geosciences

Article The Mode of Life in the Genus Pholadomya as Inferred from the Fossil Record

Przemysław Sztajner Institute of Marine and Environmental Sciences, University of Szczecin, Mickiewicza 16A, 70-383 Szczecin, Poland; [email protected]



Received: 13 July 2020; Accepted: 21 September 2020; Published: 5 October 2020


Abstract: The paper is an attempt to reconstruct the mode of life of Pholadomya bivalves, very common in the fossil record, particularly that of the Jurassic. The only extant representative of the genus is extremely rare and very poorly known. Materials from the Polish Jurassic deposits (Bajocian–Kimmeridgian; Western Pomerania and Polish Jura) and literature data were used for the reconstruction. Specifically, observations on the anatomy, taphonomy, and diagenesis of the specimens examined as well as lithology of the deposits housing the specimens were used. Shell anatomy characteristics are known for their particular utility in mode of life reconstructions, although the extremely thin-shelled and coarsely sculpted bivalves, such as the Pholadomya examined, have not been studied so far. The reconstruction suggest a diversity of the mode of life, coincident with the morphological differences between the Pholadomya species. At least the adults of anteriorly flattened species are inferred to have lived extremely deeply buried in the sediment, and were hardly mobile. The smaller, more oval in shape, species were more mobile, and some of them are thought to have preferred life in shelters, should those be available. In addition, the function of the cruciform muscle, other than that considered so far, is suggested.

Keywords: Pholadomya; Anomalodesmata; Bivalvia; life habit; deep burrowers; taphonomy; shell anatomy; cruciform muscle; Jurassic; Poland

1. Introduction The recent bivalve shell morphology is known to depend on the mode of life [1,2]. Assuming this was also the case in the distant past, similarity of certain fossil bivalves to the extant ones allows them to be used, with a high degree of probability, in palaeoecological reconstructions [3–5]. The same assumption makes it possible to reconstruct the autecology of bivalves lacking ecological equivalents in present-day environments, such as Rudista [1] or Opisoma [6]. Representatives of the bivalve genus Pholadomya Sowerby, 1823, are common fossils, present in rocks since the Uppermost Triassic. Throughout the most of Mesozoic, they were morphologically very diverse [7]. Ph. candida Sowerby, 1823, the only extant species, is extremely rare and, until recently, had been suspected of being extinct [8]. The Pholadomya (including the subgenera Procardia Meek, 1871, and Bucardiomya Rollier, 1912, occasionally treated as separate genera, e.g., by Runnegar [9] and Richter [10], respectively) feature more or less elongated shells (most probably all the specimens reported as having shells with length smaller than height are deformed) which are oval, scaphoid, ovoid or sub-triangular, with strongly convex and rounded to completely flat anterior parts. The valves are usually extremely (or at least relatively) thin. No Ph. candida valve thickness measurements have been reported in the literature, and the materials at hand are not amenable to being measured accurately, and only approximations are possible. The better preserved specimens of Ph. lirata feature valves 75–100 mm long and usually 0.2–0.4 mm thick (0.5–0.8 mm thick around the

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Geosciences 2020, 10, x FOR PEER REVIEW 2 of 23 pallialthe pallial sinus). sinus). At the At volume the volume of 80–180 of 80–180 mL, mL, the the valve valve thickness thickness index index (TI; (TI; [2 [2])]) is is then then about about 0.05–0.1. 0.05– The0.1. paper-thin The paper-thin valves valves of Ph. of concatenata Ph. concatenataAgassiz, Agassiz, 1842, 1842, usually usually two two times times smaller, smaller, seem seem to to have have still lowerstill TI.lower However, TI. However, valves valves of some of speciessome species (e.g., Ph. (e.g., protei Ph.) protei are thick) are enoughthick enough for their for internaltheir internal moulds tomoulds distinctly to showdistinctly muscular show muscular impressions. impressions. TheThe hinge hinge is is devoid devoid of of functional functional teeth.teeth. The t teeth-likeeeth-like thickenings thickenings on on the the hinge hinge plate plate differ diff erin in shape.shape. They They are are fairly fairly complexcomplex in Ph.Ph. candida candida [11,12],[11,12 compared], compared to the to thefossil fossil specimens specimens examined examined in inthis study study (Figure (Figure 1A,B).1A,B). As As the the bivalve bivalve grows, grows, the the beaks beaks usually usually erode erode each each other. other. The nymphs The nymphs are areshort short and and strong, strong, sometimes sometimes retaining retaining a short, a short, calcified calcified fibrous fibrous layer layerof the ofligament the ligament (Figure (Figure 1C). The1C). Theescutcheon escutcheon is bordered is bordered by ridges by ridges (which (which are occasionally are occasionally sharp, sharp, Figure Figure 1F) or1 F)does or not does show not showany anyborders borders (Figure (Figure 1D,H).1D,H). The The posterodorsal posterodorsal edges edges of ofthe the valves valves may may adhere adhere to toeach each other other along along a a considerableconsiderable length length (Figure (Figure1F) 1F) or or gape gape right right past past thethe beaksbeaks to a constant angle angle along along the the entire entire length length (Figure(Figure1D) 1D) or becomeor become parallel parallel (Figure (Figure1H). 1H). The The shell sh showsell shows at least at least the posteriorthe posterior gape, gape, associated associated with thewith presence the presence of a non-retractable of a non-retractable siphon. siphon. However, Howeve ther, the original original gape gape size size is is di difficultfficult to to measuremeasure in fossilin fossil specimens specimens as the as shellsthe shells are usuallyare usually pressed pressed against against each each other other by theby the sediment, sediment, more more than than they they would be in a living bivalve. The lunule is usually absent, but may be quite large (as in some would be in a living bivalve. The lunule is usually absent, but may be quite large (as in some Procardia). Procardia).

FigureFigure 1. 1.Hinges Hinges ofof PholadomyaPholadomya.(. (AA,B,B),), A A silicone silicone cast cast of of the the right right hinge hinge of of PholadomyaPholadomya protei protei (Dziwn(Dziwnówek,ówek, specimen specimen MGUS.Sz.3464) MGUS.Sz.3464) in in dorsal dorsal and and left left view. view. (C ),(C A), calcifiedA calcified fibrous fibrous layer layer of Ph.of Ph. lirata (Cz˛estochowa,MGUS.Sz.3916)lirata (Częstochowa, MGUS.Sz.3916) ligament. ligament. (D–I), Dorsal (D–I), and Dorsal ventral and margins ventral of marginsPh. lirata of(Cz˛estochowa, Ph. lirata (D(Cz)—MGUS.Sz.3733ęstochowa, (D)—MGUS.Sz.3733 and (E)—MGUS.Sz.3515), and (E)—MGUS.Sz.3515),Ph. concatenata Ph.((F, Gconcatenata)—MGUS.Sz.3917) ((F,G)—MGUS.Sz.3917) and Ph. candida ((Hand,I)—from Ph. candida Runnegar ((H,I)—from 1972, Pl.Runnegar 1, Figure 1972, e,f, Pl. modified) 1, Figure e,f., shells. modified) Scale barshells. 1 cm Scale (as bar in other1 cm (as figures, in other figures, unless otherwise stated). unless otherwise stated).

The valve ornamentation always consists of exclusively radial ribs: a few ribs only or rib traces in some cases are visible, but as many as about 40 were observed in other specimens. The ribs are

Geosciences 2020, 10, 397 3 of 23 differently shaped (with sharp or rounded ridges and symmetrical or tilted profile) folds of the shell wall (except for capillary ribs, which look as if they have been glued to the valve surface, whereby the valve thickness increases by the rib height; see below) The number of ribs is usually constant, although some additional ribs may occasionally emerge during growth. They sometimes cover the entire valve surface, but in many specimens a part of the valve may be devoid of ribs, which then usually occur in the central part of the valve. The ribs often cross the concentric folds to form a structure resembling an egg carton. The microornament occurs in the form of spikes, typical of anomalodesmatans, prefabricated in a thick, somewhat translucent periostracum, which is mineralised as well [13]. In Pholadomya, the spikes can be sharp or blunt, and are arranged chaotically or in radial rows (Figure2A–D); they are present all over the valve surface or disappear as the individual grows. However, even in the spike-less surfaces of valves (and even especially on them), a relatively thick, mineralised periostracum is visible as drapery-like folds, not aligned with growth lines, frequently on relief irregularities (Figure2B,E,F). I suspect that the origin of the capillary ribs (Figure2F,G) can be similar to that of the spikes; the ribs are rounded in cross-section, are convex only on the outer valve surface, and form crests of regular ribs or replace them. The anatomy of the Pholadomya soft parts was described in detail by Morton [11]; my fossil material does not contribute any new observations, except that I did not found the cruciform muscle impression on any individual. On the other hand, some specimens showed relief of the inner side of the valve not to be related to known soft parts. Most often, the relief consisted of delicate radial strips parallel to the ribs (Figure3A,B), or of fine concentric folds within the pallial line ([ 14,15], Figure3C), which may be additionally covered by finer wrinkles (visible on some of Arkell’s [15] and my specimens; cf. Figure3D). Terquem and Jourdy [ 16] showed identical folds to cover the entire internal surface. The nature of this relief is unknown. As it occurs in some specimens only, it could have been a pathology, but the relief is too regular to be an expression of a pathological condition. The Pholadomya mode of life has been inferred from the anatomy and the type of sediment the specimens were retrieved from. Having dissected a specimen found in the 19th century, Morton [11] concluded that the bivalve was a deep burrower (in Morton’s figure, the siphon is somewhat shorter than the shell) and a deposit feeder; the life position was oblique, dorsal side down. However, the orientation and deposit feeding of Pholadomya has been questioned [17]. It has been assumed instead (e.g., [17,18]) that the bivalve’s life position was vertical to somewhat ventrally tilted, and that the bivalves were suspension feeders. The burrowing depth shown in reconstruction drawings is equal to, or somewhat shorter than, the shell length [3,4,17]. The genus is known to be highly diverse morphologically; the high morphological diversity suggests substantial between-species differences in the mode of life. This work is aimed at reconstructing details of the mode of life of different Pholadomya species based on, firstly, morphological and taphonomic differences between them, and secondly, comparison with taphonomy of other bivalves found together with Pholadomya. As the mode of life of the extant Ph. candida (or of any other bivalve with extremely thin and highly sculpted shells) has not been sufficiently well investigated, such reconstructions should be made with caution; whenever possible, each aspect should be supported by more than one line of evidence. The effort, however, is worthwhile, as it may provide insights into the life of one of the most characteristic members of the benthos of Jurassic seas, and supply additional information useful in palaeoecological reconstructions. In addition, the knowledge of the burrowing depth might help in avoiding erroneous assignment of specimens on hand to older communities, preserved in deeper layers into which the bivalves penetrated. Geosciences 2020, 10, 397 4 of 23

