MALACOLOGIA, 2010, 52(1): 97−113

BRACHIOPOD IDENTITY OF THE ALLEGED MONOPLACOPHORAN ANCESTORS OF CEPHALOPODS

Jerzy Dzik

Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa and Instytut Zoologii Uniwersytetu Warszawskiego, Banacha 2, 02-079 Warszawa, Poland; [email protected]

ABSTRACT

Because of a serial arrangement of supposed pedal muscles and its high-conical shell, Hypseloconus from the Late Cambrian to Early Ordovician of North America is generally believed to be a monoplacophoran relative of cephalopods. The distinctive pattern of muscle attachments and shell form indicates its close relationship to roughly coeval Siberian Kirengella and several other genera classified in the order Kirengellida. Newly collected material from the Early Ordovician of Siberia shows that the bivalved Angarella, with its ventral valve cementing to a hard substratum, is closely similar to Kirengella and Hypseloconus in the arrangement of shell muscles. Permanently fixed to its substratum was also another probable member of the group, Pygmaeoconus. Musculature of Angarella in some aspects resembles that of the Early Cambrian mobergellans with phosphatic shells, but in the calcitic shell structure it is similar to the craniopsid brachiopods. Irrespective of whether the kirengellids are brachiopods or not, they should be removed from considerations on the ancestry of cephalopods. The alternative to Hypseloconus as a candidate for cephalopod ancestry is the Early Cambrian Turcutheca, an enigmatic mollusk with endogastrically curved and laterally compressed conch and relatively large subspherical embryonic conch, in both aspects resembling the earliest ellesmeroceratid nautiloids. Key words: Monoplacophora, Ordovician, Cephalopoda, Siberia, evolution.

Introduction on the shell interior. Kirengella differs from Hypseloconus in having a low-conical shell. As Embryological and comparative anatomical shown below, an even lower shell, but virtually evidence is widely used to interpret the origin of the same pattern of muscle attachment scars, aspects of cephalopod soft anatomy distinctive characterizes Angarella, fossil shells of which for them, whereas palaeontological data show abundantly occur in the Ordovician strata of how their shell, of originally hydrostatic function, the region. But at this point of reasoning a changed in the course of evolution. The most great difficulty emerges: Angarella shells are widely accepted palaeontological interpretation bivalved and were basally cemented to a hard of the origin of cephalopods, advanced by Yo- substratum! These extinct organisms appear to chelson et al. (1973) and reaffirmed in Webers be more similar to the craniopsid brachiopods & Yochelson (1989) and Kröger (2007), pro- than to the monoplacophorans. poses that they originated from a high-conical In the present paper I present data on the monoplacophoran with serially arranged pedal morphology of Angarella and discuss its evo- muscles. The only available evidence that such lutionary relationships with coeval brachiopods monoplacophorans occurred in the Cambrian, and possibly ancestral Cambrian problematica. before the appearance of earliest cephalopods, An alternative direction of search for cephalo- are specimens of Hypseloconus cf. H. stabilis pod ancestry will be proposed. Berkey, 1898, from the Davis Formation of Missouri described by Stinchcomb (1980). Webers et al. (1991) and Stinchcomb & Angeli MATERIAL (2002) transferred this species to the roughly coeval Siberian genus Kirengella, which shows Angarella shells are the most common fos- a similar disposition of muscle attachments sils in the Badaranovo Formation of the Irkutsk

97 98 DZIK

FIG. 1. Species of Angarella from the Ordovician of the Angara River region, Siberia. A: Mass occur- rence of Angarella jaworowskii Asatkin, 1932, shells in light coarse-grained calcareous sandstone of the lower Badaranovo Formation; loose block ZPAL Bp 62/12 collected at Pashino near Kodinsk. Ventral valves are marked with asterisks; B: Large ventral valves of Angarella at the bedding surface of red muddy calcareous sandstone of the lower Mamyry Formation at the bank of Vikhoreva River near Bratsk (field photo).

Amphitheatre geological structure in southern of the shell surface or interior are rarely discern- Siberia (Divina et al., 1984). Frequently they are ible. The specimens used in this study come rock-forming skeletal elements. Their preserva- from a loose block of calcareous sandstone tion is generally poor, and morphological details collected on the left bank of Angara near Ko- ANCESTRY OF CEPHALOPODS 99

