soria 2007 symposium crustaceans

8/12/2019 Soria 2007 Symposium Crustaceans

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Alice Giannetti, Paolo Monaco, Jess E. Caracuel, Jess M. Soria &

Alfonso Ybenes

Functional morphology and ethology of decapod crustaceansgathered by Thalassinoides branched burrows in

Mesozoic shallow water environments

Every part of a burrow has an important and specificfunction with regard to the ecology and trophism of theburrower. It goes from the guarantee of the oxygena-tion and irrigation of the tunnels to the protection of the

organism or the storing of organic material for feeding(Nickell & Atkinson, 1995). Regarding modern thalassi-noidean burrows, presence of surface mounds, verticaldevelopment of tunnels, geometry of the section of tun-nels and horizontal galleries, presence of chambers andof organic debris within the burrows, number and typeof apertures on the seafloor, presence of exhalant tunnelsand Y- or U-shaped burrow in vertical section, are gen-erally considered as the most important characteristicsfor ethological characterization of the burrow and ofthe trace-maker (Nickell & Atkinson, 1995). Some ofthese features can be recovered also in the trace fossilThalassinoides, usually considered a fossil counterpartof modern thalassinoidean burrows, and they allowone to formulate hypotheses about the trophism of thetrace maker. This can be particularly useful in thosecases in which body-fossil remains of the burrowersare completely absent and a direct comparison betweenthe morphology of ancient and modern organisms is notpossible. Ecologic considerations are here applied toPliensbachian and Albian Thalassinoides developed inthe lagoonal deposits of the Trento carbonate platform(Rotzo Member, Calcari Grigi Formation, northernItaly) and in the outer shelf deposits of the Scaras For-mation (Serra Gelada, Alicante Province southeasternSpain), respectively. Different morphotypes of Tha-lassinoideswere identified in these two areas, mainly onthe basis of dimensions of the branches, diameter of theturning chamber and development and geometry of themaze. Even if Thalassinoideslike burrows are nowadaysproduced also by worms and fishes, we consider crusta-ceans to be the most probable trace makers of the studiedThalassinoides. For a detailed discussion of the geologi-cal setting, palaeonvironmental models, and ichnologyconcerning the studied areas see Avanzini (1998), Gian-netti (2004), Giannetti & Monaco (2002), Masetti et al.(1998), Masetti & Posenato (2002), Monaco (2000a,2000b), Monaco & Garassino (2001), Monaco & Gian-netti (2001, 2002), Winterer & Bosellini (1981), andZempolich (1993) for the Calcari Grigi Formation, and

Caracuel et al. (2002), Castro (1998), Giannetti et al.(2005), Monaco et al. (2005), Vilas et al. (1982), andYbenes (1996) for the Scaras Formation and refer-ences therein.

In the Rotzo Member, three types of Thalassinoidessuevicus Rieth 1932 (Thalassinoides type I to type III,from the smallest to the largest form) and another formtype of Thalassinoidesisp. (type IV) were identified.

Thalassinoidessuevicustype I is a small burrow, witha diameter ranging from 2 cm to 5 cm. Branches are thinand circular in section. Turning chambers are not par-ticularly developed. Mazes are regular and geometricalin shape and developed only on the horizontal plane.Burrows are mud-filled and their surface is smooth. Whenthese burrows are in tiered with the other forms of Tha-lassinoides, they occupy the upper tier.

In Thalassinoidessuevicus type II, the diameter ofbranches ranges from 5 cm to 10 cm and diameter of turn-ing chambers ranges from 6 cm to 10 cm. Branches arelong and form geometrical, sometimes pentagonal mazes.The external surface is smooth and lining is absent. Mud-stone or bioclastic wackestone fills the burrows. Bioclastscan be minutely fragmented and grouped in the internalpart of the burrow. This is the most frequently observedtype of Thalassinoidesin the studied sections.

Thalassinoidessuevicustype III is one of the largestThalassinoidesform. Its diameter ranges between 10 cmand 16 cm, and the diameter of the turning chamber is upto 22 cm. Branches are squat with respect to the morphol-ogy of the whole burrow and bifurcate at regular angles.The outer surface is smooth and the filling is made ofbioclastic wackestones/packstones. Mottled concentra-tions of minutely fragmented bioclasts can be present inthe central part of the burrow, in correspondence to thelumen. This form always occupies the deepest tiers.

