected ( Figs 3C, D ; 4D, E ). More posteriorly, each of the branchial units shows a reversed Y-shaped profile in lateral view, which is connected ventrally with the longitudinal hypobranchial bar ( Figs 3A, C, D ; 4D, E ). Based on the position relative to the external gill openings, this fork must be positioned below the hypotrematic bar, by comparison to the arrangement of gill basket in living lampreys ( Fig. 1B ) ( Marinelli & Strenger 1954). This is different from the living lampreys in lacking the slanting rods above the junction of this bar and each extrabranchial arch as the attachment area of some external branchial constrictor muscles ( Marinelli & Strenger 1954: figs 13, 14). Although we cannot say with certainty that this suggests difference in the configuration of the branchial muscles, the available information indeed reflects some structural variations of the gill-supporting architecture between Mesomyzon and

olfactory pineal organ naso-hypophysial organ notochord atrium hypophysial pouch brain

extant lampreys. We note that these muscle-supporting bars are not seen in lamprey larvae but exist later in adult forms (Hardisty 1981; Janvier 1993, 2008). The branchial basket is relatively well preserved in the Devonian lamprey Priscomyzon Gess, Coates & Rubidge, 2006 ( Gess et al. 2006; Miyashita et al. 2021); however, its ventral part is also different from its analogue in Mesomyzon . There appears to be no bifurcation of the gill arch below the hypotrematic bar in Priscomyzon ( Gess et al. 2006; Miyashista et al. 2021).

More dorsally, the longitudinal hypotrematic bar, the epitrematic bar and the sub-chordal rod of the branchial basket are either blurred by the gill tissues or not observable in available fossil materials.

The external openings of the branchial apparatus are arranged in a posteriorly slanting line ( Figs 2C ; 3B ). Their posterior rims are usually more visible (darker) than the anterior rims, which might be resultant from the existence of the papillae in the posterior edge ( Marinelli & Strenger 1954: figs 13-15, 37, 38, 40, 41).

Relative to the eyes, the branchial apparatus is more posteriorly positioned in Mesomyzon than in its Paleozoic relatives, whose branchial region extends forward nearly below the eyes ( Bardack & Zangerl 1968; Janvier & Lund 1983; Lund & Janvier 1986; Janvier 2008) or even anterior to the eyes ( Miyashita et al. 2021). The majority of living lampreys have a slightly more anteriorly located branchial region than in Mesomyzon , except for quite a few parasitic species, i.e., Entosphenus minimus (Bond & Kan, 1973) , Lampetra lanceolata Kux & Steiner, 1972 , which display a similar postocular length (the distance between the eyes and the first gill opening) to Mesomyzon ( Renaud 2011) .

The ratio of prebranchial length/ branchial length in Mesomyzon ranges from 1.4 to 1.8, a similar state as in Paleozoic lampreys ( Bardack & Zangerl 1968; Janvier & Lund 1983; Lund & Janvier 1986; Gess et al. 2006; Janvier 2008). This ratio in six out of all 39 living lamprey species falls within this range or slightly exceeds it (see Appendices 1; 2). It is interesting that all these six species above except for Lethenteron alaskenses Vladykov & Kott, 1978 are parasitic ( Renaud 2011). Among all living species, the non-parasitic species tend to have a smaller prebranchial length/ branchial length ratio than the parasitic ones ( Renaud 2011). The prebranchial length was also assessed in another criterion. This region in Mesomyzon is no less than twice the maximal head depth. It is notably longer than that in Mayomyzon Bardack & Zangerl, 1968 , Hardistella Janvier & Lund, 1983 and Pipiscius Bardack & Richardson, 1977 ( Bardack & Zangerl 1968; Bardack & Richardson 1977; Janvier & Lund 1983; Janvier 2008). Priscomyzon Gess et al., 2006 displays a prebranchial length/maximal head depth ratio of c. 2, which actually results from the enlargement of the oral disc ( Gess et al. 2006). This ratio in half of the living lampreys is smaller than that in Mesomyzon , whereas the remaining living species (21 out of 39 species) have a ratio approximating 2 or beyond (see Appendices 1; 2).

Pericardinal cartilage

The pericardial cartilage is connected to the rear of the branchial basket ( Figs 3 A-F; 4D), and shows a profile of an equilateral triangle with little difference from that in living lampreys ( Fig. 1A, B and Marinelli & Strenger 1954: figs 59, 60; Hardisty & Potter 1981).