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FigureFigure 2. 2.Microornaments. Microornaments. ((AA),), Regular,Regular, radialradial rows of spikes. PholadomyaPholadomya fidicula fidicula, Upper, Upper Bajocian Bajocian Cz˛estochowa(specimenCzęstochowa (specimen MGUS.Sz.3880). MGUS.Sz.3880). ( B),), Spikes in in radial, radial, irregu irregularlylarly spaced spaced rows. rows. Draperies Draperies visiblevisible in thein lowerthe lower part of part the photograph.of the photograph.Ph. lirata, MiddlePh. lirata Bathonian,, Middle Cz˛estochowa(MGUS.Sz.3733). Bathonian, Częstochowa (C),(MGUS.Sz.3733). Spikes evenly and(C), denselySpikes evenly spaced, and but densely not arranged spaced, inbut rows. not arrangedPh. latirostris in rows., Middle Ph. latirostris Bathonian,, OgrodzieniecMiddle Bathonian, (MGUS.Sz.3560). Ogrodzieniec (D (MGUS.Sz.3560).), Spikes sparse and(D), Spikes chaotically sparse distributed. and chaoticallyPh. latirostrisdistributed., Middle Ph. Bathonian,latirostris, Cz˛estochowa(MGUS.Sz.3757).Middle Bathonian, Częstochowa (MGUS.Sz.3757). (E), Draperies on (E), a Draperies rib. (Same on as a rib. in ( B(Same)). (F ),as Draperies in (B)). around(F), Draperies a capillary around rib. Ph. a capillary concatenata rib. ,Ph. Lower concatenata Bathonian,, Lower Faustianka Bathonian, (MGUS.Sz.3558). Faustianka (MGUS.Sz.3558). (G), Capillary ribs.(G),Ph. Capillary latirostris ribs., Lower Ph. latirostris Bathonian,, Lower Korwin Bathonian,ów (MGUS.Sz.3571). Korwinów (MGUS.Sz.3571). Scale bar 1 mm. Scale bar 1 mm.

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Figure 3. Internal relief. (A,B), Delicate, short radial ridges. (A), Pholadomya protei, Callovian, Bolęcin Figure 3. Internal relief. (A,B), Delicate, short radial ridges. (A), Pholadomya protei, Callovian, (MGUS.Sz.3751), (B), Ph. latirostris, Lower Bathonian, Korwinów (specimen MGUS.Sz.3904). (C), Bol˛ecin(MGUS.Sz.3751), (B), Ph. latirostris, Lower Bathonian, Korwinów (specimen MGUS.Sz.3904). Concentric ridges. Ph. bucardium, Callovian, Bolęcin (MGUS.Sz.3920). (D), Wrinkled concentric ridges. (C), Concentric ridges. Ph. bucardium, Callovian, Bol˛ecin(MGUS.Sz.3920). (D), Wrinkled concentric Ph. bucardium, Callovian, Bolęcin (MGUS.Sz.3755). ridges. Ph. bucardium, Callovian, Bol˛ecin(MGUS.Sz.3755).

2. Materials2. Materials and and Methods Methods

2.1.2.1. Materials Materials Examined Examined I haveI have examined examined more more than than 1200 1200 specimens specimens from from numerous numerous sites sites in in PolandPoland (Figure(Figure4 4),), depositeddeposited in severalin several collections. collections. Literature Literature data data and and unpublished unpublished field field observations observations were were taken taken into into account account as as well. well.Most of the specimens (about 1000) were found at more than 50 sites in the Polish Jura (the Most of the specimens (about 1000) were found at more than 50 sites in the Polish Jura (the region region of Cracow-Cz˛estochowa),spanning the age range from the Lower Bajocian to the Lower of Cracow-Częstochowa), spanning the age range from the Lower Bajocian to the Lower Kimmeridgian. The specimens were found in siderites and claystone (Bajocian and Bathonian), Kimmeridgian. The specimens were found in siderites and claystone (Bajocian and Bathonian), claystone with ooids (Bajocian), mudstones, sandy mudstones (Bathonian), sandy and oolite limestones claystone with ooids (Bajocian), mudstones, sandy mudstones (Bathonian), sandy and oolite (Upper Bathonian/Lower Callovian), Callovian sandstones, marls, Lower Oxfordian sponge limestones, limestones (Upper Bathonian/Lower Callovian), Callovian sandstones, marls, Lower Oxfordian platysponge limestones limestones, (Middle platy Oxfordian), limestones chalky(Middle limestones Oxfordian), and chalky coral li reefsmestones (Upper and Oxfordian coral reefs and (Upper Lower Kimmeridgian).Oxfordian and MostLower specimens Kimmeridgian). were preservedMost specimens in life were position. preserved in life position. SeveralSeveral tens tens of of the the specimens specimens werewere collectedcollected from Czarnog Czarnogłowyłowy [19] [19 and] and Dziwnówek Dziwnówek in inWestern Western PomeraniaPomerania (NW (NW Poland); Poland); most most were were preservedpreserved in shell debris-full debris-full limestone limestone (Lower (Lower Kimmeridgian), Kimmeridgian), a singlea single specimen specimen being being found found in in glauconite glauconite marlmarl (Upper(Upper Kimmeridgian). AnAn additional additional several several tens tens ofof thethe specimensspecimens were found found in in the the Mesozoic Mesozoic margins margins of ofthe the Holy Holy CrossCross Mountains Mountains in in Poland: Poland: fromfrom BajocianBajocian and Bathonian claystones claystones near near Łę Ł˛eczyca[czyca [20,21],20,21 Lower], Lower KimmeridgianKimmeridgian shell shell debris-filled debris-filled limestoneslimestones near Ma Małogoszczłogoszcz [22], [22], Titonian Titonian chalky chalky limestones limestones near near OwadOwadówów [23 [23],], and and from from some some other other sites. sites.

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FigureFigure 4.4. DistributionDistribution ofof JurassicJurassic depositsdeposits inin Poland,Poland, withwith locationlocation ofof outcropsoutcrops mentionedmentionedin inthe thetext. text.

2.2.2.2. MethodsMethods TheThe specimens specimens were were handled handled mechanically mechanically or treated or treated with chemicals with chemicals (e.g., NaOH). (e.g., Detailed NaOH). Detailedobservations observations were made were using made a using Zeiss a Zeissstereo stereomicroscopemicroscope (Zeiss, (Zeiss, Oberkochen, Oberkochen, Germany). Germany). The Thephotographs photographs were were taken taken with with a Canon a Canon EOS EOS 550D 550D camera camera (Canon, (Canon, Tokyo, Tokyo, Japan). TheThe specimensspecimens photographedphotographed werewere frequentlyfrequently whitenedwhitened withwith ammoniaammonia chloridechloride oror (seldom)(seldom) coveredcovered withwith aa whitewhite acrylicacrylic paintpaint usingusing anan aerograph.aerograph. InIn thethe laboratory,laboratory, thethe specimensspecimens werewere measuredmeasured withwith aa calliperscallipers (linear(linear dimensions) dimensions) and and a a protractor protractor (angular (angular dimensions). dimensions). InIn thethe field,field, thethe sizesize ofof thethe specimensspecimens waswas assessedassessed usingusing availableavailable objectsobjects ofof aa knownknown size. size. MyMy conclusionsconclusions onon thethe modemode ofof lifelifeof ofindividual individual representatives representatives of ofPholadomya Pholadomyaare are based based on: on: • ShellShell anatomyanatomy andand morphology.morphology. RelationshipsRelationships betweenbetween the shellshell formform andand modemode ofof lifelife areare • relativelyrelatively well well known known in numerous in numerous extant extant bivalves bivalves [1,2]. These [1,2]. relationships These relationships are being successfully are being extendedsuccessfully onto extended fossil forms. onto Although fossil forms.Ph. candida Although, the onlyPh. candida recent large,, the deeponly burrowingrecent large, bivalve deep withburrowing a thin andbivalve copiously with a thin ornamented and copiously shell, orname has notnted been shell, so far has observed not been liveso far in observed situ (except live forin situ it having (except been for foundit having and been released; found [ 8and]), analogies released; to[8]), other analogies bivalves to showingother bivalves combinations showing ofcombinations various traits of makevarious it possibletraits make to draw it possible fairly to reliable draw conclusionsfairly reliable on conclusions the mode ofon life theof mode the diofff lifeerent of Pholadomyathe different. Pholadomya. • OntogeneticOntogenetic changes.changes. Although the collectionscollections lacklack veryvery youngyoung individuals,individuals, andand thethe earliestearliest • developmentaldevelopmental stages recorded recorded in in growth growth lines lines are are usually usually lost lost when when the beaks the beaks are eroded are eroded away away[11], [the11], ontogeny, the ontogeny, occasionally occasionally legible legible in inthe the growth growth lines, lines, makes makes it it possible possible to to inferinfer certaincertain morphologicalmorphological changes changes that that ought ought to haveto have been been reflected reflected in diff erencesin differences in the modein the of mode life between of life youngbetween and young old individuals. and old individuals. • LithologyLithology andand taphonomy.taphonomy. TheThe vastvast majoritymajority ofof specimensspecimens (at(at leastleast thosethose amenableamenable toto suchsuch • observations,observations, as as some some individuals individuals had beenhad collectedbeen collected outside outside of a profile of ora noprofile relevant or informationno relevant isinformation available regarding is available specimens regarding found specimens by other found people) by wereother preservedpeople) were in situ, preserved which makesin situ, itwhich possible makes to conclude it possible on to their conclude life position on their and life the position type of and sedimentary the type milieuof sedimentary they inhabited. milieu Inthey addition, inhabited. the shell In crackingaddition, and the deformation shell cracking induced and by non-plasticdeformation compaction induced inby the non-plastic sediment iscompaction indicative in of the diff erencessediment in is the indicative strength of of diff differenceserent parts in the of strength the shell, ofthe different differences parts beingof the relatedshell, the to thedifferences function being of those related parts to in the living function specimens. of those parts in living specimens. • Differences in taphonomy between the various Pholadomya and other deep-burrowing bivalves from the same layers are suggestive of differences in their respective modes of life, e.g.,

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Differences in taphonomy between the various Pholadomya and other deep-burrowing bivalves • from the same layers are suggestive of differences in their respective modes of life, e.g., burrowing depth and the associated probability of exhumation [24]; this might be termed the comparative taphonomy.