FIG. 2. Morphology of Angarella jaworowskii Asatkin, 1932. Shells from a loose block of sandstone of the lower Badaranovo Formation collected at the bank of Angara at Pashino near Kodinsk, Siberia. A-C: Dorsal valve ZPAL Bp 62/6 with muscle attachment scars, lateral and external views of part (A-B) and internal view of counterpart (C); D: Ventral valve ZPAL Bp 62/8 in internal view; E: Same for ventral valve ZPAL Bp 62/7; F-G: Lateral and external view of extremely high-conical dorsal valve ZPAL Bp 62/25 with thin wall; H-I: Same views of small-conical dorsal valve ZPAL Bp 62/27 with thick wall. 100 DZIK dinsk upstream of Verkhnaya Kezhma Creek, bellerophonts indicative of more open marine where formerly the village Pashino was located. conditions than lower in the section. The material is relatively informative owing to Specimens are housed at the Institute of Pa- weathering of the rock, which then easily dis- leobiology of the Polish Academy of Sciences integrates along the shell surfaces. The block in Warszawa, Poland (ZPAL). probably derived from the 1.5 m thick bed 43 of the Badaranovo Formation, where a mass oc- currence of Angarella was reported by Kniazev Geological Age of the Fauna (1978). There was little sorting, as ventral and dorsal valves seem to be equally frequent in the In the Ordovician, the Siberian Platform was rock (Figs. 1A, 2). The fossil assemblage in the a separate continent with faunas biogeographi- block is monospecific, but in other beds of the cally distinct from those of other regions of the formation Angarella is sometimes associated world. Only some species of pelagic organisms with sinuitid bellerophonts. had their distribution wide enough to enable age This material of Angarella Asatkin, 1932, correlation. In the Angara section, the conodonts almost certainly represents the type species of may serve this purpose, but our knowledge the genus, A. jaworowskii Asatkin, 1932, and about them is at a rather preliminary stage, with derives from its type stratum, the Badaranovo the standard apparatus-based taxonomy applied Formation. The type locality is within the same to only a few species, none from the Badaranovo region on Angara at the mouth of the Koda Formation. A brief overview of the evidence thus River. Each sample of Angarella shells from seems necessary. the Badaranovo Formation shows so much Conodont samples rich enough to enable the variability in the shape of both ventral and apparatus study have been obtained from two dorsal valves that discrimination seems hardly glauconitic limestone beds near the base of possible. Originally four species of the genus the formation. They are underlain by mudstone were named, and Yadrenkina (1984) reduced beds, and overlain by reddish sandstone, with their number to three, but all occur together in the Angarella-rich horizon a few meters higher. various horizons of the formation. Glauconite is the main component of the rock In relatively pure reddish sandstone of the in the basal bed of the Badaranovo Formation lower Badaranovo Formation on Angara, speci- (Ang-3), whereas quartz sand dominates in mens with marginal concentration of growth the bed 40 cm higher in the section (Ang-2). increments are mostly of 3 cm diameter and Conodont faunas from these beds are closely rarely approach 4 cm. Specimens possibly similar to each other. representing a more advanced segment of the The most common conodont is a species of Angarella lineage or a local population living Drepanoistodus (141 specimens in Ang-3; 122 in a different environment occur in olive-gray in Ang-2) with rather robust elements lacking marly sandstone of the upper Badaranovo For- any additional ribs (Moskalenko, 1984). Some mation exposed near Kodinsk a few kilometers elements resemble Drepanodus and were clas- upstream from the place where specimens with sified as such by Moskalenko (1984), but no muscle scars were collected (near the former symmetrical S0 or geniculate M elements in the village Rozhkovka). The stratum probably rep- samples represent this genus. The second in resents the basal horizon in the upper Badara- number (19 specimens in Ang-3; 30 in Ang-2) is novo Formation. Specimens of Angarella reach a similarly generalized species of Acodus with there oyster size, up to 12 cm long (Fig. 3B). At minute elements (possibly A. deltatus). Rozhkovka the large Angarella is a member of Conodonts with coniform elements in the ap- a rather diverse fossil assemblage with cono- paratus are not common in the assemblages donts, suggestive of an open-sea environment studied, the most important of them being Glyp- with normal salinity. Similar, but usually poorly toconus, probably G. quadraplicatus (there is a preserved specimens of Angarella occur in the possibility that “Oneotodus” rotundus Moskalen- cliff of Vikhoreva River, a tributary of Angara ko, 1967, is a P element of the same apparatus) near Bratsk (Fig. 1B). The stratigraphically (29 specimens in Ang-3; 24 in Ang-2). Hyaline highest occurrence of Angarella at Vikhoreva Drepanodus-like specimens with the S0 element is in the marly oolithic limestone of the lower distantly resembling Ulrichodina may be related Mamyry