Intestine

The accurate morphology of the intestine cannot be determined; however, its position can be traced by the arrangement of the detrital gobbets occasionally preserved in some large adult specimens. Judging from the detritus preserved immediately behind the last gill pouch ( Fig. 3C, D ), the intestine is arranged in a similar pattern as in living adult lampreys ( Fig.1A and Marinelli & Strenger 1954). The preserved part of the detrital crumbs consists of coarse sand grains of variable sizes and their presence in the digestive tube possibly hints at the fossil lamprey’s habit of attaching on stones or moving stones with their oral disc for nesting ( Hardisty 1979; Renaud 2011), or alternatively the habit of burying themselves in the substrate when the mucus secreted by the oral fimbriae trapped the sand and silt, just like the case in some living lampreys ( Khidir & Renaud 2003; Renaud 2011). In IVPP V 15449 , there is a mass of detritus preserved between the buccal funnel and the pharyngeal cavity, which might locate the mouth ( Fig. 3C, D ). The ending of the intestine, the cloaca (anus) is located at the level of the peak of the “posterior dorsal fin”.

The liver can be detected just behind the pericardial cartilage in several specimens ( Fig. 3A, E, F ). It shows a triangular profile and this part is always in a brightly red in color on the matrix (e.g., in IVPP V 15448 A), which might be related with the oxidized iron ion (of the blood) originally concentrated in the liver organ. This is a similar preservation pattern interpreted in several agnathan fossils ( Janvier 1996; Newman & Trewin 2001).

FINS

Dorsal and caudal fins

Here we hold the assumption proposed by Janvier (2008: 1051) that the so-called “posterior dorsal fin” in lampreys should represent the anterior part of the caudal epichordal lobe inserted on the markedly hypocercal tail and this issue will be referred to in the discussion section below. The anterior dorsal fin is so long that it extends anteriorly as far as to the level of the anterior edge of the fourth gill pouch. Its occupies 44%-48% of the total body length and the height occupies c. 50% of the maximal body depth. The fin changes little in height along its whole length, except that it is beveled smoothly at both ends. The anterior dorsal fin and the “posterior dorsal fin” are usually separated by a distance slightly smaller than the body depth in this position, except for one specimen (IVPP V 15448 A), where the base of the “posterior dorsal fin” extends anteriorly to touch the anterior dorsal fin and this connecting part lacks fin radials. This variation of “posterior dorsal fin” might be correlated with a sexual maturity or mating behavior, because this fin in living lampreys serves as a “brake” to present the male’s tightly wound tail from gliding backward over the female’s body during the mating act (Vladykov 197) and before the breeding season, the female (and sometimes the male) develops a swelling in the anterior edge of this fin ( Janvier & Lund 1983: fig. 4f1). The chordal lobe of the tail is titled downwards at the position of the cloaca (anus). This feature is more marked than in extant lampreys ( Renaud 2011). The “posterior dorsal fin” is mostly surmounted on the tail, roughly triangular in shape and notably higher than the anterior dorsal fin. More posteriorly, it is continuous with the posterior lobe of the caudal fin via a smoothly round notch. The posterior lobe is spade-like in shape with the ventral lobe slightly longer than the dorsal one ( Fig. 2 ).

The fin “rays” (actually cartilaginous radials, see Marinelli & Strenger 1954; Janvier 2008; Miyashita et al. 2019 for interpretation for lampreys and other jawless vertebrates) are easily observable and closely arranged, with a density of about two rays per millimeter, which is more closely arranged than at least in the anal fin of a female Petromyzon marinus Linnaeus, 1758 (31 mm in length supported by 30 rays) ( Vladykov 1973). There is a continuous dark band in the ventral part of the anterior and “posterior dorsal” fins ( Figs 2 ; 3C, D ), possibly reflecting the imprints of the radial muscles ( Sansom et al. 2010: fig. 2a) and/or the adipose stratum ( Marinelli & Strenger 1954: (figs 43, 44, 49). It is noticed that in IVPP V 15027 A, there is a dorsal swelling of the body at the anterior base of the “posterior dorsal fin”, which is not seen in other specimens ( Fig. 2A ). This fin is not completely preserved in the holotype and referred to as emerging above the posterior portion of the body ( Chang et al. 2006).

Anal fin (fold)

The anal fin (fold) is present just behind the anus (cloaca), which is best preserved in IVPP V 15449 ( Fig. 3G ). It is shallow and elongate but separate from the ventral lobe of the caudal fin. It is more a skin fold than a “true” fin because no traces are detected as the supporting rays for this fin as in living form ( Vladykov 1973).

Preanal skin fold

An unexpected finding is the long preanal skin fold that emerges immediately before the anus (cloaca) and extends on the ventral midline ( Figs 2A, C ; 3H ). This structure is common in fossil lampreys from different localities of Jehol Biota. It reaches the level of the front end of the branchial apparatus (i.e., beyond the anterior termination of the anterior dorsal fin), at which point it becomes very shallow. This fold can be seen in IVPP V 14981 B, 15027 and 15448, where it is much lighter in color than and clearly distinct in texture from the impressions of the trunk myomeres. Moreover, it is separated from the trunk myomere imprints by a clear black line delineating the ventral midline ( Figs 2C ; 3H ). In contrast, there is no additional structure below the trunk myomeres in living lampreys as evidenced by the transverse section of the body ( Marinelli & Strenger 1954: figs 42-44). In fossils at hand this fold is occasionally preserved in the way of wrinkling, suggestive a thin and soft nature, distinct from the stiff and straight ventral edge of the body. Additionally, its distal line is always clear rather than the obscure outline in decayed carcass ( Sansom et al. 2013). According to current observable information, this fold is most likely median in position and lacks supporting radials.

