A new look at the Cretaceous Lamprey Mesomyzon Chang, Zhang & Miao, 2006 from the Jehol Biota

ABSTRACT Lampreys, one of the remaining two living jawless vertebrates, carry great weight in the study of vertebrate evolution. They have a long history dating back to the Devonian but left a scarce fossil record. So far, only five unequivocal fossil lampreys have been described, of which four are Paleozoic. Mesomyzon Chang, Zhang & Miao, 2006, the only known Mesozoic lamprey bridging the Paleozoic and extant relatives, was originally considered similar in morphology and life history to modern forms. Although being repeatedly referred to in early vertebrate phylogeny, the morphology of Mesomyzon is far from adequately known. Based on extensive investigations on numerous new and well-preserved specimens, we present herein more details of the morphology of this fossil lamprey, thereby releasing a package of new information of the cranial nerves, some associated structures of the oral disc, and the naso-hypophysial complex, which are barely preserved in previously known fossil lampreys. Mesomyzon shows peculiarities in having an extremely long anterior dorsal fin and a ribbon-like preanal skin fold, and hence being restored in a distinct profile from the formerly claimed look. Additionally, it shares with some Southern Hemispheric species the widely separated dorsal fins, posteriorly positioned cloaca and enlarged oral papillae. In the light of these new data, the feeding ecology of Mesomyzon was tentatively discussed and this fossil lamprey was considered a likely blood feeder, judging from the reinforcement of the attachment and sensory structures on the periphery of the oral disc.


INTRODUCTION
Cyclostomes, the only extant jawless vertebrates (agnathans) consisting of lampreys and hagfishes, are generally considered as the most primitive lineage of vertebrates (Janvier 1996(Janvier , 2008(Janvier , 2015Oisi et al. 2013). Despite the long-standing morphology-molecular conflict on the phylogeny of this group (Miyashita et al. 2019), these animals are vitally important in understanding the early evolution of jawed vertebrates (gnathostomes) (Janvier 2008(Janvier , 2015Miyashita et al. 2019). Compared to the hagfishes, the lampreys seem to bear greater resemblance with jawed vertebrates and deeper fossil record dating back to the Late Devonian (Hardisty 1979;Gess et al. 2006;Janvier 2008). However, their fossil record is poor. Only five unequivocal lamprey taxa and a few other enigmatic candidates are known in the fossil record, which are all Paleozoic with the sole exception of Mesomyzon from the Cretaceous of China (Janvier 2008; Miyashita et al. 2021). Albeit so ancient a lineage, the crown group is considered generally conservative in anatomy (Janvier 2006;Chang et al. 2014). The Paleozoic stem lampreys already have the characteristic crown lamprey morphology, including a long eel-like body, an oral disc, non-mineralized (presumed to be keratinous) teeth, while lacking paired fins (Bardack & Zangerl 1968;Janvier & Lund 1983;Gess et al. 2006;Sallan et al. 2017). Despite this, they still have some features, e.g., the extremely small body equipped with a teeth-bearing oral disc and prominent eyes, which suggest an ancestral life history pattern without the larval stages and hence distinct from that of their living counterparts (Miyashita et al. 2021). Mesomyzon, the only Mesozoic representative of this group, reduces the gap between the Paleozoic and modern lampreys (Chang et al. 2006). It shows not only a fairly modern look in external morphology, but also a three-phased (larva, metamorphosis and adult) life cycle interposed with a stage of radical metamorphosis (Chang et al. 2006(Chang et al. , 2014, a particular adap-tive strategy well known in living lampreys (Hardisty 1979). However, since only a young adult specimen was used in the original description of Mesomyzon (Chang et al. 2006) and the subsequent study merely focused on the larvae and transformers (Chang et al. 2014), the knowledge of the morphology of this fossil lamprey is still limited. Inspired by the collection during the past ten years of dozens of exquisitely-preserved specimens, we present here an extensive morphological investigation of Mesomyzon. In comparison with the organic decay process of a modern lamprey (Sansom et al. 2013), these fossilized animals were preserved in a superb state, affected little by the decay process. Consequently, they expose delicate anatomical details of the naso-hypophysial complex, oral fimbriae and papillae, cranial nerves, as well as some unique features of the fins, which have never been clearly preserved or documented in the lamprey stem. With this wealth of new information, some previous descriptions are also revised. New data are synthesized and compared among fossil and living lampreys ( Fig. 1) to tentatively restore the feeding habit of Mesomyzon. Therefore, this study has no doubt refined the existing knowledge of the morphology and ecology of this fossil lamprey and paved the way for the updated phylogenetic analyses to test its systematic status.