As this work does not address taxonomy, I do not revise the nomenclature referred to by other authors. The taxonomy of the Jurassic Pholadomya species will be dealt with in another paper, therefore sub-generic affinities are not touched upon here.

3. Results and Discussion

3.1. Burrowing Depth As the shell of deep-burrowing bivalves shows a posterior gape, deep burrowing of Pholadomya cannot be questioned. However, it is difficult to tell what the actual burrowing depth was. Several years ago, M. Zato´n[25] saw, in the wall of a post-excavation pit of a Cz˛estochowa brickyard, a Pholadomya specimen with the pyritised trace of the siphonal canal. More such specimens were observed there in the 1970s by Franz Th. Fürsich and Ryszard Marcinowski [25]. Unfortunately, these observations were not recorded. Of the four brickyards (“Gnaszyn”, “Kawodrzanka”, “Knopik” and “Anna” [26]) in which silts of the Middle and Upper Bathonian were exposed, only the first is still operating. When I was visiting it lately, only the lowest layers (i.e., Lower Bathonian, then not yet exposed) were freshly exposed. The numerous specimens of Ph. lirata found in the wall did not bear any trace of the siphonal canal. The specimen examined by M. Zato´n,most probably also Ph. lirata, showed an about 20–30 cm long canal, a few cm thick. The sediment was considerably compacted [27]. The Ph. lirata specimens I examined (more than 100), preserved in situ in silts are shortened by about half (see also [28], text-Figure 8D,E). The compaction of the anterior part of the shell provides a better indication of the sediment compaction, as the posterior part (thicker and more linear) resisted the sediment compaction to some extent (although not completely). Moreover, the sediment above the burrowing depth was most likely more hydrated, which resulted in a still more shortened siphonal canal. The sediment could have been also obliterated in its upper part; the Gnaszyn profile reveals very abundant traces of storm-related obliterations [27]. The factors affecting the final results are summarised in Figure5. In any case, based on M. Zato´n’sobservations [25], the absolute minimum burrowing depth can be determined as 50 cm above the upper shell edge (the Ph. lirata specimens found usually retained about 10 cm of the original shell length). This would place Pholadomya among the deepest-burrowing bivalves, such as the extant Panopaea japonica [29]. At that depth, they would be virtually inaccessible to predators, even if the siphon could not be completely retracted. It is very seldom that traces of siphonal canals can be found. In shallow burrowing bivalves, the depth of the siphonal sinus is a reliable indicator of the siphon length and burial depth. Unfortunately, in clams that were unable to completely withdraw the siphon (as can be identified by the permanent posterior opening), the depth of the mantle sinus is no longer such an indicator [29]. Certain conclusions on the relative burrowing depth can be occasionally drawn from taphonomic observations of various taxa co-occurring in the same deposit. At Gnaszyn, despite frequent storm events, specimens of numerous species, even those partly buried (Pinna, Trigonia), have been often found in life position ([28]; own observations); such was the case of all Pholadomya and other Pholadomyoidea (Goniomya spp., Pleuromya alduini). The deposits at Korwinów (Lower Bathonian) are more sandy and contain more shell debris, which indicates a higher energy of the habitat. The taphonomy there is more diverse, suggesting selective removal of specimens burrowing to shallower depths. Table1 shows numbers of specimens of various burrowing species preserved (and usually deformed) in life positions and exhumed. Ph. lirata accounted for the lowest percentage (22%) of the exhumed specimens. The percentage of the other congener, Ph. latirostris, was much higher (58%). The exhumation probability depends on the absolute depth of the shell in the deposit [24], which in turn is related Geosciences 2020, 10, 397 8 of 23 to the specimen size; Figure6 illustrates a pattern of distribution of average-sized individuals of different species, assuming a linear correlation between exhumation probability and burrowing depth. Noteworthy is the total absence of Trigonia sp., which would have been preserved in situ. Compared toGeosciences those of 2020Goniomya, 10, x FOR literata PEER REVIEW, a species most frequently exhumed, their shells are much larger8 of 23 and massive, which in itself would enhance retention in the sediment [2]. For them to be exhumed, the sedimenttheir shells would are much have tolarger be obliterated and massive, to which more thanin itself half would their enhance length, and retent it ision there in the that sediment the origin [2]. of theFor percentage them to be scale exhumed, should the be sediment placed, rather would than have at to the be sediment obliterated surface. to more Although than halfG. their literata length,had a veryand shallow it is there pallial that sinusthe origin (own of observation, the percentage but scale cf. [9 should] for another be placed, species), rather it wasthan removedat the sediment fairly far awaysurface. from Although the posterior G. literata margin had (pallial a very sinusshallow length pallialsensu sinusKondo (own[ observation,29]). Table1 andbut cf. Figure [9] for6 show another also species), it was removed fairly far away from the posterior margin (pallial sinus length sensu Kondo analogous data for Ph. acuminata and Goniomya moeschi from the Mid-Oxfordian platy limestones of [29]). Table 1 and Figure 6 show also analogous data for Ph. acuminata and Goniomya moeschi from the the Polish Jura. Mid-Oxfordian platy limestones of the Polish Jura.

FigureFigure 5. 5.Compaction Compaction of of fossilfossil inin sediment. Factors Factors affe affectingcting the the difference difference in in the the degree degree of ofshortening shortening ofof the the shell shell and and the the channel channel left left byby thethe siphon.siphon.

Table 1. Frequency of exhumed specimens of burrowing species in two specific lithologies (described Tablein the 1. text).Frequency The number of exhumed in parentheses specimens indicate of burrowing the number species of specimens in two specificpreserved lithologies with dorsal (described edge inhorizontal. the text). The Platy number limestones in parentheses are from Rz indicateędkowice the (22 number specimens) of specimens and several preserved other exposures with dorsal with edge horizontal.the same lithology. Platy limestones are from Rz˛edkowice(22 specimens) and several other exposures with the same lithology. Life Position Exhumed % Exhumed Korwinów—mudstones,Life Position Lower Exhumed Bathonian % Exhumed PholadomyaKorwin lirata ów—mudstones,31 Lower Bathonian9 22 Ph. latirostris 5 (3) 7 58 Pholadomya lirata 31 9 22 Goniomya literata 5 (5) 25 83 Ph. latirostris 5 (3) 7 58 GoniomyaPleuromya literata jurassi 5 (5)17 2511 39 83 PleuromyaP. tenuistria jurassi 1717 (1) 11 48 74 39 GresslyaP. tenuistria gregaria 17 (1)10 486 37 74 GresslyaTrigonia gregaria sp. 100 24 6 100 37 Trigonia sp. 0 24 100 platy limestones, Middle Oxfordian Pholadomya acuminataplaty limestones, 7 Middle Oxfordian3 30 PholadomyaGoniomya acuminata moeschi 78 14 3 64 30 Goniomya moeschi 8 14 64

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FigureFigure 6.6. Distribution of the burrowing species (listed(listed in Table1 )1) in in the the sediment; sediment; a a schematic. schematic. BurrowingBurrowing depthdepth shownshown asas relativerelative toto the the frequency frequency of of non-exhumed non-exhumed specimens. specimens. SpecimenSpecimen sizessizes averagedaveraged for for the the species species and and proportional proportional to to each each other other rather rather thanthan toto thethe estimatedestimated burrowingburrowing depth. depth.

It is probable that many factors can influence the correctness of such an inference. It is difficult to It is probable that many factors can influence the correctness of such an inference. It is difficult assess the impact of predators—ribbed individuals could more effectively protect themselves against to assess the impact of predators—ribbed individuals could more effectively protect themselves being pulled out (see below). The ribs could also, to a minimal extent, hinder the washing of the empty against being pulled out (see below). The ribs could also, to a minimal extent, hinder the washing of shell out of the sediment. the empty shell out of the sediment. 3.2. Orientation in the Sediment 3.2. Orientation in the Sediment Most of the Pholadomya specimens examined were preserved in their life position. This state is Most of the Pholadomya specimens examined were preserved in their life position. This state is indicated by both the positions of the specimens found in profiles and the deformation manner of the indicated by both the positions of the specimens found in profiles and the deformation manner of the vast majority of deformed specimens observed outside of the profiles. It is difficult to imagine a factor vast majority of deformed specimens observed outside of the profiles. It is difficult to imagine a factor that could affect the orientation of shells after the death of the clams without their exhumation, so that could affect the orientation of shells after the death of the clams without their exhumation, so if if they had been diagenetically compacted, the deformation—even in a specimen found outside of they had been diagenetically compacted, the deformation—even in a specimen found outside of the the profile—is indicative of the shell’s orientation in the deposit. The degree of deformation can be profile—is indicative of the shell’s orientation in the deposit. The degree of deformation can be measured to calculate the original angles. The mathematical formulae were given by Rollier [30], but measured to calculate the original angles. The mathematical formulae were given by Rollier [30], but they are applicable to ideally plastic deformations, present only in exceptional situations. Figure7 they are applicable to ideally plastic deformations, present only in exceptional situations. Figure 7 shows orientation of specimens representing two species found at Gnaszyn (Cz˛estochowa) and shows orientation of specimens representing two species found at Gnaszyn (Częstochowa) and corrected using the Rollier formulae. Although the specimens had not been entirely plastically crushed, corrected using the Rollier formulae. Although the specimens had not been entirely plastically the bias in this case is not large, and the corrected values are closer to the original ones than those crushed, the bias in this case is not large, and the corrected values are closer to the original ones than measured directly on the deformed specimens. those measured directly on the deformed specimens.