Formation in an abandoned quarry to “Drepanodus” concavus as interpreted by Ji on the other side of the road (exposure 755 in & Barnes (1990). Divina et al., 1984). Large Angarella are asso- Of the greatest correlative value are two ciated with the raphistomatic gastropods and species of conodonts with incipient denticula- ANCESTRY OF CEPHALOPODS 101 tion of some elements. The more elaborate of but field observations suggest that there is a them is Jumudontus sigmoidalis (Moskalenko, complete gradation in the shell geometry. The 1984), different from its cosmopolitan relative most common shell outline is oval, somewhat J. gananda in having a narrow base and sinu- elongate, and with the greatest width at the ous orientation of denticles in P1 elements (22 level of the apex (Fig. 1A). Specimens circular specimens in Ang-3; 16 in Ang-2). The Siberian in outline or slightly wider than long also occur. material does not support identification of M The apex is located in the posterior third of elements by Nicoll (1992), who attributed to the dorsal valve in most specimens, but it may Jumudontus probable specimens of Protopri- overhang the margin in the tallest ones (I apply oniodus. Instead, associated elements of the here descriptives as for brachiopod shells, not Siberian Jumudontus have a much less derived molluscan conchs). Usually the length of the appearance (Moskalenko, 1984: pl. 16:18). valve is about 3.5 times its height, but in the Jumudontus gananda is widespread in the tallest specimens it is about 2.5, whereas in the late Oepikodus evae Zone of the Arenig. The flattest it may reach 4.0. Siberian species, as suggested by its narrow The apex of the dorsal valve, smooth and base, may be transitional between J. brevis hemispherical in shape, is about 1 mm in diam- and J. gananda within the same lineage (see eter. Its surface is too crudely preserved, with Nicoll, 1992), or represents a separate lineage imprints of sand grains, to allow discrimination that split out near this point. This interpretation of the larval stage. is supported by the co-occurrence of an early The ventral valve was cemented to hard ob- member of the Histiodella lineage in Siberia (11 jects, as shown by attachment scars replicating specimens in Ang-3; 9 in Ang-2). Histiodella the rough substratum surface of undetermined sibirica (Moskalenko, 1967) (= Histiodella an- nature. Rarely it is cemented to other Angarella gulata Moskalenko, 1982) may be ancestral to shells. Prominent irregular growth increments H. altifrons, known to occur above the range of are discernible on an area of variable extent J. gananda (e.g., Pyle & Barnes, 2003). in the posterior region of the valve. This area Among the numerous relatively shallow-water is more or less concave, in connection with conodonts reported from the lower Mamyry For- a slight dorsoventral curvature of the whole mation by Moskalenko (1984), the most distinc- valve. Generally, the valve is almost flat and tive is the pterospathodontid “Polyplacognathus” near its posterior margin it attains the greatest angarense Moskalenko, 1984, of limited vertical thickness. In specimens from marly facies (pos- distribution, but unknown outside Siberia. Higher sibly separate species) this thickness exceeds up in the section it co-occurs with Phragmodus 1 cm (Fig. 3B). flexuosus Moskalenko, 1973, which indicates the Pygodus serra Zone of the Llanvirn. Shell Structure Angarella is thus known to occur from the Mid Arenig to Late Llanvirn in terms of the British The shell thickness is clearly correlated with subdivision of the Ordovician. its mode of growth. Tall-conical specimens are Phosphatized ellesmeroceratid nautiloids thin-walled (Figs. 2F, G, 3A), whereas low- were extracted from a detrital limestone inter- conical dorsal valves and all ventral valves are calation in the stromatolithic Ust-Kut Formation thick-walled (Fig. 2H, I). at Pashino, stratigraphically much below the Internal structures of the Angarella shell are Badaranovo Formation. Associated specimens well preserved even in strata where aragonitic of Cordylodus rotundatus indicate its own Zone, gastropod conchs are recrystallized into calcitic that is early Tremadoc. sparite or dissolved, which proves its originally calcitic composition. There is no evidence of secondary mineralisation or recrystallisation Shell Morphology and Structure in specimens from Pashino but, as a result of of Angarella mechanical weathering, the shell is cut by nu- merous cracks obliterating the original structure. The shell of Angarella was bivalved, the up- The shell wall structure is laminar and imporous. per (dorsal) one being of conical appearance Because of this structure, the shell matrix of and the lower (ventral) almost flat. Dorsal weathered specimens easily splits into fine valves are very variable in outline and height glossy blades. In the SEM, rhythmic banding is of the cone (Figs. 2A, F, H, 3A). The sample recognizable, with laminae thickness about 1.5 taken to the laboratory is too small to offer a µm (Fig. 3E), with rhythmicity suggestive of daily reasonable biometric evaluation of variability, increments (e.g., Goodwin et al., 2001). 102 DZIK