SYSTEMATIC PALEONTOLOGY
Class CYCLOSTOMI Duméril, 1806Order PETROMYZONTIFORMES Berg, 1940 Genus Mesomyzon Chang, Zhang & Miao, 2006 Mesomyzon mengae Chang, Zhang & Miao, 2006 (Figs 2-4) Holotype 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 by Chang 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 3A-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 ( The imprints of the styliform cartilage is preserved posteroventral to the otic capsule (Figs 3A-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 3C-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 3A-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 3A-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 3B-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

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; 3A-D). The branchial basket is partially preserved in IVPP V 15027A 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  extant lampreys. We note that these muscle-supporting bars are not seen in lamprey larvae but exist later in adult forms (Hardisty 1981;Janvier 1993Janvier , 2008. The branchial basket is relatively well preserved in the Devonian lamprey Priscomyzon Gess, Coates & Rubidge, 2006(Gess et al. 2006Miyashita 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, 1983and Pipiscius Bardack & Richardson, 1977(Bardack & Zangerl 1968Bardack & 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 3A-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 15448A), 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 15448A), 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 15027A, 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 14981B, 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).

COMPARISONS AND DISCUSSION
Our study presents a set of characters not recorded in the original descriptions of Mesomyzon (Chang et al. 2006), and reveals additional traits shared with extant lampreys. The differentiation of the major cranial cartilages, i.e., the anterior lateral cartilage, the anterior and posterior dorsal cartilage, as well as the possible subocular arch, show no significant difference from those in living adult lampreys (Marinelli & Strenger 1954;Janvier 1993). The nasohypophysial organ displays many folds, indicating a pattern to increase of the surface area of the nasal epithelium, nearly identical with the modern counterpart in morphology (Marinelli & Strenger 1954). This organ was never preserved so well in other fossil lampreys (Bardack & Zangerl 1968;Bardack & Richardson 1977;Janvier & Lund 1983;Lund & Janvier 1986;Chang et al. 2006Chang et al. , 2014Gess et al. 2006). Additionally, the general configuration of the naso-hypophysial duct and its external opening, and the hypophysial pouch closely resembles their counterparts in modern lampreys (Marinelli & Strenger 1954). Some cranial nerves, exemplified by the trigeminal and facial nerves, are also arranged in a modern pattern. The recurrent ramus of the facial nerve looping over the inner ear is characteristic in lampreys (Marinelli & Strenger 1954), and is not developed in living hagfishes (Marinelli & Strenger 1956;Oisi et al. 2013).
Despite resembling living forms in many aspects, Mesomyzon displays considerable peculiarities that distinguish itself from its modern and other fossil lamprey relatives. The anterior dorsal fin extends anteriorly above the middle branchial region, and is much longer than its counterpart in all living lampreys (Renaud 2011) and any other known fossil lampreys (Bardack & Zangerl 1968;Bardack & Richardson 1977;Janvier & Lund 1983;Lund & Janvier 1986;Chang et al. 2006Chang et al. , 2014Gess et al. 2006), even with Pipiscius Bardack &Richardson, 1977 andGilpichthys Bardack &Richardson, 1977 included in this group (Janvier 2008). Considering the similarly long dorsal fin fold of the larval and metamorphosing individuals (Chang et al. 2014) and the developmental process of the dorsal fins in modern lampreys (Potter et al. 1982;Richardson & Wright 2003), the long dorsal fin of adult Mesomyzon was likely inherited from the larval dorsal fin fold, with the addition of the later emergence of the supporting cartilaginous fin rays (radials). It should be kept in mind that the cartilaginous rays (radials) are deeply embedded in the dorsal portion of the body (Marinelli & Strenger 1954: figs 43, 44, 46). Moreover, the anterior dorsal fin and "posterior dorsal fin" are widely separated in Mesomyzon. In other fossil lampreys with anterior dorsal fin developed, it is very close, if not in contact, to the "posterior dorsal fin" in Hardistiella  The relative position of these two fins in most living lampreys is variable, except the southern hemisphere taxa Geotria Gray, 1851 and Mordacia Gray, 1851, whose the dorsal fins remain separate throughout the lifespan. Whereas in other species except for those with a single dorsal fin (e.g., Ichthyomyzon spp.) they progressively become closer to each other as the animal becomes sexually mature and eventually come into contact at the base (Renaud 2011). It cannot be determined for now whether this specimen with weak fin connection mentioned above fits this condition or not. Another important feature refers to the unique ribbon-like preanal skin fold. This is not