Geosciences 2020, 10, 397 10 of 23 Geosciences 2020, 10, x FOR PEER REVIEW 10 of 23 Geosciences 2020, 10, x FOR PEER REVIEW 10 of 23

Figure 7. Ventral tilt of the hinge axis in Pholadomya lirata and Ph. concatenata from Middle Bathonian, CzFigureęstochowa, 7. Ventral with tilt two of thespecimens hinge axis of Ph. in Pholadomya lirata preserved lirata inand extreme Ph. concatenata orientations. fromfrom Middle Middle Bathonian, Bathonian, Cz˛estochowa,withCzęstochowa, with two specimens ofof Ph.Ph. lirata preserved in extreme orientations. Many of the compacted specimens were, in addition to the ventral tilt of the hinge axis, tilted— Many of the compacted specimens were, in addition to the ventral tilt of the hinge axis, tilted—to to a varyingMany of degree—tothe compacted the specimensside. The maximumwere, in addition tilt of tothe the deformed ventral tilt Ph. of lirata the hinge individuals axis, tilted— from a varying degree—to the side. The maximum tilt of the deformed Ph. lirata individuals from Gnaszyn Gnaszynto a varying was degree—toalmost 12° (usuallythe side. much The less),maximum with atilt similar of the proportion deformed of Ph. tilt lirata to the individuals right and to from the was almost 12 (usually much less), with a similar proportion of tilt to the right and to the left. Such left.Gnaszyn Such wassmall ◦almost tilt does 12° not(usually seem muchto be biologicalless), withly a important.similar proportion The case of is tilt different to the rightwith andrespect to the to small tilt does not seem to be biologically important. The case is different with respect to specimens of specimensleft. Such small of Ph. tilt cf. doesambigua not Sowerby,seem to be 1821, biological from thely important. Blanowice TheBajocian case is[31]. different Of the withsix specimens, respect to Ph. cf. ambigua Sowerby, 1821, from the Blanowice Bajocian [31]. Of the six specimens, one is slightly onespecimens is slightly of Ph. crushed cf. ambigua in the Sowerby, horizontal 1821, plane from (it thehad Blanowice been exhumed), Bajocian two [31]. are Of lacking the six the specimens, anterior crushed in the horizontal plane (it had been exhumed), two are lacking the anterior part on account of partone ison slightly account crushed of resting in the at horizontalthe border planewith (ita layerhad beenof poorly exhumed), lithified two fine-grained are lacking ferruginousthe anterior resting at the border with a layer of poorly lithified fine-grained ferruginous sandstone, and three show sandstone,part on account and three of resting show anteriorat the border crushing with indicati a layerng oftheir poorly having lithified been tilted fine-grained to the side ferruginous by about anterior crushing indicating their having been tilted to the side by about 45 (Figure8). The anterior 45°sandstone, (Figure and8). The three anterior show anteriorpart of the crushing two indivi indicatidualsng mentioned their having above been were tilted◦ cut to offthe atside an byangle about as part of the two individuals mentioned above were cut off at an angle as well, the angle being somewhat well,45° (Figure the angle 8). beingThe anterior somewhat part smaller. of the two The indivi specimendualss arementioned relatively above small, were but thecut disappearanceoff at an angle ofas smaller. The specimens are relatively small, but the disappearance of the shell ornamentation later thewell, shell the ornamentationangle being somewhat later during smaller. growth The isspecimen indicatives are of relativelytheir having small, completed but the disappearancegrowing. All the of during growth is indicative of their having completed growing. All the specimens were found close specimensthe shell ornamentation were found close later to during the bottom growth of is an indi aboutcative 10-cm of their thick having layer completedof limonitised growing. fine-grained All the to the bottom of an about 10-cm thick layer of limonitised fine-grained sandstone. The layer must sandstone.specimens wereThe layer found must close have to the been bottom shallow of anand about the underlying 10-cm thick stratum layer of of limonitised a more coarse-grained fine-grained have been shallow and the underlying stratum of a more coarse-grained sandstone (poorly lithified sandstonesandstone. (poorlyThe layer lithified must haveat present) been shallow too difficult and the to underlyinglend itself tostratum penetration of a more for thecoarse-grained bivalves to occupyatsandstone present) a preferred (poorly too diffi depth,cultlithified to for lend at which present) itself reason to penetrationtoo theydifficult rested forto obliquely thelend bivalves itself in to the to penetration occupysediment apreferred tofor be the able bivalves depth,to extend for to theirwhichoccupy siphons. reason a preferred they rested depth, obliquely for which in reason the sediment they rested to be obliquely able to extend in the theirsediment siphons. to be able to extend their siphons.

Figure 8. DeformationDeformation of of the the specimen specimen from from Blanow Blanowiceice preserved preserved in in a alaterally laterally inclined inclined position, position with (A), Figure 8. Deformation of the specimen from Blanowice preserved in a laterally inclined position, with awith situational a situational reconstruction. reconstruction (B). a situational reconstruction.

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3.3. Sediment Type Most of the Pholadomya specimens examined, including the extant representative, were found in fine-grained, originally marshy, deposits such as clays, silts and carbonate silts, occasionally with an admixture of a coarser material. The very thin shell is (among the other factors—see below) an adaptation to flotation in such a low-density and low-viscosity substratum [2]. The thinnest shell mentioned by Stanley [2] was that of Macoma tenta, with TI = 0.11 and a diameter of up to 2 cm. The estimated TI range of Ph. lirata mentioned above (0.05–0.1) seems not to be drastically different from that of M. tenta, but considering the exponential growth of bending stress relative to the shell length, bivalves that are a few times larger should have relatively thicker shells to maintain a relatively similar resistance to bending forces. Most likely, the ornament in the form of folds afforded much more rigidity to the shell which was, however, still very vulnerable to point forces. Representatives of some species were found in sediments of a different type. Ph. protei (Brongniart, 1821) (perhaps a group of species because of a large variability shown by the specimens) is commonly found in the Callovian deposits of the Polish Jura as well as in the Kimmeridgian deposits in Pomerania and on the margins of the Holy Cross Mountains. The Callovian deposits usually consist of sandstone and sandy limestone with a high admixture of shell debris and/or gravel; the Kimmeridgian deposits mentioned consist of limestone, also with numerous other fossils. The two deposit types were formed under high energy conditions; numerous specimens found in both show traces of at least partial exhumation (numerous Callovian specimens had been exhumed and transported). It is characteristic that individuals of Ph. protei, judging by the very pronounced inner relief, had shell valves exceptionally thick for the genus. This was related to the necessity of burrowing in the coarser sediment, which exerted point pressure on the shell as the bivalve was burrowing. The same deposits also supported other species, with thin shells typical of the genus. I collected a few specimens of Ph. aequalis, each in the shell debris-rich Kimmeridgian limestone at Czarnogłowy and Dziwnówek as well as in the Polish Jura’s uppermost Oxfordian. The bivalves are highly variable in shape and occasionally exhibit small deformations of the shell margins incurred during growth. One Pomeranian finding sheds light on the cause of the deformations (Figure9). It is the internal mould of Ph. protei with three individuals of Ph. aequalis inside. The Ph. protei specimen has retained the siphonal gape width of 18 mm (as the edges have not been preserved, the width must have been somewhat smaller), whereas the Ph. aequalis specimens are 23, 20 and 14.5 mm wide. They must have moved inside when considerably smaller, and grew when already there. The smallest specimen had been pushed down and blocked by the intermediate-sized one and, unable to move out, suffered deformation of the shell edge where it contacted the intermediate-sized bivalve. The largest specimen had moved in earlier than the other two and died before they grew: while growing and being pushed down, the smallest individual damaged the shell of the largest, which shows no trace of repair. Deformations or small size of some other specimens of the species indicate their preference to living in sheltered spaces which, with time, eventually restricted growth [1,2]. Such spaces, although surrounded by hard reef components or a larger shell, usually contained fine-grained calcareous silt. However, I have also found specimens of Ph. concatenata in Callovian sandstones. Although they are poorly preserved internal moulds, the specimens from Bathonian silts have extremely thin shells and, according to Stanley [2], should not have been able to live in a coarser sediment. Their presence there cannot thus be explained. Geosciences 2020, 10, 397 12 of 23