In Thalassinoidestype IV the diameter of the branchescan reach 16 cm. The outer surface is mammillary, withsmall mounds distributed without particular orientationwith respect to the burrow. Branches are short, subcircu-lar in section and mazes are irregularly developed. On theouter surface, crinoids and other bioclasts are frequentlygrouped. Bioclastic wackestone fills the burrows.

Other cylindrical burrows, indicated as Thalassi-noides? isp., exhibit simple tubes up to 80 cm in length,are gently arched, 6 to 8 cm in diameter, and show shortaborted branches, without turning chambers. Thesetraces are very similar in horizontal plane view to thoseillustrated by BROMLEY(1990). These atypical, branched

tunnels are very different from I-II-III-IV types describedabove, and are analogous to some burrows of stomatopodsfrom the Seychelles Islands. For their morphological andtaphonomic characteristics these simple cylindrical tubes


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are not closely related to burrows of shrimps or otherdecapods, but resemble to those of carnivorous crusta-ceans (stomatopods, Malacostraca), indicated as activepredators, now populating the shallow waters mainly oftropical and subtropical seas.

In the Scaras Formation, we identified three typesof Thalassinoidessuevicus Rieth 1932 (Thalassinoides

type A to type C) and another poorly branched form(Thalassinoidestype D), resembles the Thalassinoides?isp. described for the Rotzo Member, while the type IV isvery rare.

Thalassinoidessuevicus type A is a small burrow,with a maximum diameter of 5 cm. Turning chambers arepresent but not particularly developed. It does not showlaterally developed mazes, but only burrows with shortbranches developed only on the horizontal plane. Bur-rows are mud-filled and their surface is smooth. Similarlyto the Rotzo examples, these burrows can be frequentlyobserved tiered with the other forms of Thalassinoidesand they are placed in the most superficial layers.

In Thalassinoidessuevicus type B, branch diameterranges from 6 cm to 8 cm, turning chambers, can reach adiameter of 9 cm. Regular mazes can be well developedon the horizontal plane. The outer surface is smooth,lining is absent and filling is homogenous and made up ofbioclastic wackestone.

Thalassinoidessuevicustype C is the largest Thalassi-noidesform, with a mean diameter of around 8 cm, andup to 12 cm in size for the turning chamber. When tier-ing is present, this form is always located in the deepestlayers. Filling is homogeneous and made up of bioclasticwackestone.

Thalassinoidesisp. type D is a very large, horizontallydeveloped burrow, following an irregular zigzag path. Itis up to 1 m long, with a diameter ranging between 10cm and 14 cm and a circular to slightly elliptical section.It does not show true branching, but only short protru-sions, extending in correspondence of the bending pointof the burrow. This trace resembles Thalassinoides? isp.of Calcari Grigi. Filling is generally homogeneous andis made up of fine-grained calcarenites or by bioclasticwackestone/packstone, according to the lithology of thehost rock.

The following interpretation of the trophism of thetrace makers and of the ecological significance of thedifferent Thalassinoides forms are based on the modelproposed by Nickell & Atkinson (1995; fig. 3, p. 195)for modern Thalassinoides-like burrows, considering themain features common for both, modern and fossil bur-rows. The 12 diagnostic features suggested by Nickell andAtkinson (1995) and their ecological significance are herebriefly examined.

1) Surface mounds. They are produced by the trace-maker removing sediment from the burrow and transport-ing it to the surface. Surface mounds formed after theconstruction of the burrow are indicative of deposit feed-ing trophic mode. Cone-shaped, muddy mounds, locallycrossed by narrow vertical shafts are represented by afew specimens in the Rotzo Member and are completelyabsent in the Scaras Formation, maybe due to the higher

intensity of bottom currents.2) Horizontal mazes (it replaces the tightly layered

lattice feature of Nickell & Atkinson, 1995). They sug-gest the intense exploitation of the sediment by a deposit

feeder. Although all the studied Thalassinoidesare hori-zontally developed, this feature is particularly character-istic of Thalassinoidestype II, forming geometrical mazeswidely developed at the base of the beds. As suggested bySuchanek et al.(1986), laterally extended mazes wouldallow the trace makers to maximize the capture of organicmaterial when the substratum has a low nutritional value.