recorded in any other known lampreys (Bardack & Zangerl 1968;Bardack & Richardson 1977;Janvier & Lund 1983;Lund & Janvier 1986;Chang et al. 2006Chang et al. , 2014Gess et al. 2006), nor in the original description of Mesomyzon (Chang et al. 2006). A preanal fold of such a dimension is also never developed among living lampreys except that a very short preanal fold is occasionally seen in a few living species, e.g., Ichthyomyzon greeleyi Hubbs & Trautman, 1937(Renaud 2011. This preanal skin fold is also present in living and the recently reported fossil hagfishes (Miyashita et al. 2019, and personal communications with Miyashita) but arranged in a much more restricted extent (personal observations on living hagfish). It is worthy to note that the decay halo is occasionally seen in some cyclostome or other jawless vertebrate fossils (Newman & Trewin 2001; Miyashita 2020), which might raise the suspicion that the preanal skin fold here is just part of the decay halo, just like the case in the Carboniferous hagfish Myxinikela siroka Bardack, 1991(Bardack 1991, 1998Miyashita 2020). However, unlike the decay halo around the carcass, both along the ventral and dorsal edges of the body in Myxinikela (Miyashita 2020), there is no "decay halo" along the dorsal edge of the body in all Mesomyzon's fossil materials with the preanal skin fold clearly observable. Therefore, it seems more possible that this fin fold in Mesomyzon is not a taphonomic artifact.
We also found a small distinct anal fin fold in Mesomyzon, which is again not seen in other fossil lampreys (Bardack & Zangerl 1968;Bardack & Richardson 1977;Lund & Janvier 1986;Gess et al. 2006) except for Hardistiella, which has a small anal fin (Janvier & Lund 1983;Janvier 2008). However, the anal fin in the latter is supported by radials, which are likely not present in this anal fold of Mesomyzon. A female sea lamprey (Petromyzon marinus) was reported to develop a true anal fin during the spawning time (Vladykov 1973). However, we think this might not be the case for Mesomyzon. This radial-free fin fold in Mesomyzon is present in several individuals of different sizes, and the individuals with this structure display few female secondary sexual features but instead some characters, e.g., the relatively long prebranchial length, oral disc and tail, which are regarded as male sexual features in living forms (Renaud 2011), were they related with the sexual maturity of Mesomyzon.
The tail is hypocercal in Mesomyzon (Fig. 2). The exact status of the tail of Priscomyzon is not clear as all known specimens of this taxon are preserved in dorsoventral compression (Miyashita et al. 2021),the feature of the caudal fin in unclear either according to the restudy on this taxon (Miyashita et al. 2021). The tail is nearly straight in Mayomyzon (Bardack & Zangerl 1968;Janvier & Lund 1983;Janvier 2008) and in Pipiscius (Janvier 2008), whereas it is gently tilted downwards in Hardistiella (Janvier & Lund 1983), with a more marked inclination than in modern lampreys (Renaud 2011). The condition of the tail of Mesomyzon is therefore more similar to that in some other Paleozoic fossil jawless vertebrates, e.g., most "anaspids" and "thelodonts" (Janvier & Lund 1983;Janvier 2008) than in its modern and fossil lamprey allies.
The myomeres of the preanal trunk of Mesomyzon show a Z-shaped profile, and those in the tail region are typically W-shaped. The trunk myomeres in modern lampreys are W-shaped (Renaud 2011).
The oral fimbriae and papillae were never documented in previously known fossil lampreys (Bardack & Zangerl 1968;Bardack & Richardson 1977;Janvier & Lund 1983;Lund & Janvier 1986;Chang et al. 2006Chang et al. , 2014Gess et al. 2006) except in the recently revisited Late Devonian lamprey Priscomyzon, where theoral fimbriae were recognized as the ridges along the edge of the oral funnel (Miyashita et al. 2021). These structures are relatively decay-resistant according to experimental taphonomy (Sansom et al. 2010(Sansom et al. , 2013. The development of oral fimbriae was documented in living lampreys by Lethbridge & Potter (1981). Mesomyzon's oral fimbriae and papillae show some peculiarities in their shape and arrangement. The number of the oral fimbriae in Mesomyzon falls within the range exhibited by some living lampreys. The fimbriae are morphologically simpler in Mesomyzon than in their modern counterparts (Renaud 2011). The oral fimbriae are absent only in Mordacia which is currently confined to the southern hemisphere (Renaud 2011). The oral papillae of Mesomyzon are conspicuous in size and arrangement. In morphology, they are reminiscent of those of the Southern Hemispheric lamprey Mordacia lapicida (Gray, 1851) (Renaud 2011: fig. 20) but much larger in size than those in this species or other modern lampreys (Khidir & Renaud 2003;Renaud 2011). Additionally, they are more closely arranged and continuous at their base, and likely enclose the outer rim of the oral disc. The enlargement of the oral papillae was reported in another Southern Hemispheric taxon, Geotira australis Gray, 1851(Potter 1980 . 4); however, only a pair of anterolateral papillae are involved in such a specialization and are proportionally much smaller than those in Mesomyzon described here.