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FigureFigure 9. Pholadomya 9. Pholadomya aequalis aequalis fromfrom the the Lower Lower Kimmeridgian Kimmeridgian in Western in Western Pomerania Pomerania (NW Poland): (NW Poland):(A– C), distribution of three specimens of Ph. aequalis inside a specimen of Ph. protei (Dziwnówek or (A–C), distribution of three specimens of Ph. aequalis inside a specimen of Ph. protei (Dziwnówek or Czarnogłowy, specimens MGUS.S/267 and 307) in dorsolateral (A), posterior (B) and lateral (C) view. Czarnogłowy, specimens MGUS.S/267 and 307) in dorsolateral (A), posterior (B) and lateral (C) view. (D), the largest specimen of Ph. aequalis with a hole eroded by the smallest specimen. (E), the smallest D Ph. aequalis E ( ), thespecimen largest of specimen Ph. aequalis of with a deformedwith postero-ventral a hole eroded edge. by ( theF), another smallest malformed specimen. specimen ( ), the from smallest specimenCzarnog of Ph.łowy aequalis (MGUS.Sz.3460).with a deformed postero-ventral edge. (F), another malformed specimen from Czarnogłowy (MGUS.Sz.3460). 3.4. Mobility 3.4. Mobility The manner of a bivalve valve’s mobility can be inferred from the structure, and basically from Thethe length, manner of ofthe a hinge. bivalve A long valve’s hinge mobility restricts can valve be movements inferred from to rotation the structure, around andthe hinge basically axis, from the length,whereas of a theshort hinge. hinge enables A long the hinge movement restricts around valve the movements dorso–ventral to axis, rotation typical around of many the efficient hinge axis, whereasburrowers a short [2]. hinge In the enables Laternulidae, the movement the slit from around the shell the apex dorso–ventral to the ventral axis, side, typical combined of manywith shell effi cient burrowerselasticity, [2]. makes In the it Laternulidae, possible for the the anterior slit from and the post shellerior apex part toof thethe shell ventral to rotate side, combinedaround that with axis shell elasticity,independently makes it to possible some degree; for the however, anterior this and is not posterior the case part in representatives of the shell to of rotate Pholadomya around. that axis independently to some degree; however, this is not the case in representatives of Pholadomya. Geosciences 2020, 10, 397 13 of 23

Hinges of Pholadomya have no functional teeth, and the beaks above them abrade each other. In many species, the fibrous and lamellar layers of the ligament are very short, although longer in Ph. candida, as reported by Morton [11]. Thus, the hinge movement around the dorso–ventral axis was dependent on the length and width of the fused periostracum connecting the dorsal edges of the valves, the latter varying highly in form (Figure2). Where the edges separated beyond the beak, a potential for rotation around the dorso–ventral axis was high, but no such potential is evident where the edges are adjacent to each other. The ventral margin shape of the valves—rounded and touching at one point in the first and adjacent to each other along a longer stretch in the other—seems to support such an inference [2]. I suspect that a third valve rotation plane, not observed in bivalves so far, does exist, namely around the transverse (lateral) axis. The two planes described above require that the shell hinge acts like a ball hinge, which by itself enables rotations in all directions, so there is only the question of an appropriate muscle system. This, in the simplest form, should consist of two muscle bundles, each connecting the two valves, crossed and attached opposite to each other, situated on a plane more or less perpendicular to a line running from the point of rotation (hinge) to a more removed point (to attain a longer lever). The similarity of this theoretical description to the cruciform muscle found in Ph. candida by Morton [11] is striking. Valve rotation around the transverse axis could have been an aid for burrowing in a relatively poorly hydrated medium, which seems particularly important for thin-valved bivalves, the shells of which would not withstand the high stress exerted by strong movements of the foot or adductors. The Ph. candida foot is indeed so small that Runnegar [32] suspects that it could not have been used to move in a manner typical of bivalves, and—according to Morton [11]—functioned mainly as a valve. To aid in motility, the cruciform muscles must have facilitated valve rotation by at least one between-rib distance (half a distance each side from the neutral position). Moving in such “strides” should have required less effort (and exerted weaker stress on the valves) than the typical bivalve foot movement described by Trueman et al. [33]. The mechanism of movement involving the cruciform muscle is shown in Figure 10. Alternative views on the function of the cruciform muscle, proposed by Runnegar [32] and Morton [11], involve shifting of the anterior mantle gape and assisting in its closure, respectively. The first activity would have been achieved by alternating contractions of the muscles below and above their crossing; the second—exclusively by contractions of the lower parts. Both situations would require a functional division of each bundle (both authors describe them as continuous and crossing) into a lower and an upper part, one contracting independently of the other. Moreover, according to Runnegar, “the muscles are attached to the mantle at the point where they cross beneath the foot”, which most probably would not have ensured a tight adherence of the foot to the edges of the gape, as postulated by Morton. This could have been sufficiently achieved by the lower parts of the muscles joined to form an inverted U and connected with the mantle along the entire length. Bringing the anterior margins of the valve together doubles the function of the anterior adductor. Nevertheless, the function of the cruciform muscle as presented here is unusual in bivalves. In the Tellinoidea, a similar muscle is seen on the proximal end of the posterior fusion of the inner mantle lobes [34]. Interestingly, most genera with the cruciform muscle (except Tellina) listed by Yonge [34] show reduced lateral teeth, which would theoretically enable the valve to rotate around the transverse axis. Still more strikingly, the presence of the postero-ventral carina, and/or of diagonal sculpture with asymmetric ridges in Macoma, Gari and Solecurtus, would make it possible for the bivalves to move forward with such rotation even without assistance from the foot, the use of the foot rendering this movement much more efficient. Doubtless, the origin of the cruciform muscle in the Tellinoidea is independent from that in Pholadomya, but should such function be involved in the former, it would be confirmed in the very rare, and therefore difficult to observe, Ph. candida. Perhaps the manner of the cruciform muscle contraction (bundles contracting independently vs. their lower parts contracting together) could be explored by analysing their nerve pattern. Unfortunately, traces of the cruciform Geosciences 2020, 10, 397 14 of 23 muscle attachment in fossil forms have been found so far only in Ph. virgulosa from the Eocene [12,32]. No such structures could be found in my Jurassic material, despite examining specimens of a few species with well-visible internal morphology. Therefore, the muscle was either still not separated from the anterior adductor and/or the mantle muscles, or it had not appeared yet. Its utility in forms with tightly adhering hinge margins would be doubtful anyway, so it might have not been present in allGeosciences the forms. 2020, 10, x FOR PEER REVIEW 14 of 23

FigureFigure 10. 10. Mechanism of of cruciform cruciform muscle-aided muscle-aided movement movement in the insediment, the sediment, as exemplified as exemplified by: (A), by:Ph.(A ),candidaPh. candida (from(from Moesch Moesch 1874–1875); 1874–1875); and and(B), ( BPholadomya), Pholadomya protei protei (reconstructed(reconstructed specimen specimen MGUS.Sz.3577,MGUS.Sz.3577, Callovian, Callovian, Zalas, Zalas, PolishPolish Jura).Jura). The rota rotationtion axis axis and and the the hypothetical hypothetical position position of ofthe the cruciform muscle indicated. Contracted bands drawn thicker. The range of motion may be much cruciform muscle indicated. Contracted bands drawn thicker. The range of motion may be much smaller in Ph. candida due to smaller spaces between the ribs. smaller in Ph. candida due to smaller spaces between the ribs.

RegardlessNevertheless, of the the assistance function of provided the cruciform by the muscle cruciform as presented muscle, here there is unusual are other in bivalves. hints to inferIn thatthe at Tellinoidea, least some Pholadomyaa similar musclespecies is wereseen on capable the proximal of active end movement. of the posterior On account fusion of of its the small inner foot, Runnegarmantle lobes [32] suspects [34]. Interestingly,Ph. candida ofmost having genera burrowed with the deeper cruciform as it grew,muscle and (except of having Tellina been) listed otherwise by Yonge [34] show reduced lateral teeth, which would theoretically enable the valve to rotate around virtually immobile in the sediment. Morton [11] assumes a higher potential for movement, as much the transverse axis. Still more strikingly, the presence of the postero-ventral carina, and/or of diagonal as it is necessary for a deposit feeder. My fossil material does not lend itself to inferences regarding sculpture with asymmetric ridges in Macoma, Gari and Solecurtus, would make it possible for the the foot structure. The anterior gapes, when visible (fossilised specimens are frequently found with bivalves to move forward with such rotation even without assistance from the foot, the use of the valves non-anatomically pressed against each other), are narrow and long. The likely range of shell foot rendering this movement much more efficient. Doubtless, the origin of the cruciform muscle in circumferencethe Tellinoidea within is independent which the from foot that could in havePholadomya moved, but is inferredshould such from function the thickness be involved of muscular in the impressionsformer, it would on the be pallial confirmed line, in observed the very inrare, some and specimens therefore difficult of the Ph. to observe, protei group. Ph. candida The. linePerhaps is thin the manner of the cruciform muscle contraction (bundles contracting independently vs. their lower parts contracting together) could be explored by analysing their nerve pattern. Unfortunately, traces of the cruciform muscle attachment in fossil forms have been found so far only in Ph. virgulosa from the Eocene [12,32]. No such structures could be found in my Jurassic material, despite examining specimens of a few species with well-visible internal morphology. Therefore, the muscle was either still not separated from the anterior adductor and/or the mantle muscles, or it had not appeared yet.

Geosciences 2020, 10, 397 15 of 23 beneath the anterior adductor, and becomes a series of thicker imprints from the first strong lateral rib and just beyond it, suggesting reinforcement of the pallial muscles, which could have assisted the adductors (the first, strongest impression could have been left by the cruciform muscle) (Figure 11).

Figure 11. Imprints of anterior adductors and anterior parts of mantle muscles. Anterior (A) and antero-ventral (B) views of Pholadomya protei, Callovian, Bol˛ecin,Polish Jura (specimen MGUS.Sz.3921, (A), whitened with ammonium chloride, (B), with marked anatomical features).