FIG. 3. Wall structure of Angarella from the bank of Angara. A, C-E: Angarella jaworowskii Asatkin, 1932, high-conical specimen ZPAL Bp 62/24 from the lower Badaranovo Formation at Pashino; pol- ished section (A), thin section showing shell detritus in the sandstone (C), and SEM pictures of shells in transverse section etched with dilute acetic acid (D, E) note polished quartz sand grains in the upper part of the area shown (D) and fine lamination of the shell matrix, possibly daily increments (E); B: Very large and thick Angarella sp. ventral valve ZPAL Bp 62/13 from the upper Badaranovo Formation at Rozhkovka; thin section of the anterior margin of diageneticaly altered shell; note detritus of thinner parts of shells in the rock matrix below. ANCESTRY OF CEPHALOPODS 103

FIG. 4. Restoration of interiors of Angarella jaworowskii Asatkin, 1932, from the Badaranovo Formation of the Angara River region, Siberia. A: Interiors of dorsal valves based on ZPAL Bp 62/6; B: Interiors of ventral valves based on ZPAL Bp 62/8.

Muscle Attachments No ventral valve of comparable quality in rep- resentation of muscle attachments is available. Muscle attachment scars of Angarella ja- The two best preserved specimens with the in- worowskii were identified and illustrated by terior of the valve visible (Fig. 2D, E) show only Yadrenkina (1984, pl. 8: 1a, b, pl. 9: 3). In her indistinct areas of muscle attachments. They paper, they are shown as large dark-stained are clearly delimited by the thickened posterior fields separated from each other by a narrow region of the valve and the low medial ridge. strip of lighter matrix. Their exact shape is hard An indistinct, very low transverse elevation may to visualize based on published photographs. suggest that two pairs of large oval attachments More informative is the part of a mature are represented in the posterior part of the ven- dorsal valve from the Pashino block split tral valve floor, but their boundaries cannot be along its internal surface (Fig. 2A–C). The traced with confidence. There are two tear-like valve represents the flat morphotype that has elongated belts of different coloration along the a relatively thick shell. Consequently, muscle lateral margins of the valve floor that may also attachment scars are relatively deep for the represent muscle attachments. This is definitely species. Moreover, roughness of the surface not enough to make restoration of ventral at- of the muscle attachment area, as compared tachments reliable, and I hesitate to speculate with the rest of the shell interior, permitted the about the relationship between dorsal and shell matrix to remain in places on the nucleus, ventral muscle attachments in Angarella. demarcating the limits of muscle attachments. These are symmetrically distributed on both sides of the shell, excluding a possibility of Relatives of Angarella being artifactual. There are five paired scars and a probably Siberian Species continuous, narrow posterior band. The three lateral scars are of similar size (Fig. 4). A pair of Conical shells with muscle attachments virtu- large scars occupies the anteriormost position. ally indistinguishable from those of Angarella These seem to be internally subdivided into are known from other rock units of the Early smaller fields, perhaps five in each scar. Minute Ordovician and Late Cambrian of Siberia. The scars are near the midline of the valve at the stratigraphically and geographically closest level of posterior margin of the largest scars. is probably Nyuella bjalyi Rosov, 1975, col- 104 DZIK lected from sandy limestone near the base of attachments there are deep, which indicates the Ust-Kut Formation of the Kima stage, left reasonable thickness of the shell. Its original bank of the Nyuia River, a tributary of the Lena mineralogy remains undetermined. Specimens River. The Ust-Kut Formation underlies the were separated to two genera because on Badaranovo formation in the region, but it is not specimens attributed by her to Kirengella two certain that everywhere they are of the same additional small anterior (posterior, if the mol- age. Muscle attachments are preserved as a luscan interpretation is preferred) attachments dark carbonaceous(?) staining (Rosov, 1975) are discernible. Otherwise the distribution of of the same kind as in the Angarella specimens scars is virtually the same in all specimens. of Yadrenkina (1984). There is a possibility Kirengella kultavasensis, based on four speci- that, like the Yadrenkina (1984) material, not mens, Romaniella aebitensis, R. getlingi and all muscle scars are there reproduced with the R. zverevi of Doguzhaeva, 1972, are all from stain and that these are actually specimens of the same stratum. By analogy with Angarella, Angarella. with which these Uralian form shares a similar Another form collected from the lower Ust- shell appearance, one may suggest that they Kut Formation in the same area is Lenaella represent variability in shell form within a single octobinaria Bjaly, 1973. The type locality is population. Three separate small scars of Ro- on the Kirenga River, 7 km upstream from maniella appear to correspond to each of the Kluchi Village. Lenaella octobinaria differs large scars of the anterior pair in Angarella and from Angarella jaworowskii only in having a Kirengella. The admedial scars are between taller conical shell with the apex located in its them, that is, somewhat anteriorward in respect center. It may well be ancestral to the lineage. to their position in Angarella. In fact, Bjaly (1973) reported the species also Still within the Siberian Platform but near its from the Angara River at Rozhkovo, that is northern margin is the type locality of Moyero- from the same locality from which the material kania miagkovae Rosov, 1970, found on the of Angarella described here comes. He refers right bank of the Moyerokan River, a tributary to concentric growth increments on the exter- of Moyero, near the mouth of the Superechi nal surface of the shell, which means that it is Stream. The species is known only from nuclei preserved in at least some specimens. with muscle attachment scars (Rosov, 1970). The stratigraphically oldest Siberian member They show a characteristic subquadrate out- of the group and its name-giver is Kirengella line, with the posterior margin almost straight. ayaktchica Rosov, 1968. Its shell form is closely Moyerokania miagkovae is reported to co-occur similar to that of Lenaella, and the muscle with Angarella. The main difference between attachments pattern is virtually the same as these genera is that in Moyerokania admedial in Angarella. Kirengella ayaktchica occurs in scars are located anteriorward of the largest sandy limestone of the Khandin Formation muscle scars. on the Kirenga riverbank near the mouth of Ayaktcha stream in the same area of southern European Species Siberia. Based on the co-occurrence of the tri- lobite Delgeuloma, characteristic for the Mansy Such disposition of muscle attachments char- regional stage, the age of the strata is believed acterizes also Pygmaeoconus porrectus (Bar- to be Late Cambrian (Rosov, 1968). Specimens rande, in Perner, 1903) from the Llanvirn Šárka frequently preserve the shell, with concentric Formation of Bohemia (Horný, 2006). The shell growth increments visible on the surface and, of P. porrectus was thick, highly variable in its when the external shell layer is exfoliated, the geometry and with irregular growth. Interest- surface is shiny and smooth (Rosov, 1968: ingly, the shell of this animal was precisely fit 1429). This description suggests that the shell to the complex geometry of the substratum. is of lamellar structure, presumably originally This clearly resulted from its sessile growth calcitic, as in Angarella. mode. The ventral valve was apparently thin Doguzhaeva (1972) named four species or even non-mineralized. A similar fossil, 11 attributed to Kirengella and a new genus, Ro- mm long, still attached to the shell of a Mid maniella, from the Tremadoc (thus coeval to the Ordovician orthoconic nautiloid, was identified southern Siberian Ust-Kut Formation) strata of as Archinacella by Rudolph (1997: 33, fig. 11). the southern Urals, on the bank of the Ebit River The shell aperture in those specimens appar- near the mouth of Kultabasay Creek based on ently replicate the substratum morphology, and nuclei preserved in sandstone. Muscle scar this may explain unusual geometry of the shell ANCESTRY OF CEPHALOPODS 105