During the mating activity of living lampreys, the male warps its tail around the female and slides posteriorly to approximate the female's urogenital region for fertilization (Johnson et al. 2015), and the female's "posterior dorsal fin" acts as a "brake" to hold a proper mating gesture of the male (Vladykov 1973). Therefore, it is interesting to note the position of the cloaca relative to the "posterior dorsal fin" among lampreys. The cloaca opens at the level of the peak of the "posterior dorsal fin" in Mesomyzon. It is more posteriorly positioned than in other fossil lampreys where the cloaca can be located. In Hardistiella, it is located slightly anterior to the base of the "posterior dorsal fin" (Janvier & Lund 1983), whereas in Mayomyzon, although not clearly preserved, judging from the position of the confluent anal fin, it must be positioned before the peak of the "posterior dorsal fin" (Bardack & Zangerl 1968;Janvier 2008). The situation is not clear in Priscomyzon where the fins cannot been clearly recognized (Miyashita et al. 2021). The cloaca of Mesomyzon opens more posteriorlythan in most living lampreys, where it is usually located at the level of the emergence of the "posterior dorsal fin". Mesomyzon resembles some Southern Hemispheric species (Mordacia), whose cloaca is positioned at a level slightly behind the peak of the "posterior dorsal fin" (Renaud 2011 In the literature, lampreys are generally described as having two (one anterior and one posterior) dorsal fins (Hubbs & Potter 1971;Hardisty 1979Hardisty , 1981Potter et al. 1982Potter et al. , 2015Renaud 2011). Actually, the so-called "posterior dorsal fin" is mostly inserted on the tail region (the part behind the cloaca) and its internal cartilaginous radials are continuous with those of the posterior lobe of the caudal fin in living lampreys (Marinelli & Strenger 1954: fig. 46;Renaud 2011). It is also the case in Hardistiella, whose "posterior dorsal fin" is completely confined in the tail region and there is even no mark between it and the more posterior part that was traditionally called "caudal fin" (Janvier & Lund 1983 fig. 109) and Devonian jawless fish Euphanerops longaevus Woodward, 1900(Janvier & Arsenault 2007. Therefore, we insist on the presumption that lampreys' confluent "posterior dorsal fin" and the caudal fin should be the homolog of the markedly hypocercal tail in other fossil jawless fishes, e.g., euphaneropids and anaspids (Janvier 2008), which is compatible with the recognition of stem cyclostomes of these fossil forms (Miyashita et al. 2019). Namely, lampreys should have only one dorsal fin when present.
a likely blood feedeR? When Mesomyzon was originally described (Chang et al. 2006), the possession of an oral disc with tooth plates did not warrant definitely a parasitic mode of life, as the holotype is relatively small, c. 8.4 cm in total length and no larger than some later transforming individuals and the other specimen is a metamorphosing individual (Chang et al. 2006(Chang et al. , 2014. Additionally, the living non-parasitic species also retain oral disc and teeth (Renaud 2011).
The parasitic and non-parasitic lampreys show radically different modes of life in the post-metamorphosis period. The non-parasitic species no longer feed and shrink in size relative to their larval stages, whereas the parasitic species feed and naturally grow longer and larger than their larvae (Hardisty 1979;Potter et al. 1982;Renaud 2011). The individuals described here all display adult features in the oral disc, eyes, branchial apparatus and fins. Moreover, they are all much larger than the metamorphosing individuals and the young adult (the holotype) (Chang et al. 2014), which entails a post-metamorphic feeding habit and sufficient food ingestion. Therefore, it can be confirmed with confidence now that Mesomyzon is very likely a parasitic form.