Stanley [2] found the deepest-burrowing extant bivalves to burrow very slowly, the burrowing depth being adapted to changes in the sediment surface and the bivalve size; some, when removed from the sediment, are not capable of re-burrowing. Panopea japonica, arguably the deepest-burrowing extant bivalve, was studied by Kondo [24,29], who found it incapable of burrowing when removed from the sediment. However, he observed it to be able to perform efficient vertical movements. Despite the similarity with respect to deep burrowing, there is no extant equivalent of Pholadomya, a deep burrower with non-retractable siphon and rich shell ornamentation. In my opinion, the mode of life of Pholadomya cannot be regarded identical to that of the smooth-shelled species. At present, thin shells are observed in both slow- and fairly fast-burrowing bivalves. The thinnest shell examined by Stanley [2] is that of the slow-burrowing Macoma tenta. It seems that bivalves with very thin shells could not burrow fast, because the shell would not be then able to withstand both the stress exerted by energetic movements of the foot, adductors, and ligament, as well as encounters with harder, coarser sediment particles. The genus Pholadomya is very variable in terms of the shell shape. The length to height ratio ranges from about 1 to more than 2 (at a low variability of the width to height ratio); according to Stanley [2], the range accommodates both slow and fairly fast burrowers. Burrowing efficiency must have been highly dependent on the shape of the anterior part, which intercepts most of the stress during movement. Those forms assigned to the sub-genus Bucardiomya are short and their anterior part is high and more or less flattened, which should have greatly hindered movements and rendered the bivalves virtually immobile in the sediment. Representatives of the sub-genus Procardia show, mostly, still a more flattened anterior part, and occasionally even concave on account of large lunulae, which presented a hindrance as well unless they moved antero-ventrally. The remaining Pholadomya are oval or somewhat cylindrical in outline, usually with a rounded anterior, and spindle-shaped in the dorsal view, which renders them relatively streamlined in shape (Figure 12). Geosciences 2020, 10, x FOR PEER REVIEW 16 of 23

during movement. Those forms assigned to the sub-genus Bucardiomya are short and their anterior part is high and more or less flattened, which should have greatly hindered movements and rendered the bivalves virtually immobile in the sediment. Representatives of the sub-genus Procardia show, mostly, still a more flattened anterior part, and occasionally even concave on account of large lunulae, which presented a hindrance as well unless they moved antero-ventrally. The remaining Pholadomya Geosciencesare oval2020 or, somewhat10, 397 cylindrical in outline, usually with a rounded anterior, and spindle-shaped16 in of 23 the dorsal view, which renders them relatively streamlined in shape (Figure 12).

FigureFigure 12. 12.Shapes Shapes of of anteriors anteriors ofof variousvarious PholadomyaPholadomya..( (AA),), Ph.Ph. latirostris latirostris (Rudniki(Rudniki near near Zawiercie, Zawiercie, LowerLower Bathonian, Bathonian, Institute Institute of of Paleobiology Paleobiology of Polish Academy Academy of of Sciences Sciences collection, collection, specimen specimen ZPAL ZPAL Mo.XII-38)—roundedMo.XII-38)—rounded anterior, anterior, traditionally traditionally referredreferred to as subgenus PholadomyaPholadomya s.str.s.str. (B (–BD–D), Ph.), Ph. lirata lirata (Upper Bathonian, Częstochowa, specimen MGUS.Sz.3587)—changes of shape during growth of one (Upper Bathonian, Cz˛estochowa,specimen MGUS.Sz.3587)—changes of shape during growth of one individual, species of subgenus Bucardiomya. (E), Ph. acuminata (Wysoka, Middle Oxfordian, specimen individual, species of subgenus Bucardiomya.(E), Ph. acuminata (Wysoka, Middle Oxfordian, specimen MGUS.Sz.3584, right valve, reversed)—flat front, subgenus Procardia. MGUS.Sz.3584, right valve, reversed)—flat front, subgenus Procardia. In deep-burrowing bivalves, the shell loses its protective function. To reduce the amount of effort In deep-burrowing bivalves, the shell loses its protective function. To reduce the amount of effort associated with valve movement during burrowing and filtration, there are two gapes (even at closed associated with valve movement during burrowing and filtration, there are two gapes (even at closed valves): for the siphon and for the foot. In most bivalves, the anterior gape is large, evidencing a valves): for the siphon and for the foot. In most bivalves, the anterior gape is large, evidencing a powerful foot. The gape in Pholadomya is narrow; in the extant species, the foot is regarded as powerful foot. The gape in Pholadomya is narrow; in the extant species, the foot is regarded as relatively relatively small [11], and poorly adapted to burrowing. Runnegar [32] proposed that burrowing small [11], and poorly adapted to burrowing. Runnegar [32] proposed that burrowing could have could have been assisted by hydrating the sediment in front of the bivalve (to be aspired and released beenat the assisted rear, with by hydrating the siphon?) the with sediment a water in jet front ejecte ofd thethrough bivalve the (toanterior be aspired mantle and gape. released In my opinion, at the rear, withthe the same siphon?) mechanism, with aregardless water jet of ejected its details, through must the have anterior been applied mantle also gape. by In the my extinct opinion, forms. the same mechanism,Among regardless representatives of its details, of the mustfamily, have Pholadomya been applied is distinct also byin thehaving extinct radial forms. ribs. Stanley [2] demonstratedAmong representatives the ornamentation of the (particularly family, Pholadomya radial) tois distinctassist in instabilising having radialan individual ribs. Stanley in the [2] demonstratedsediment, and the in ornamentationburrowing in the (particularly shallow-burrowing radial) tospecies assist with in stabilising shapes far an from individual streamlined in the sediment,(which, andhowever, in burrowing were inusually the shallow-burrowing relatively thick-shelled). species with A shapesparticularly far from good streamlined indicator (which, for however,ornamentation were usually as an aid relatively in burrowi thick-shelled).ng is the rib A profile particularly asymmetry: good the indicator steeper for posterior ornamentation sides of the as an aidribs in burrowingfacilitate moving is the ribonwards profile an asymmetry:d prevent themoving steeper back. posterior Indeed, sides such of ribs the are ribs visible facilitate in movingsome onwardsindividuals and preventof Ph. protei moving (cf. Figures back. Indeed,10 and 13), such but ribs their are shel visiblel shape in somemust have individuals been a hindrance of Ph. protei for(cf. Figuresthe movements. 10 and 13), The but ribs their in shell most shape species must are have fairly been symmetrical a hindrance and forstrongest the movements. at the widest The spot, ribs in mostwhich species would are have fairly made symmetrical them quite and useful strongest for burrowi at theng. widest On the spot,other whichhand, th woulde ribs may have fulfil made other them quitefunctions. useful I for have burrowing. no doubtsOn that the thick other ribs hand, (even themore ribs so, may those fulfil criss-crossing other functions. the folds) I have rendered no doubts the thatshell thick considerably ribs (even stiffer, more similarly so, those to criss-crossing flutings on gothic the folds)plate armouries rendered or the folds shell of considerablycorrugated sheet. stiff er, similarlyAlthough to Stanley flutings [2] on did gothic not regard plate the armouries ornamentatio or foldsn as ofimportant corrugated for the sheet. shell Althoughconstruction, Stanley he did [2] not study such thin sculpted shells. Nevertheless, this was most probably not the only function of the did not regard the ornamentation as important for the shell construction, he did not study such thin ribs; they could have both strengthened the valves and aided in burrowing. Another function sculpted shells. Nevertheless, this was most probably not the only function of the ribs; they could mentioned by Stanley is stabilisation of valves interlocked with dentition of edges, which was not the have both strengthened the valves and aided in burrowing. Another function mentioned by Stanley is case here: the ribs usually oppose one another, and the valves are barely touching each other. stabilisation of valves interlocked with dentition of edges, which was not the case here: the ribs usually oppose one another, and the valves are barely touching each other. The assistance in burrowing may be indicated by the correlation between the rib thickness and the sediment fraction [2]. My materials did not show such a correlation, but it is mentioned by Aberhan [35] with respect to Ph. fidicula (see also below), and is indicative of a substantial activity, at least for the genus. The finding of Ph. aequalis in the shell of Ph. protei and in reef limestone suggests that representatives of the species, having found a site appropriate for them, did not move any further. The gape in the shell of one individual, eroded by the other, is indicative that they, despite their very thin shells, could have—at least to some extent—burrowed among not particularly solid obstacles. A similar strategy in the remaining species cannot be ruled out. However, I think that the presence of specimens preserved with the dorsal margin more or less horizontal, suggests that they were more capable of moving in the horizontal plane than those always preserved with the more or less vertical axis (Ph. latirostris vs. Ph. lirata from Korwinów; Table1), which most probably can be reflected in the morphology (oval vs. anteriorly flattened) and corresponds to groups of species traditionally assigned to the subgenera of Ph. (Pholadomya) and Ph. (Bucardiomya), respectively. Geosciences 2020, 10, 397 17 of 23

Another argument for the ability to move, and even re-burrow after surfacing, is seen in traces of shell damage repair. The Ph. protei specimen with such healing traces is shown in Figure 13. Such a serious mechanical damage was produced most likely on the sediment surface (after the specimen had been washed out by current or removed by a predator) and the bivalve, having repaired the damage andGeosciences continued 2020, 10 growing,, x FOR PEER must REVIEW have re-burrowed. 17 of 23

Figure 13.13. HealedHealed shell shell damage damage in Pholadomya protei, Callovian, Bol Bol˛ecin,Polishęcin, Polish Jura (specimen(specimen MGUS.Sz.3924, whitened with ammonium chloride).