This probably could be related to different rates of nutri-ent input within the sediment in the various phases ofevolution of the lagoon.

3) Deep burrows. Depth of the burrow developmentwithin the substratum can indicate the importance of sur-face sediments as a nutritional source for the trace-maker.The deeper the maze is developed within the sediment, theless the trace maker depends directly on organic materialsand current destruction present on the seafloor. Due to theabsence of vertical tunnels and to the vertical distributionof the different Thalassinoidestypes within the beds, wehave hypothesized that the studied Thalassinoides, and inparticular the smallest forms, were probably developed

at a shallow depth within the substratum. This wouldindicate a quite strong dependence of the trace makers onorganic material present on the seafloor (surface depositfeeders or omnivorous scavengers). We must consider thepossibility that parts of burrows were eroded by bottomcurrents and depth of burrowing within the substratumcould have been therefore underestimated. This consider-ation is particularly important in the Serra Gelada section,where burrows are developed in a relative high-energyenvironment.

4) Sub-circular tunnel cross section. According toNickell & Atkinson (1995), sub-circular tunnels occur inestablished burrows, after the phase of exploitation, andare the result of crustacean movement. Oval or irregu-lar shapes are less suitable for efficient water flow thana perfectly circular cross-section (Nickell & Atkinson,1995). The narrowing in cross section, as occurs in someThalassinoides? isp. (such as in some modern burrows ofstomatopods), is another characteristic which is not idealto water flow. According to this hypothesis, the occupantof the burrow would be strongly dependent on the sur-face sediments as a nutritional source and less to currentflows. With the exception of Thalassinoides type IV, allthe studied Thalassinoidesboth in the Calcari Grigi andin the Scaras Formation have a sub-circular cross sectionor narrowing, thus confirming the hypothesis suggestedby the other characters that the primary nutritional sourcewas the organic material present on the seafloor ratherthan that transported passively by current flow within thetunnels.

5) Chambered burrows. Chambers can have differentpurposes: they can be used for sub-surface deposit feed-ing or to store coarse grained material and fragmentedbioclasts, found within the substratum or coming from thesea-floor. Another purpose maybe the change the direc-tion of crustacean by 180, by performing a somersaultfollowed by a roll around the body axis, as observed inmodern examples of thalassinoideans. Therefore, the sub-spherical shape of chambers maybe important to recognizebehaviour patterns, in the relationship with charateristics

of the substrate (Monaco & Giannetti, 2002). In Thalassi-noidestype III, type IV and secondarily in Thalassinoidestype II of the Calcari Grigi Formation and type B and C ofthe Scaras Formation, turning chambers are particularly


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developed and large. Stored coarse grained material suchas fragments of crinoids, coated grains and large bivalves,algae and foraminifers were sometimes observed, in par-ticular in Thalassinoidestype IV.

6) Organic detritus in burrows. Organic detritus in bur-rows can be the result of scavenging on the surface, butalso of passive falling of material within an open burrow.

In the fossil record, the different origin of organic detri-tus can be sometimes identified through taphonomic andsedimentologic criteria. In modern Thalassinoides-likeburrows, organic material is also used by the crustaceanto cultivate micro-organisms for nutritional purposes.Organic detritus in burrows is particularly abundant inThalassinoidestype III and type IV.

7) Oblique tunnels were very rarely recorded in thestudied sections (in some Thalassinoides? isp.) and there-fore they are not considered here.

8) Burrow openings, tubular tempestites. In the studiedsections, burrow openings can be directly observed onlyrarely. Their existence can be deduced by the presence

of tubular tempestites inside the bed, but their number,dimensions and density cannot be defined. The presenceof many burrow openings would indicate a continuousand necessary contact of the trace maker with the seafloor,for deposit feeding or scavenging.

9) Funnel-shaped openings. According to Nickell &Atkinson (1995), the presence of funnel-shaped openingswould facilitate the capture of prey or any other nutri-tional particles, as they fall down to the sides of the funneland into the burrow. In the studied section, this feature isrepresented only by a specimen from the Rotzo Member.The funnel is asymmetric in shape, the back being nearlyvertical and the front being flattened probably due to thecontinuous movement of the trace maker. The bottom ofthe funnel is connected to a short vertical tunnel, which isnot directly related to a preserved Thalassinoidesmaze.