Myomeres

The number of the trunk myomeres are counted in the range of 70-80. The trunk myomeres show a zig-zag profile ( Figs 2 ; 3H ) with the ventral section lacking the posteriorly-pointing apex of living forms ( Sansom et al. 2010; Renaud 2011),

whereas those caudal myomeres show a typical W-shaped profile, with the median apex pointing anteriorly ( Figs 2 ; 3G ). The impressions the trunk muscular fibers are clearly observable and arranged in paralleling longitudinal direction as in living forms ( Marinelli & Strenger 1954: fig. 6).

Figures

FIG. 2. — Mesomyzon mengae Chang, Zhang & Miao, 2006, new adult individuals and morphological restoration:A, photograph of IVPP V 15027; B, photograph of IVPP V 15449; C, photograph of IVPP V 14981B; D, restoration of the general appearance of Mesomyzon, dotted circle behind the eyes representing the otic capsule. Abbreviations: adf, anterior dorsal fin; bp, branchial pouches; cf, caudal fin; cl, cloaca; e, eye(s); egp, external gill pores; od, oral disc; pdf, posterior dorsal fin. Scale bars: 10 mm.

FIG. 3. — Mesomyzon mengae Chang, Zhang & Miao, 2006, head, branchial region, and fins. Photographs of: A, IVPP V 15027A; B, IVPP V 15035A; photograph (C) and line drawing (D) of IVPP V 15449; photograph (E) and line drawing (F) of IVPP V 15450.3; G, photograph of the tail of IVPP V 15449, the arrow points to the anal fold; H, close-up of middle trunk of IVPP V 14981B. Abbreviations: ac, annular cartilage; adc, anterior dorsal cartilage; adf, anterior dorsal fin; ao, anterior pillar of subocular cartilage; ap, apicalis and the laminae it carries;atr, naso-hypophysial atrium; ba, imprints of brain;bb, branchial basket; cot, circumoral teeth; da, dorsal aorta; dt, detritus in intestine; eh, extra hyal arch; m, mouth; gp1, gp7, 1st and 7th gill pouch; il, infraoral lamina; lv, liver; no, external narial opening; nhp, hypophysial pouch; gl, gill filaments;gf(l, r), gill filaments of left and right gills; gp, infillings of gular pouch; lo, olfactory organ; my, myomeres; nlp, posterior lateral nerve; ns, nasal sac; oc, otic capsule; oca, oral cavity; op, oral papillae; of, oral fimbriae; p, piston cartilage; paf, preanal skin fold; pc, pericardial cartilage; pdc, posterior dorsal cartilage; sl, supraoral lamina; st, styliform; tel, telencephalon; vs, velar skeleton; I, olfactory nerve or tract; V1, ophthalmic ramus of trigeminal nerve; VII. uc, buccal ramus of facial nerve; VII.rec, recurrent ramus of facial nerve surrounding labyrinth;IX, glossopharyngeus nerve; X, vagus nerve. See Figure 2 for other abbreviations. Anterior facing left in all figures. Scale bars: 10 mm.

FIG. 4. — Close-up of some anatomical structures of Mesomyzon Chang, Zhang & Miao, 2006: A, head of IVPP V 15035A; B, naso-hypophysial complex and adjacent structures of IVPP V 15449; C, oral disc and associated structures of IVPP V 15027A; D, details of gills and branchial skeleton of IVPP V 15449; E, details of the branchial apparatus of IVPP V 15027A; F, imprints of the brain and some cranial nerves of IVPP V 15450.3. Abbreviations: alc, anterior lateral cartilage; nhd, naso-hypophysial duct; fr, presumed position of fossa rhomboidea; g1, gill filaments of the first gill; nsc2, ventral spino-occipital radix nerve. See Figures 1 and 2 for other abbreviations. Scale bars: 5 mm.

FIG. 1. — Generalized anatomy of living lampreys: A, schematic sagittal section through head and pharyngeal region; the vessels, esophagus, respiratory tube, and parts of the heart are shown with portions cut away, rather than in sagittal section; redrawn after De Iuliis & Pulerà 2011, with the structures of the nasohypophysial complex labelled after Janvier 1974; B, the cranial skeleton of an adult lamprey, redrawn after Hardisty & Potter 1981; C, arrangement pattern of the cranial nerves, redrawn after Marinelli & Strenger 1954; D, oral disc of an adult lamprey showing the keratinous teeth and other associated structures. The dotted lines delineate the various fields. Redrawn after Renaud 2011.