3.5. PredationThe assistance in burrowing may be indicated by the correlation between the rib thickness and the sedimentFilter-feeding fraction animals [2]. My bury materials themselves did in not the sh sedimentow such to a avoid correlation, predators. but Forit is deep-burrowing mentioned by bivalves,Aberhan the[35] sediment with respect takes to over Ph. thefidicula protective (see also function, below), and and the is shell indicative becomes of reduceda substantial to the activity, minimum at thatleastis for mechanically the genus. necessary to move and/or to enlarge the burrow in which the animal stays. The efindingfficacy of of such Ph. a strategyaequalis isin di thefficult shell to assess. of Ph. Zato´nand protei and Salamon in reef [36 limestone] found no remainssuggests of that the Anomalodesmatarepresentatives of inthe remains species, regurgitated having found by durophages.a site appropriate My observations for them, did of not similar move accumulations any further. ofThe remains gape in at the Gnaszyn shell of (also one individual, those I had eroded just uncovered) by the other, proved is indicative negative asthat well, they, despite despite the their presence very ofthin fragments shells, could of delicate have—at shells least similar to some to thoseextent—burrowed of Pholadomya among, e.g., resembling not particularly those solid of the obstacles. ammonites A Oxyceritessimilar strategy. The absence in the remaining may suggest species that memberscannot be of ruledPholadomya out. However,were not Iroutine think that diet the items presence for those of predators.specimens However,preserved itwith cannot the bedorsal ruled margin out that, more because or less their horizontal, shells were suggests so delicate, that they they were were more not regurgitatedcapable of moving but digested in the horizontal whole, as opposed plane than to emptythose always (thus of preserved no nutritive with value) the more shells or of less ammonites. vertical axis (TwoPh. latirostris specimens vs. only Ph. lirata bore repairedfrom Korwinów; traces of Table mechanical 1), which damage, most whichprobably might can have be reflected been inflicted in the bymorphology predators. (oval The shellvs. anteriorly of one specimen, flattened)Ph. and lirata correspondsfrom silt, wasto groups about 1.5–2of species cm high traditionally when its ventralassigned margin to the subgenera became damaged; of Ph. (Pholadomya the subsequent) and Ph. shell (Bucardiomya growth was), respectively. disturbed. It seems significant that suchAnother a small argument specimen for the could ability have to been move, attacked—my and even re-burrow collection after does surfacing, not contain is seen any in equally traces smallof shell representative damage repair. of theThe species Ph. protei (nor specimen do I have with any othersuch healing such small traces representative is shown in of Figure the sub-genus 13. Such Bucardiomyaa serious mechanical), which may damage suggest was that, produced once the most bivalves likely survivedon the sediment the diffi cultsurface early (after period the when specimen they werehad been not able washed to burrow out by down current to a safeor removed depth in by the a sediment, predator) and and the the shells bivalve, were having so thin asrepaired to become the amenabledamage and to continued digesting ingrowing, a durophage’s must have intestine, re-burrowed. they were safe later in life. The other case is the already mentioned specimen of Ph. protei which, with a much larger shell, was severely damaged but managed3.5. Predation to repair the shell (Figure 13). It was found in shell debris-rich sandy-oolithic limestone and was exhumedFilter-feeding in taphonomic animals bury process, themselves suggesting in the itse coulddiment have to avoid been predators. similarly exhumed For deep-burrowing when alive, andbivalves, the damage the sediment could have takes been over accidental the protective (if the damage function, was and inflicted the shell by a becomes predator, reduced why would to the latterminimum abandon that itsis mechanically prey?). The repaired necessary shell to damagesmove and/or (interpreted to enlarge as predationthe burrow traces) in which are muchthe animal more frequentstays. in the remaining molluscs in the collection. The efficacy of such a strategy is difficult to assess. Zatoń and Salamon [36] found no remains of the Anomalodesmata in remains regurgitated by durophages. My observations of similar accumulations of remains at Gnaszyn (also those I had just uncovered) proved negative as well, despite the presence of fragments of delicate shells similar to those of Pholadomya, e.g., resembling those of the ammonites Oxycerites. The absence may suggest that members of Pholadomya were not routine diet items for those predators. However, it cannot be ruled out that, because their shells were so delicate, they were not regurgitated but digested whole, as opposed to empty (thus of no nutritive value) shells of ammonites.

Geosciences 2020, 10, 397 18 of 23

Although the vital organs were, with the adult individual’s shell, at a depth safe from predators, the siphons reached the sediment surface. Predators could have attempted to pull the bivalve out by the siphon. The defence could have involved a rapid siphon retraction, whereby the siphon tip could have been bitten off at most. The predator could have given up trying to pull the prey out because that proved overly difficult, hardly promising any success, and/or because of a high energy expenditure. This is observed also today [37,38]. Could the smaller length of the inhalant siphon and the lack of sensors in the Ph. candida specimen examined by Morton [11] have been a result of such healed, but not fully regenerated, siphon bite-off? Obviously, pulling out a whole bivalve comes as a big benefit for the predator, so it should try to do it, whereas preventing this is, for the bivalve concerned, a question of survival. Siphon contraction results in its being hidden in the sediment, but when the siphon has already been grabbed, the bivalve can be pulled out. The bivalves prevent this by wedging themselves in the sediment or increasing friction if the sediment is plastic or loose, and by creating a negative pressure beneath the shell so that it is pulled in a direction opposite to that of the predator’s pull. Friction increase or wedging occurs by shell valves being extended due to siphon contraction with closed pallial gapes, which increases the pressure within the mantle cavity [2,11]. The excess of water can be directed by the siphon towards the attacker. Valve extension facilitates wedging in the sediment, which is assisted by the shell shape narrowing posteriorly, and by the radial ribs. The properties of the ribs, already mentioned—which are usually at their strongest in the widest part of the shell, orientated horizontally in the filtering position (i.e., perpendicular to the direction of the predator’s pull), and occasionally an asymmetrical profile—makes them ideal for the purpose. The shell is being kept in place by the negative pressure beneath, just as with mud sucking in a person’s shoes. This is related to the sediment viscosity and the geometry of the lower (anterior) part of the shell. The higher the viscosity and the wider and flatter (or even concave) anteriorly the shell, the more effective the sucking (Figure 12). Although the well-developed ribbing may assist in motion, the anterior flattening seems to bring no other benefit. The species traditionally assigned to the sub-genera Procardia and Bucardiomya are clearly flattened anteriorly and, in my opinion, this is a result of their being extremely well adapted to the defence strategy described above, at the expense of mobility.

3.6. Parasitism and Diseases As opposed to predation, diseases (and perhaps other causes of health deterioration; not always can the cause of adverse changes be identified) have left very numerous traces on the individuals examined. The most common pathology involves mud blisters, described by Sztajner [39], found in several tens of Pholadomya specimens and in some other deep-burrowing anomalodesmates, but in no other bivalve in the collection. Not uncommon were also half-pearls (also mentioned by Sztajner [39]) and growth retardation as well as other developmental malformations (except for the already mentioned deformations of Ph. aequalis), traces of pallial inflammation (?), and irregular ornamentation. Growth retardation and irregular ornamentation could have been provoked by fasting and other factors, and not necessarily by pathogens [40], whereas the remaining symptoms evidence effects of parasites and diseases. Some specimens showed a series of pathologies, experienced both during growth and on maturity. The frequent occurrence of the parasite-produced pathologies seems to be obvious because, although not every free-living animal falls prey to a predator, each is affected by parasites [41]. In addition, the predator, having devoured its prey whole, makes it impossible to detect fossil traces of its attack. On the other hand, of the multitude of bivalve diseases, only very few leave their traces on the shells [42]. Indeed, such traces are rarer in the non-infaunal bivalves in the collection than traces of predation, and even in the remaining anomalodesmatans (also infaunal) are less frequent than in Pholadomya (the numbers are not given, because the collection is not representative owing to the preferential acquisition). As reported by Lauckner [42], parasitic infestation usually intensifies with age, and the longevity of otherwise safe, deeply burrowed, adult bivalves seems to be the best explanation. Geosciences 2020, 10, x FOR PEER REVIEW 19 of 23

ornamentation. Growth retardation and irregular ornamentation could have been provoked by fasting and other factors, and not necessarily by pathogens [40], whereas the remaining symptoms evidence effects of parasites and diseases. Some specimens showed a series of pathologies, experienced both during growth and on maturity. The frequent occurrence of the parasite-produced pathologies seems to be obvious because, although not every free-living animal falls prey to a predator, each is affected by parasites [41]. In addition, the predator, having devoured its prey whole, makes it impossible to detect fossil traces of its attack. On the other hand, of the multitude of bivalve diseases, only very few leave their traces on the shells [42]. Indeed, such traces are rarer in the non-infaunal bivalves in the collection than traces of predation, and even in the remaining anomalodesmatans (also infaunal) are less frequent than in Pholadomya (the numbers are not given, because the collection is not representative owing to the Geosciences 2020, 10, 397 19 of 23 preferential acquisition). As reported by Lauckner [42], parasitic infestation usually intensifies with age, and the longevity of otherwise safe, deeply burrowed, adult bivalves seems to be the best explanation.As mentioned by Sztajner [39], pathological structures (mud blisters, most probably produced by Gymnophallidae-likeAs mentioned by Sztajner digeneans) [39], pathological must have astructuresffected the (mud functioning blisters, most of some probably individuals produced very heavily,by Gymnophallidae-like to the extent of making digeneans) defence must from have predators affected di ffithecult, functioning e.g., by impeding of some individuals siphon contraction very heavily, to the extent of making defence from predators difficult, e.g., by impeding siphon contraction with a much reduced diameter of the posterior gape (Figure 14). Effects of parasites on the bivalve with a much reduced diameter of the posterior gape (Figure 14). Effects of parasites on the bivalve behaviour, facilitating attack and transfer, is well-known today [42]. It may be presumed that the behaviour, facilitating attack and transfer, is well-known today [42]. It may be presumed that the bivalves examined had been so affected, although this is difficult to decipher from the fossil record. bivalves examined had been so affected, although this is difficult to decipher from the fossil record.

FigureFigure 14. 14.Specimen Specimen extremely extremely aaffectedffected byby pathologypathology in in fo formrm of of mud mud blisters blisters (o (orr pockets) pockets) (described (described inin detail detail in in [39 [39]).]). The The restriction restriction ofof thethe posteriorposterior part part of of the the shell shell interior interior must must have have impaired impaired the the functioningfunctioning of of the the siphon; siphon;Ph. Ph. liratalirata (Lower(Lower Bathonian, Cz Cz˛estochowa,Polishęstochowa, Polish Jura, Jura, MGUS.Sz.3888), MGUS.Sz.3888), posteriorposterior and and right right view, view, and and approximate approximate schemescheme ofof the longitudinal section section of of the the specimen. specimen.