10) Narrow exhalant shaft. The presence of a narrowexhalant shaft indicates the necessity of current genera-tion. This feature was observed only in one cone-shapedmound in the Rotzo Member, but its origin is uncertain.

11) U- or Y- shaped burrows in vertical section. Thisfeature, typical of many modern thalassinoidean burrow sys-tems (Bromley, 1990), was not observed in the studied fossilThalassinoidesand therefore it is not considered here.

12) Circular tunnel cross section. Their presencewould facilitate the water flow within and through theburrow. Due to the importance of a constant water flowthrough the burrow, circular tunnel cross sections arepresent also in burrows produced by primarily depositfeeders and therefore this cannot be considered diag-nostic of a particular trophic mode (Nickell & Atkinson,1995). In the studied sections this feature is only rarelyrepresented in Thalassinoides type II and in the largestThalassinoides type III, mainly where mazes are wellpreserved and regularly developed.

In summary, features representing sediment processingand storage of material for nutritional and maintenancepurposes are the most common in the Thalassinoidesofthe Rotzo Member.

The only characteristic shared by the five Thalassi-

noides forms is the horizontal development of the bur-rows. Branches with a sub-circular section are typicalof Thalassinoidessuevicus type I, type II, and type III.Enlarged chambers are absent in Thalassinoidessuevicus

type I and Thalassinoides? isp., present in type II and typeIII and well developed in type IV.

According to the scheme proposed by Nickell &Atkinson (1995), the most commonly observed featuresin all the Thalassinoidesof the Calcari Grigi Formationindicate intense sediment processing. In Thalassinoidessuevicus type II, type III, and especially in the type IV,

storage of organic material (chambered burrows andorganic detritus in burrows, characters 5 and 6) is alsoevidenced. The presence of organic detritus is representedby coarse-grained to fine-grained crushed bioclasts withinThalassinoidessuevicus type II, type III, and type IV. Itindicates both biogenic sorting and storage of bioclas-tic debris coming from the seafloor and fallen down ortrasported into the burrow. The local concentration ofcoarse-grained fragmented shells in ThalassinoidestypeIV, is probably due to the need of the trace maker to main-tain other parts of burrow free from debris.

Vertical tunnels and apertures on the seafloor (char-acters 8, 9 and 12) are very rarely observed and it is

sometimes impossible to state if vertical tunnels are notpreserved due to sedimentologic or diagenetic processesor if they were really poorly developed in the burrows.Actually, we have no evidences of deep burrows with aU- or Y- shaped vertical morphology. We have hypoth-esized, therefore, that mazes were burrowed on horizontalplanes only at shallow depth in the substratum. The fewevidences of vertical tunnels and of exhalant shafts aredifficult to relate to Thalassinoidesmazes. The presenceof tubular tempestites inside the beds proves the existenceof direct apertures on the seafloor, even if only the hori-zontal part of the filled burrows can be usually observed.The only clearly observable vertical element has a circu-lar section (character 12), but it is an isolated feature. Thistunnel closely resembles the burrows of modern crusta-ceans and therefore we have considered it separately. Inthe category of the vertical elements, we have consideredalso inhalant and exhalant shafts, represented in the RotzoMember only by an unclear example. Actually, shaftsare so small that they could be produced also by worms.These rare vertical elements indicate the presence of sus-pension feeders, whose primary nutritional source is thewater column, and of surface deposit feeders. Accordingto the scheme proposed by Nickell & Atkinson (1995), allthe studied Thalassinoidesbelong to the category of thesub-surface deposit feeders (searching for organic parti-cles within the substratum) and secondarily to that of theomnivorous scavengers (Vaugelas, 1990), which ingestorganic debris present on the seafloor and deriving fromother animals and algae.

Similar interpretations can be proposed for the Tha-lassinoides studied in the Scaras Formation, even ifonly four of the twelve characters described by Nickell& Atkinson (1995), namely those related the horizontaldevelopment of the burrow (characters 2-5), are wellpreserved. The problem concerns the vertical develop-ment of the burrows, because we have neither directnor indirect proofs of the existence of vertical tunnels(except Ophiomorpha specimens of calcarenites at thetop of parasequences of Serra Gelada, see Monaco et

al., 2007) and of the type and number of apertures onthe seafloor. Anyway, we can hypothesize the pres-ence at least of some vertical tunnels linking the big-gest Thalassinoides, placed in the deepest tiers within


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the sediment, to the seafloor. Intense bioturbation ofthe more superficial tiers probably obliterated them.