FIG. 5. Stratigraphic distribution of members of the Kirengellidae with known muscle attachments in the dorsal valve; diagrammatic representation of Craniops inserted. Restorations of muscle attachments distribution are based on literature sources reviewed in the text. in the Uralian Romaniella aebitensis, to which patellacean gastropod Floripatella rousseaui aspect of morphology Doguzhaeva (1981) Yochelson, 1988, from the Whiterockian paid much attention. She interpreted the pres- Kanosh Shale of Utah may be even more ence of a sinus at the adapical shell margin as derived. Numerous muscle attachments form evidence for a shell curvature opposite to that there an almost continuous horseshoe belt in Kirengella. (Yochelson, 1988). Serial muscle scars associated with conical North American Midcontinent Species shell shape have also been identified by Stinch- comb (1980) in another North American form, Pygmaeoconus is geologically younger than Hypseloconus cf. H. stabilis Berkey, 1898, from Romaniella and Moyerocania, and the anterior the Late Cambrian Davis Formation of Mis- disposition of its admedial attachment scars souri. Although rather superficially described, may be a derived character. If so, the alleged the disposition of muscle attachments in this 106 DZIK form seems similar to that in the type species subjective synonyms. The same refers to the of Kirengella, and because of this Webers et al. orders Kirengellida Rosov, 1975, Romaniellida (1991) and Stinchcomb & Angeli (2002) placed Doguzhaeva, 1981, and Hypseloconida Peel, Hypseloconus stabilis Berkey, 1898, and other 1991. Ironically, the arrangement of muscle at- American species earlier classified in this ge- tachments in the Kirengellida is rather different nus in Kirengella, namely H. washingtonensis from that in unquestionable Monoplacophora, Stinchcomb, 1975 (nom. corr. herein; Hypselo- which appear to be unknown before the Late conus is of masculine gender, not neuter), and Ordovician. Instead, there are some Cambrian H. simplex Berkey, 1898. Stinchcomb & Angeli fossils exhibiting much similarity to the kirengel- (2002) also added to Kirengella Hypseloconus lids in this respect. compressus Ulrich & Bridge, 1931, H. ozarken- sis Ulrich & Bridge, 1931, Gasconadaeconus expansus Stinchcomb, 1986, and Kirengella Cambrian Problematica Possibly oregonensis Stinchcomb & Angeli, 2002. The Related to Angarella family Kirengellidae was supplemented with his new genus and species Irondalia irondalensis. In the Early Cambrian several species of These all are forms of generalized morphology phosphatic shells with muscle attachment scars with unknown musculature, and their con- of disposition similar to that in Angarella occur generity with Kirengella does not seem well (Fig. 6). They are referred to as mobergellans substantiated. (e.g., Conway Morris & Chapman, 1997; Sk- If these nomenclatorial transfers were ac- ovsted, 2003) with the family name Mobergel- cepted, the muscle scars of true Hypseloconus lidae Missarzhevsky, 1976, available. Although (its type species is H. recurvus Berkey, 1898), originally described as brachiopods, they are as well as the original shell mineralogy, would known exclusively from their “dorsal” valves remain unknown. However, shells of all the that make such affinity difficult to support. Their American species differ from the Siberian type brachiopod affinities are generally also consid- species of Kirengella in being much taller, hav- ered unlikely because in some species shells ing an ovate cross-section and an anteriorly are concave, which makes them similar rather displaced apex. In stratigraphically higher Emi- to opercula of tubular skeletons (Bengtson, nence and Gasconade Formations of Trempea- 1968). No such tubes fitting mobergellid and co- leauan and earliest Ordovician age in Missouri occurring with them have been identified yet. more elongate and septate Shelbyoceras oc- However, some mobergellans have high curs. Muscle scars are unknown in it, but gen- vaulted shells (e.g., Aktugaia and Thorslun- eral shell form makes the ancestor-descendant della) and with irregular outline and growth relationship between Hypseloconus and Shel- bands distribution making them similar to the byoceras likely. The North American lineage kirengellids such as Pygmaeoconus. The lack is likely to be separate of the Siberian one. A of ventral valves in fossil assemblages could wide range of hypseloconid shell morphologies have resulted from their being unmineralized is known in the North American Midcontinent or cemented to hard bottom environments not province (Greenland) as early as in the late Mid represented in the fossil record. A taphonomic Cambrian (Peel, 1988). analogue may be offered by the Ordovician The only other Cambrian fossil shell with pre- acrotretid Eoconulus, which cemented to the served serial muscle attachments is that identi- substratum by its ventral valve. Although its fied as Scenella sp. undet. by Rasetti (1954; phosphatic dorsal valves are common fossils, Peel, 1991: 56). The almost centrally located known after incomparably more numerous shell apex and the arrangement of muscles specimens than mobergellas, it took some make it more similar to the kirengellids than to time until the rare ventral valve was identified the tergomyan monoplacophorans. In the latter, (Holmer, 1989: 147). One may speculate that the “muscle scars form a ring located posterior the ventral cementing valve of mobergellans to the shell apex, such that the apex lies outside was flat or reduced in a manner suggested here of the muscle field” (Peel, 1991: 14). for the kirengellid Pygmaeoconus. The unavoidable consequence of the above Despite the similarity in overall form, the review is that the family-rank taxa Hypseloconi- phosphatic composition of the mobergel- dae Knight, 1956, Kirengellidae Starobogatov, lans makes them basically different from the 1970, Romaniellidae Rosov, 1975, Hypseloco- kirengellids. The difference is original (Nyers, nellacea (recte Hypseloconacea) Stinchcomb, 1987; Skovsted, 2003), because at least in 1986, and Pygmaeoconinae Horný, 2006, are Aktugaia triangula Missarzhevsky, 1976, from ANCESTRY OF CEPHALOPODS 107