3.7.3.7. Ontogeny Ontogeny I amI am not not aware aware of of a aPholadomya Pholadomyaspecimen specimen smaller than than about about 13 13 mm. mm. Smaller Smaller juvenile juvenile stages stages are are well-visiblewell-visible in in growth growth lines lines of of larger larger specimens;specimens; however, the the umbone umbone abrasion abrasion made made it itimpossible impossible toto make make inferences inferences on on the the very very earliest earliest developmentaldevelopmental stages. stages. On On the the other other hand, hand, changes changes visible visible in in thethe observable observable size size range range seem seem significant significant enoughenough to enable enable inferences inferences on on ontogenetic ontogenetic changes changes in inthe the lifelife habits. habits. There There are are species species that that dodo notnot showshow significantsignificant changes changes in in the the size size range range at athand, hand, but but the the developmentdevelopment is is usually usually somewhat somewhat anisometric. anisometric. In different species, certain trends can be detected, frequently independently of the species’ In different species, certain trends can be detected, frequently independently of the species’ affinities: affinities: • Increased number of ribs: very rare in the size range at hand. In Ph. fidicula, the ribs appear at • Increased number of ribs: very rare in the size range at hand. In Ph. fidicula, the ribs appear at thethe length length of of about about 3 3 mm mm and and most most areare alreadyalready present present in in a a 14-mm 14-mm long long specimen; specimen; very very few few ribs ribs appear,appear, by by intrusion, intrusion, later later in in life. life. ThisThis is also, to a a different different extent, extent, observed observed in in representatives representatives ofof the the sub-genus sub-genusProcardia Procardia(Figures (Figures 1212EE and 15A).15A). The The increase increase in in the the number number of ofribs ribs results results in in retainingretaining their their more more stable stable size size andand density.density. According to to [2], [2], when when the the ribs ribs assist assist in inburrowing, burrowing, theirtheir thickness thickness correlates correlates with with thethe dominantdominant sediment grain grain size size fraction; fraction; the the thickness thickness should should thereforetherefore be be stable stable to to maintain maintain optimaloptimal functionality.functionality. Most Most of of the the specimens specimens examined examined do donot not show ribs of stable thickness, as they must have lived in silts enriched with shell debris, or in calcareous silts, i.e., too fine-grained sediments. It is only the Callovian specimens of Ph. fidicula that come from sandstones or oolithic limestone. Similarly, specimens of the species examined by Aberhan [35] (dated to the Sinemurian–Aalenian) were found in similar sediment fractions. Interestingly, they are usually larger than those found in Poland and show more ribs, including those added by intrusion. Moreover, as reported by the author mentioned, ribs in the specimens from coarser sediments are thicker. Geosciences 2020, 10, x FOR PEER REVIEW 20 of 23

show ribs of stable thickness, as they must have lived in silts enriched with shell debris, or in calcareous silts, i.e., too fine-grained sediments. It is only the Callovian specimens of Ph. fidicula that come from sandstones or oolithic limestone. Similarly, specimens of the species examined by Aberhan [35] (dated to the Sinemurian–Aalenian) were found in similar sediment fractions. Interestingly, they are usually larger than those found in Poland and show more ribs, including Geosciencesthose2020 added, 10, 397 by intrusion. Moreover, as reported by the author mentioned, ribs in the specimens20 of 23 from coarser sediments are thicker.

Figure 15. 15. AppearancesAppearances of ribs. of ribs. (A), All (A ),ribs All almost ribs almostsimultaneously; simultaneously; Ph. cf. acuminataPh. cf. (Oxfordian,acuminata (Oxfordian,Rzędkowice, Rz˛edkowice,Polish Polish Jura, MGUS.Sz.3607)—but Jura, MGUS.Sz.3607)—but compare compare with withFigure Figure 12E. 12 (E.B), (B ),Gradually, Gradually, by by intercalations; Ph. fidicula fidicula (Upper Bajocian, Cz˛estochowa,PolishCzęstochowa, Polish Jura, MGUS.Sz.3880).

• Disappearance of ornamentation late during th thee growth is, on the other hand, frequent and • involves both ribs and the regular concentric folds.folds. The The ornamentation ornamentation may disappear even long before the finalfinal size isis attained;attained; the bestbest exampleexample is providedprovided byby Ph.Ph. transversa Seebach, 1864, 1864, which shows poorly visible ribs on the top part of the shellshell only.only. The disappearancedisappearance of ribs is usually not uniform; uniform; the the first first to to vanish vanish are are the the posterior posterior ones ones (e.g., (e.g., Ph.Ph. lirata lirata), and), and frequently frequently not notall disappear. all disappear. Stanley Stanley [2], [ 2who], who referred referred to toshallow-burrowing shallow-burrowing bivalves bivalves only, only, associated associated the ornamentation primarily with stabilisation in the sediment and explained the disappearance by the factfact that that large large individuals individuals are moreare more stable stable due to due their to size their alone. size In alone. this case, In asthis the case,Pholadomya as the arePholadomya deep-burrowers, are deep-burrowers, this could pertain this could to the pertain earliest to stages the earliest of growth stages only. of In growth my opinion, only. In as my the bivalvesopinion, as grew, the thebivalves ribs were grew, losing the ribs their were importance losing their as importance locomotory as aids, locomotory because theaids, larger because the bivalve,the larger the the deeper bivalve, in sedimentthe deeper (in in absolute sediment depth) (in absolute it penetrated depth) to, it andpenetrated thus was to, less and exposed thus was to predatorless exposed attack to orpredator to being attack washed or away,to being and washed was not away, as dependent and was on not rapid as burrowing.dependent on I suspect rapid alsoburrowing. that the I role suspect of ribs also as a that shell the construction role of ribs reinforcement as a shell lostconstruction its importance reinforcement in adults because,lost its firstly,importance the tension in adults in the because, shell was firstly, weakened the tensio inn organisms in the shell moving was weakened slowly, and, in organisms secondly, themoving most hazardousslowly, and, tensions, secondly, bending the most moments, hazardous are low tens onions, free bending edges not moments, affected byare pointlow on forces—there free edges werenot affected no muscle by point attachments, forces—there and most were soft no sediments muscle attachments, lack coarser and components most soft thatsediments could havelack producedcoarser components point forces. that Once could at have a safe produced depth in poin thet sediment,forces. Once the at bivalves a safe depth could in athefford sediment, to slow downthe bivalves their growth could afford (as occasionally to slow down inferred their from growth increased (as occasionally density and inferred thickening from ofincreased growth linesdensity and and periostracum thickening folds)of growth and tolines invest and more perios intotracum shell thickening.folds) and to However, invest more owing into to shell their thinnessthickening. and However, poor preservation owing to state, their I was thinness unable and to observe poor preservation growth increments state, I on was cross-sections unable to toobserve test the growth hypothesis increments of such on a cross-sections growth model. to test the hypothesis of such a growth model. • Disappearance of spikes, present at the earliest stages of shellshell growth.growth. Spikes occur in all the • species in which I observedobserved wellwell preservedpreserved shellshell onon beaks.beaks. It is only in Ph.Ph. fidicula fidicula that spikes cover the shell surface until the termination of growth (although the specimens I examined are small compared to to those those presented presented by by Aberhan Aberhan [35] [35);]); in in the the other other species, species, the the remaining remaining part part of ofthe the shell shell is iscovered covered by by periostracal periostracal folds folds and and growth growth lines. lines. I Ihave have no no suggestion suggestion as as to their function or the cause of their disappearance. The species with flattened anterior parts show that, as may be inferred from the growth lines, The species with flattened anterior parts show that, as may be inferred from the growth lines, the the anterior part was more rounded at early growth phases (Figure 15). This could have been only anterior part was more rounded at early growth phases (Figure 15). This could have been only the the legacy of the ancestors, but in my opinion this legacy has been maintained to increase the mobility legacy of the ancestors, but in my opinion this legacy has been maintained to increase the mobility of of young individuals which, when buried to shallower (in absolute terms) depths, had to be more young individuals which, when buried to shallower (in absolute terms) depths, had to be more mobile. mobile. 4. Conclusions The morphological diversity within the genus Pholadomya most likely reflects the diversity of the genus members’ modes of life. With the absence of physical constraints in the sediment, the bivalves were always deep burrowers, frequently burrowing to extreme depths. When filtering, their posterior end was turned upwards, with the hinge axis tilted somewhat ventrally. When not forced to move, they were sluggish; some were Geosciences 2020, 10, 397 21 of 23 occupying a limited space and were immobile. Others, when necessary, moved—not only vertically, as they grew and the sediment was setting down or eroded, but also horizontally. Even the least streamlined forms were able to bury themselves back when exposed, but it is difficult to imagine how they would do it. Most probably, streamlined individuals were more mobile, including juveniles of the species which, when adult, were virtually sessile. Young individuals buried shallow in the sediment were virtually defenceless against predators. On the other hand, large individuals were out of reach for predators, except when pulled up by the siphon. They resisted that by expanding the shells and wedging them in the sediment, as well as by rapidly contracting the siphon and ejecting water through it. In adulthood, they could live for a relatively long time, but were suffering from parasitic infections, which occasionally led to serious dysfunctions, perhaps making it easier for predators to pull them out and facilitating the parasite transfer to the definite host.

Funding: The research was supported by the Polish National Science Centre grant No. N N307 375639. The publication was financed from the statutory funds of Institute of Marine and Environmental Sciences, University of Szczecin (grant No. 503-1100-230342). Acknowledgments: I am grateful to Marek Trzepizur (Cz˛estochowa)and Janusz Kucharski for donating some specimens. Adam T. Halamski (Polish Academy of Sciences—PAS, Warsaw) granted access to the collection of the PAS Institute of Palaeobiology. Thanks are due to managers of the numerous brickyards and quarries in the Polish Jura for permission to search the pits. Sincere thanks are extended to the library staff at the Polish Geological Institute (PGI) in Warsaw, the PAS Institute of Palaeobiology, and the PAS Institute of Parasitology for their invaluable assistance with the literature search. This manuscript benefited from the suggestions of Michał Zato´n(University of Silesia, Sosnowiec), Adam Bodzioch (Opole University), Andrzej Kaim and Jerzy Dzik (PAS Institute of Palaeobiology), and especially Teresa Radziejewska (University of Szczecin), who also provided linguistic support and critically evaluated the initial draft of the manuscript. Conflicts of Interest: The author declares no conflict of interest.

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