All the studied Thalassinoidesare horizontally devel-oped (horizontal mazes, character 2) and show a sub-cir-cular cross section (character 4), both characters indica-tive of intense sediment processing.

Presence of enlarged chambers (as in Thalassinoides

suevicus type B and in Th.suevicus type C, character5) could indicate storage of organic material for feed-ing purposes or changes in the direction of locomotion.Concentration of skeletal remains (character 6) is rarelyobserved, due to the high bioclastic content of the hostrock and of the burrow filling. Because of the absence ofvertical tunnels, we cannot formulate hypotheses aboutconditions of irrigation and importance of the watercolumn for nutritional purposes (characters 7-12). It isevident that burrowers were closely dependent on nutri-ents present within and on the substratum as the main

feeding source. Therefore, they can be included in thetrophic categories of the sub-surface deposit feeders andsecondarily in those of the surface deposit feeders and ofthe omnivorous scavengers.

This is only a first step in the ecological analysis of theThalassinoidestrace fossils and of the trophism of theirtrace-makers. However, an approach which considers the

functional morphology of the parts making up modernThalassinoides-like burrows can be useful for understand-ing of the ecological meaning of ancient burrows producedby extinct crustaceans and can give further informationfor a more complete paleoenvironmental reconstruction(Monaco et al., 2007). However, this method has to berefined, elaborating a detailed model which compares dif-ferent types of Thalassinoidesin different environmentalcontexts and considering also the functional morphologyof modern branchedburrows produced not only by crus-tacean decapods but also by other organisms.

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Bromley R. G. C., 1990 Trace fossils, biology andtaphonomy. Special topics in paleontology. UnwinHyman Ltd, London: 1-280.

Caracuel J. E., Monaco P., Ybenes, A. & Giannetti A.,2002 Trazas afines aImbrichnus wattonensisHallamde edad Albiense en el Prebtico de Alicante (SerraGelada)Geogaceta, Zaragoza, 31: 171-174.

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shell concentrations of Lithiotis facies enhanced bymicrostratigraphy and taphonomy, Early Jurassic,Gray limestones Formation, Trento area (NorthernItaly). GeoResearch Forum, Zurich, 6: 473-485.

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Monaco P., Caracuel J. E., Giannetti A., Soria J. &Ybenes A., 2007 Thalassinoidesand Ophiomorphaas cross-facies trace fossils of crustaceans from shal-low-to-deep-water environments: Mesozoic and Terti-ary examples from Italy and Spain. In: 3rdSymposiumon Mesozoic and Cenozoic Decapod Crustaceans.Museo di Storia Naturale di Milano, May 23-25, 2007.A. Garassino, R. M. Feldmann & G. Teruzzi (eds.).Memorie della Societ italiana di Scienze naturali edel Museo civico di Storia naturale di Milano, Milano,XXXV (II): 79-82.

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52 EDITED BY ALESSANDRO GARASSINO, RODNEY M. FELDMANN & GIORGIO TERUZZI

Alice Giannetti - Geologisches Institut, Bonn Universitt, Nuallee 8, 53115 Bonn, Germany.e-mail: [email protected]

Paolo Monaco - Dipartimento Scienze della Terra, Universit Perugia, piazza dellUniversit, 06100 Perugia, Italy.e-mail: [email protected]

Jess E. Caracuel - Dpto. Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain.e-mail: [email protected]

Jess M. Soria - Dpto. Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain.e-mail: [email protected]

Alfonso Ybenes - Dpto. Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain.

e-mail: [email protected]

3rdSymposium on Mesozoic and Cenozoic Decapod Crustaceans - Museo di Storia Naturale di Milano, May 23-25, 2007Memorie della Societ Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano

Volume XXXV Fascicolo II

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Gelada. Cuadernos de Geografa de la Universidad deValencia (60): 201-222.Zempolich W. G., 1993 The drowning succession in

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