FIG. 6. Comparison of muscle attachments in the dorsal valve of Angarella with those of various members of the Mobergellidae. Restorations are based on literature sources reviewed in the text. the Geres Member of the Shabakhty Forma- Brachiopod Affinities of Angarella tion in Malyi Karatau Range, Kazakstan, the shell differs from secondarily phosphatized co- Originally Angarella was attributed to the occurring mollusk shells in its shiny appearance Craniidae. This affinity was questioned by (Skovsted, 2003). Yadrenkina (1984), because, unlike recent Genera of the mobergellans differ from each craniids, the four pairs of large muscle attach- other in number of muscle attachments (Sk- ment scars she identified inside the dorsal ovsted, 2003). The pattern of attachments, with valve of Angarella leave only one third of the a circle of oval attachments surrounding apex, internal shell volume for the mantle cavity. She makes them remotely similar to the “cyclomyan” noticed that there was virtually no space for the bellerophontids, and such affinity has been lophophore and that the virtually flat ventral proposed for them by Nyers (1987). However, valve did not protect the animal against inges- in their roughly similar size decreasing poste- tion of the mud from the sea bottom. Therefore, riorly (brachiopod orientation applied), and the she proposed that Angarella represents a new presence of a pair of distinctly smaller scars at unknown group of animals with bivalve shells the anterior end, they more closely resemble of the brachiopod ground plan. the pattern represented in the kirengellids. The small anterior attachments pair are in the ring, Lophophore of Heliomedusa thus in position like Hypseloconus rather than Kirengella. There is a long time span separating The problem of the space for the lophophore the oldest kirengellids from the mobergellans, in the shell of Angarella, raised by Yadren- which makes a relationship between them only kina (1984), has been indirectly solved by a highly hypothetical possibility. However, the Heliomedusa, an Early Cambrian mickwitziid mobergellans remain the only fossil organ- (Holmer & Popov, 2007) with a thin organic or isms preceding the kirengellids with a similar organophosphatic shell. This brachiopod had body plan. a surprisingly simple lophophore, with a row of 108 DZIK radially oriented tentacles forming a relatively in the advanced Recent Notosaria (Williams et narrow belt along the anterior part of the shell. al., 1997: fig. 11.1). Its extent was virtually the same as, or at least The main and possibly fundamental differ- closely similar to, the area outside the muscle ence between the Craniopsida Gorjansky & scar belt in Angarella (Chen et al., 2007: fig. 3: Popov, 1985, and the Kirengellida is in the dis- 4). A large visceral mass may be plesiomorphic tribution of shell muscles. In Angarella, instead for brachiopods, as suggested by its entering of three pairs of lateral muscle scars recognized the base of the pedicle in some Early Cambrian in Craniops (Popov & Holmer, 2000), there are lingulids (Zhang et al., 2005, 2007; Holmer & four. Instead of the single posterior attachment Caron, 2006). Heliomedusa, as restored by there is a posterior belt. Presence of paired Chen et al. (2007), had a simple organization posterior and anterior adductors is considered of adductors, with a pair of large anterior and plesiomorphic to all brachiopods by Holmer et another pair of small posterior muscles sur- al. (1995) and rather conservative in their evo- rounding the large visceral mass with pedicle lution. If the kirengellids are truly brachiopods, in the center. Although Heliomedusa is not a a secondary complication of their musculature craniopsid, its ancestral position with respect took place in their early evolution, perhaps in to craniopsids and perhaps also kirengellids connection with cementation to the substratum seems likely (at least it is a more parsimonious and under selection pressure, of a kind similar solution than assuming completely indepen- to that acting on limpet-like molluscs. dent origin of calcareous and organophosphatic skeletons). Transition from a shortened pedicle Ancestry of Kirengellids to shell cementation may have occurred in a way analogous to the shift from a massive To resolve the question of the origin of the shortened byssus to cementation in the anomiid discrepancy in musculature, the evolution of the bivalves (Fürsich & Palmer, 1982). kirengellids and craniopsids has to be traced backward up to the identification of their puta- Kirengellid and Craniopsid Shells tive common ancestor. This does not seem possible at the moment. Apart from the disposition of the dorsal valve Probably the geologically oldest craniopsid muscle attachments, the Angarella shell is with well-known musculature is Pseudop- similar to that of the craniopsid inarticulate bra- holidops stolleyana (Huene, 1900) from the chiopods. In particular, the general appearance Caradoc (Baltic Nabala-Vormsi stages) of of shells of the Devonian Lingulapholis, free Belorussia (Popov & Pushkin, 1986). It is lying on the sediment surface in its postlarval still much younger geologically than the last stage, is like that of Angarella. The Mid to Late kirengellids. Juvenile shells with a shape Ordovician (Caradoc-Ashgill) Pseudopholidops similar to those of Angarella from the latest differs from Lingulapholis only in the distribution Early Ordovician (Kunda-Aseri boundary) were of muscles, whereas Craniops (Caradoc-Tour- tentatively attributed to the craniids (Madison, naisian) was attached apically by cementation 2007). The craniids, which are more derived (Popov & Holmer, 2000). than craniopsids, are known already from the The calcitic secondary shell of Craniops has Tremadoc (Mergl, 2002; Sdzuy et al., 2001), a laminar structure (Williams, 1997: fig. 249.1), which implies a “ghost” lineage of the crani- thus similar to that of Angarella. The Angarella opsids exists before this date. Only putative shell matrix shows increments similar to those craniopsid relatives, with strongly convex