The oral fimbriae and papillae are mucus-producing structures of the oral disc and closely associated with the feeding behavior of lampreys, or even linked to the variations of food types among the adults (Lethbridge & Potter 1981;Khidir & Renaud 2003;Clark & Uyeno 2019: 221). They secrete mucus that protect the oral disc from abrasion by sand particle and aid to ensure the sealing of the oral disc onto the body of the prey. More specifically, among the parasitic lampreys, the blood feeders (e.g., Petromyzon marinus Linnaeus, 1758) require an effective attachment of the oral disc more than do the flesh eaters (e.g., Lampetra fluviatilis (Linnaeus, 1758)), because they have to prevent any loss of the blood when they feed (Khidir & Renaud 2003). They also depend on these structures with possible sensory function (Lethbridge & Potter 1981) to sense the appropriate (i.e., more vascularized) site to feed on the prey (Khidir & Renaud 2003). In contrast, the flesh feeders (e.g., Lampetra fluviatilis) do not have such a strong requirement in these aspects (Khidir & Renaud 2003). Mesomyzon is well equipped with both the oral fimbriae and oral papillae, outstanding among all lampreys in the large and continuous series of oral papillae. This, as a natural consequence, endows the oral disc of Mesomyzon a considerable efficiency of attachment. With this in mind, we assume that Mesomyzon could be more a blood feeder than a flesh feeder. On the other side, the complete set of both oral fimbriae and papillae, as well as the enlargement of the latter, might merely be the compensation of the less well-developed teeth on the oral disc, which would be embedded in the host and prevent the slippage of the oral disc along the prey's body when the lamprey feeds (Clark & Uyeno 2019). Besides, the general feature of Mesomyzon's oral disc is also compatible with the assumption of thr blood feeding habit, because the relative size of the oral disc (6.6% of total body length) falls in the range of that in some blood feeders (e.g., Ichthyomyzon unicuspis Hubbs & Trautman, 1937, I. castaneus Girard, 1858, Mordacia mordax (Richardson, 1846) but is much larger than that of those typical flesh feeders (e.g., 4.5-5.5% of total body length in Lampetra fluviatilis) (Potter & Hilliard 1987;Khidir & Renaud 2003).
If Mesomyzon was indeed a land-locked predaceous lamprey (Chang et al. 2006), then it must have fed on some GEODIVERSITAS • 2021 • 43 (23) local aquatic vertebrates. The aquatic vertebrates in the Jehol Biota include some swimming reptiles, amphibians, and fishes (Zhou 2014; Zhou & Wang 2017). Among them the fishes are potentially suitable hosts, e.g., the highly diversified acipenseriform fishes (sturgeons), as they are naked, with the integument delicate enough to penetrate for Mesomyzon, and they could grow to fairly large sizes, thereby facilitating the suctorial attachment of the lampreys (Jin 1995). The Lycoptera Müller, 1848 (a primitive bony-tongue) are also a potential food source for Mesomyzon, as they are abundant in that biota and their cycloid scales are relatively thin (Zhang et al. 1994). The abundant and naked salamanders (Zhou & Wang 2017) from this biota are also potential prey of lampreys. However, admittedly, no cases of bite marks on the skin by lampreys on these animals are documented yet. And moreover, the possibility of the anadromy cannot be excluded from the life history of Mesomyzon, as there might be some drainage links between region hosting the Jehol Biota and the nearby Bohai Bay from the Late Jurassic to early Early Cretaceous (Liu et al. 2015). a laRge oRal cavity (pHaRynx) and its functional and evolutionaRy significance The backward-shifted branchial apparatus and the resulting proportionally longer prebranchial region together make an enlargement of the pharynx (oral cavity) and hence more space for the development of the feeding structures in Mesomyzon. This recalls the expansion of the oral cavity from the larval to adult stages in modern lampreys (Potter et al. 1982: 155).
The oral cavity plays an important role in the feeding of living parasitic lampreys as they bite in the absence of moveable jaws (Clark & Uyeno 2019). It serves as the pump to generate volumetric fluctuations to facilitate the adhesive suction of the oral disc and rasping (incising) prey tissues of the teeth-bearing tongue (Clark & Uyeno 2019). It also actively expands to induce a vacuum to drag the fluid and food items into the mouth. In this sense, equipped with an enlarged oral cavity (pharynx), Mesomyzon would have reaped some selective advantage in feeding. Conversely, the anteriorly shifted branchial region in Paleozoic lampreys (Janvier 2008) squeezed out the room for the pharynx and hence the oral cavity, which had consequently caused a proportionally shorter piston cartilage (Hardisty 1979: 37). Although Priscomyzon has a relatively large prebranchial length (Gess et al. 2006); however, when leaving aside the question of the large oral disc, the length of the pharynx shows no significant difference from that in Mayomyzon and Hardistiella (Bardack & Zangerl 1968;Janvier & Lund 1983). This divergence should be partially related with the feeding efficiency between these Paleozoic forms and Mesomyzon, and in turn, accountable for in part the disparate body sizes of these lampreys, were they all parasitic.