larval in the Ordovician Petrocrania (Cusack & Wil- shell, are known from the Early Cambrian Up- liams, 2007: fig. 1648a) or RecentNovocrania per Bastion Formation of Greenland (Skovsted (Cusack & Williams, 2007: fig. 1644). It is possi- & Holmer, 2005). Other Cambrian brachiopods ble that the shell of Angarella underwent some with calcareous shells (Balthasar, 2008) are diagenetic changes of a similar kind as those rather unlikely to be related to the craniopsids in fossil craniids, but its originaly calcitic and or kirengellids. laminar structure seems to be not in doubt, be- A possibility remains that the mobergellans ing visible in thin sections, on etched surfaces are near the ancestry of both the craniopsids and in the pattern of exfoliation. Under SEM, and kirengellids. The Mid Cambrian Scenella the shell matrix of Angarella (Fig. 3E) shows of Rasetti (1954) may represent the connecting growth banding resembling that in the primary link, although it is too incompletely known for calcitic shell layer of brachiopods, for instance this purpose. The mobergellan and kirengellid ANCESTRY OF CEPHALOPODS 109 anterior admedial pair of small muscles would by muscular suckers in more advanced forms represents the brachial protractor (Popov & (von Byern & Klepal, 2006). Such interpreta- Holmer, 2000), homology of other muscles tions of the evolution of soft parts are difficult remaining obscure (Fig. 6). to test with the paleontological evidence, but The difference in shell composition may not the modifications of shell in the evolution of be of great importance, as in the early bra- cephalopods have a rather good fossil record. chiopods organic, phosphatic, aragonitic and An exception is the origin of the phragmocone, calcitic shells are known, and the gap between which probably took place not later than in the lineages characterized by different skeletal min- early Late Cambrian. In this respect we have eralogy does not need to be wide (Balthasar, to rely only on speculations. 2008). This is suggested also by the apparent Peel (1991) reaffirmed Yochelson et al.’s craniopsid affinities of the trimerellid brachio- (1973) hypothesis that cephalopods originated pods with presumably originally aragonitic shell from a benthic monoplacophoran similar to (Gorjansky & Popov, 1985, 1986). Probably Hypseloconus. Based on endogastric curvature, also shells of some of the kirengellids were ara- he proposed aenigmatic helcionellid molluscs as gonitic. This could have been the case with un- possible ancestors to the cephalopods, the idea named forms I collected from the upper part of supported by molecular phylogenetic proximity Ust-Kut Formation at Angara near Kudinsk and of scaphopods (probable distant successors the Krivaya Luka Formation at the Lena River. of the helcionellids) to cephalopods (Steiner In shell shape they are similar to Kirengella & Dreyer, 2003). The main argument used by or even taller, resembling the North American Yochelson et al. (1973) and Webers & Yochelson Hypseloconus. They frequently show their shell (1989) was the presence of diaphragms (septa) matrix recrystallised into calcitic sparite, as if in the apex. Dzik (1981), Chen & Teichert (1983), originally aragonitic. Obviously, a possibility and Teichert (1988) pointed out that it is the remains that the aragonitic Kirengella-like Or- siphuncle, not septa, that makes a cephalopod dovician fossils are truly monoplacophorans. and that septa occur widely in high-conical Nevertheless, additional evidence for this has shells of unrelated invertebrates. Cephalopods to be provided, as the serial distribution of from the beginning of their palaeontologically muscle attachment is misleading. documented history differ from the monopla- Whether the bivalved kirengellids are brachio- cophorans in the shape of their larval shells. pods or not, they have to be removed from con- In their subspherical apex and cylindrical ap- siderations on the early evolution of mollusks, pearance, they resemble rather larval shells of in which these fossils were assumed to play the gastropods. A flat discoidal larval shell is known main role. It appears that the pre-Late Ordovi- to occur in the tryblidiid monoplacophorans at cian history of the tergomyan Monoplacophora least since the Silurian (Dzik, 1994: 294, fig. has to be rewritten. More specifically, the bra- 30A). Possibly they had such shells already in chiopod affinity of the celebrated high-conical the Early Cambrian, although mollusks of that Hypseloconus calls for reconsideration of the age with low cap-like larval shells have not been theory about the ancestry of Cephalopoda. shown to have serial pedal musculature (Dzik, 1994: 265, fig. 12A). Such shell morphology of the Cambrian limpet-like mollusks means that Origins of the Cephalopods the early development within the egg capsule without free-living larva characterized tryblidiid It is generally accepted that the body plans monoplacophorans for all their evolution. Thus, of cephalopods, monoplacophorans, and basal according to the Yochelson et al. (1973) theory, gastropods are closely similar. There is a signif- the pelagic mode of life was a complete novelty icant progress in understanding the origin and in the lineage of benthic monoplacophorans early evolution of the nominal cephalopod or- adapted to life among stromatolites. Such was gans: head and arms. Embryological evidence the ecology of Hypseloconus and related forms is now available that the unique digital tentacles (Stinchcomb, 1975) interpreted as molluscs. of Nautilus or arms of other cephalopods de- On the contrary, I proposed (Dzik, 1981) veloped from the foot of an ancestral mollusk that the origin of the phragmocone in the (Shigeno et al., 2008). Comparative anatomical early evolution of cephalopods was a result data suggest that the adhesive function of car- of extension of the pelagic mode of larval bohydrates and proteins secreted on tentacles life of their ancestors to mature stages, in a of Nautilus was probably gradually taken over way similar to pteropods. I suggested that the 110 DZIK

FIG. 7A-C. Possible relative of earliest cephalopods, Turcutheca crassaecochlia Syssoiev, 1962, from the Tommotian (Dokidocyathus lenaicus Zone) at Bydyangaia on the Lena River, central Yakutia; phos- phatized specimens photographed under SEM. A: Specimen with well preserved surface of embryonic part (destroyed while removed from the stub); B, C: Probably mature specimen (largest known) with partially phosphatized shell wall and displaced embryonic part preserved as glauconitic internal mold, lateral and posterior (‘ventral’) views; D-F: Pieces of phosphatized siphuncles (mineralized fibres of the connecting ring enlarged in F) of ellesmeroceratid nautiloids from the earliest Ordovician (early Tremadoc) Ust-Kut Formation at Pashino, Angara River, southern Yakutia; note that inferred larval conch size was similar to that of Turkutheca but mature specimens were many times larger. positive buoyancy by replacing liquid with gas & Yochelson, 1989: fig. 3), which suggests in the phragmocone chambers (Greenwald the presence of a minute pelagic larva. From et al., 1982, 1984; Greenwald & Ward, 1987) the earliest Ordovician Ust-Kut Formation at was preceded by a plain removal of sodium Pashino fragmentary phosphatized conchs of ions from the liquid inside the conch tip as a ellesmeroceratid nautiloids were recovered result of the siphonal epithelium sodium pump with siphuncle diameter 160 μm; the conch action, in a way analogous to that of the Re- apex diameter was thus less than 0.8 mm. cent cranchiid squids. This requires that the The size of the embryonic conch was not much cephalopod ancestor already had a pelagic larger than in other early mollusks. Larvae with veliger-like larva with subspherical embryonic elongate cylindrical conches were also typical and cylindrical larval conchs. Such conchs for the Cambrian bellerophontids, gastropods, characterized the orthoceratid and presum- and hyoliths (Dzik, 1994). ably also ellesmeroceratid cephalopods (Dzik, Possible ancestors of the lineage leading 1981); note that the smallest piece of the oldest to the cephalopods are known from the Early known Late Cambrian cephalopod Plectronoc- Cambrian. Turcutheca is an Early Cambrian eras was less than 1 mm in diameter (Webers mollusk with the general conch form closely ANCESTRY OF CEPHALOPODS 111 resembling that of the earliest ellesmerocer- brachiopod Heliomedusa orienta Sun and Hou, atid cephalopods: endogastrically curved and 1987a as a discinid. Journal of Paleontology, laterally compressed, with relatively large 81: 38–47. CONWAY MORRIS, S. & A. J. CHAPMAN, 1997, subspherical embryonic part (Fig. 7). As usual Lower Cambrian halkieriids and other coelosc- for any high-conical shells, diaphragms may oc- leritomorphs from Aksu-Wushi, Xinjiang, China. cur in the apex of Turcutheca or related forms, Journal of Paleontology, 71: 6–22. but no sign of anything similar to a siphuncle CUSACK, M. & A. WILLIAMS, 2007, Biochemistry is known. Despite its unusual conch form Tur- & diversity of brachiopod shells. Pp. 2373–2395, cutheca seems related to hyoliths (they have in: P. A. Se l d e n , ed., Treatise on Invertebrate Paleontology, Part H, Brachiopoda, Vol. 6. Geo- a depressed and straight or exogastric conch). logical Society of America, Boulder, Colorado. Perhaps it had a morphologically simple calci- DIVINA, T. A., A. V. KANYGIN, V. G. MATUKHINA, fied operculum, like its relativeL adatheca (Dzik, T. A. MOSKALENKO & A. G. YADRENKINA, 1994: 268). If it is truly related to cephalopods, 1984, Opisanie razrezov. Trudy Instituta Ge- not only did the siphuncle have to develop in ologii i Geofiziki SO AN SSSR, 529: 5–46. DOGUZHAEVA, L. A., 1972, Novyie tremadoks- the course of evolution to the Late Cambrian kiye monoplakofory Yuzhnogo Urala. Paleonto- cephalopods but also the size of the conch, logicheskiy Zhurnal, 1972(1): 24–28. both mature and embryonic, had to increase. DOGUZHAEVA, L. A., 1981, Sinus kolpachkovid- Together with reduction of the foot, the oper- nykh monoplakofor. Doklady Akademii Nauk culum was probably transformed into the lower SSSR, 258: 209–211. jaw (Dzik, 1981; the idea presented also without DZIK, J., 1981, Origin of the Cephalopoda. Acta Palaeontologica Polonica, 26: 161–191. attribution to its source in Seilacher, 1993). DZIK, J., 1994, Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic. Acta Pa- laeontologica Polonica, 39: 247–313. ACKNOWLEDGEMENTS FÜRSICH, F. T. & T. J. PALMER, 1982, The first true anomiid bivalve? Palaeontology, 25: 897–903. I am grateful to Aleksandr Kanygin (Institute GOODWIN, D. H., K. W. FLESSA, B. R. SCHÖNE & D. L. DETTMAN, 2001, Cross-calibration of of Petroleum Geology and Geophysics, Novosi- daily growth increments, stable isotope variation, birsk) and Andrey Dronov (St. Petersburg State and temperature in the Gulf of California bivalve University) for introducing me into the Irkutsk mollusk Chione cortezi: implications for paleoen- Amphitheatre geology. My participation in the vironmental analysis. Palaios, 16: 387–398. expedition to the Cambrian exposures near GORJANSKY, W. J. & L. Y. POPOV, 1985, Morfologia, sistematicheskoye polozhenie i Yakutsk in 2006 (led by V. M. Sundukov) and to proiskhozhdenie bezzamkovykh brachiopod the Angara and upper Lena Rivers in 2007 (led s karbonatnoy rakovinoy. Paleontologicheskiy by Taras Gonta) was possible owing to the ex- Zhurnal, 1985 (3): 3–19. change program between the Polish Academy GORJANSKY, W. J. & L. Y. 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