Richard G. Bromley & Kurt S. S. Nielsen

Annales Societatis Geologorum Poloniae (2015), vol. 85: 453–455. doi: http://dx.doi.org/10.14241/asgp.2015.033

BIOEROSIONAL ICHNOTAXA AND THE FOS SIL IZA TION BAR RIER

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RÝnnevej 97 3720 Aakirkeby, Den mark

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LyshÝj alle 15 2500 Valby Co pen ha gen, Den mark; e-mail: knieslen@ya hoo.dk

Bromley, R. G. & Niel sen, K. S. S., 2015. Bioerosional ichnotaxa and the fos sil iza tion bar rier. Annales Societatis Geologorum Poloniae, 85: 453–455.

Ab stract: For the es tab lish ment of a new ichnogenus or ichnospecies, the type ma te rial shall be fos sil, not unfossilized ma te rial. This is not al ways pos si ble, be cause the tran si tion be tween the two states, the fos sil iza tion bar rier, is ex tremely vague de fined. In most fos sil ma te rial, this is not a prob lem. How ever, in the case of bioerosion struc tures (bor ings, rasp ing traces, at tach ment scars in hard sub strates), the prob lem is se ri ous. For ex am ple, when does a sponge bor ing in an oys ter shell be come fos sil ized? The ques tion arises when Re cent and sub-Re cent ma te ri als are con sid ered. Two ex am ples are dis cussed. (1) Microborings are de scribed and named in foraminifera dredged from the sea floor. In this ma te rial, it is not pos si ble to dis tin guish be tween “fos sil ized” and “unfossilized” foraminifera. Bioturbation and other pro cesses may have mixed re cently dead, Pleis to cene and older foraminifera in the sea-floor sed i ments. (2) Small, char ac ter is tic bor ings are made by slip per lim pets in pagurized gas tro pod shells. The struc tures would con sti tute a new ichnospecies of Oichnus, but these bor ings have not been found in “fos sil ized ma te rial” and the bor ings there fore re main name less. Be cause bioerosion struc tures con sti tute “ready-made fos sils”, it is sug gested that the on set of fos sil iza tion be equated with the death of the bioeroding tracemaker.

Key words: Fos sil ized, unfossilized, bioerosion, trace fos sils, microborings. Manuscript re ceived 21 October 2014; ac cepted 11 September 2015

IN TRO DUC TION

Since its sec ond edi tion, the In ter na tional Code of Zoo -log i cal No men cla ture has only al lowed the use of names for mod ern traces es tab lished prior to 1931 (Stoll et al., 1964; Ride et al., 1999, Art. 1.2.1). The no men cla tur al re stric tion of Art 1.2.1 in the ICZN ap plies for the es tab lish ment of new names only, i.e., for the es tab lish ment of a new ichno-ge nus or ichnospecies, the type ma te rial shall be fos sil ized, not unfossilized ma te rial. Even so, Bertling et al. (2006) rec om mended the use of ex ist ing and well de fined trace fos -sil names (based on fos -sil ma te rial) for “mod ern” traces, fol low ing Bromley and Fürsich (1980) and Bromley (1990). Hence, it is up to the re search com mu nity to ac cept nam ing of mod ern traces within al ready ex ist ing trace fos sil names. The rea son for ex clud ing unfossilized traces is that tracemakers make traces in a wide va ri ety of sub strates, rang ing from unpreservable via rarely pre serv able to em i -nently pre serv able. Birds dis turb air and fish wa ter in ways com pa ra ble to the dis tur bance a rab bit pro duces in sand. Nam ing unpreservable struc tures would pro duce a pleth ora of use less names (Seilacher, 1953).

THE FOS SIL IZA TION BAR RIER

The above sit u a tion ap pears to be clear and straightfor -ward, but in fact con tains a se ri ous flaw: it is im pos si ble to

de fine the fos sil iza tion bar rier. Dif fer ent dic tio nar ies de fine “fos sil” in very vague terms, em pha siz ing dif fer ent as pects: “usu ally pre his toric” (McIntosh & Friedrichsen, 1971), “be -fore the be gin ning of his toric time” (Bates and Jack son, 1984), “dug out of su per fi cial de pos its” (Tweney and Hughes 1968), “whether con sol i dated or not” (Nel son and Nel son, 1967). “Fos sil” and the fos sil iza tion bar rier re main un de fined (see Bromley 1990 for fur ther dis cus sion).

ICHNOTAXA FOR BIOEROSION TRACE

FOS SILS

In one group of trace fos sils, those re sult ing from the bioerosion of lithic and skel e tal sub strates, the fos sil iza tion bar rier prob lem may be solv able. The tracemakers in clude pro duc ers of bor ings into the sub strate, or rasp ers scratch ing it surficially, or at tach ing or gan isms etch ing the sur face. The prob lem is solv able here, be cause the sub strate, be ing al ready hard, has a high pres er va tion po ten tial. When the bor ing tracemaker has fin ished its bioerosion, lit tle or no change may oc cur af ter in ter mit tent or fi nal burial. Mod ern and Me so zoic bioerosion can show ex tremely sim i lar modes of pres er va tion. It may be con sid ered that the fos sil -iza tion bar rier co in cides with the death of the bioeroder. Bertling et al. (2006) men tioned this pos si bil ity, but ex pressed doubts about its use ful ness. Nev er the less, two ex

-am ples of bioerosion stud ies are pro vided here to show how this con sid er ation would greatly ben e fit bioerosion stud ies.

EX AM PLES OF BIOEROSION

NO MEN CLA TURE

Microborings in foraminifera in seafloor sed i ments Qua ter nary sed i ments from a num ber of lo cal i ties, both on shore and from the sea floor, have yielded a sur pris ingly var ied se ries of bioerosion traces within the tests of fora-minifera. These have been treated ichnotaxonomically (Fig. 1; Niel sen 1998, 1999; Niel sen and Niel sen 2001; Niel sen et

al., 2002, 2003). The prob lem is that, in the seafloor ma te

-rial, the ages of in di vid ual dead foraminifera are not known. In most forms of these bor ings, the struc ture is re garded as

pred a tory, and so was pro duced dur ing the life of the fora-minifer; but at what time was that?

Ob vi ously, it would be coun ter pro duc tive to leave these var ied struc tures un named, be cause their sta tus rel a tive to the fos sil iza tion bar rier is un known. But if the death of the borer and/or the prey is re garded as the mo ment of fos sil iza -tion, this dif fi culty is over come.

Small slits bored in pagurized gas tro pod shells In coastal wa ters of the Aegean Sea, Greece, gas tro pod shells con tain ing her mit crabs are com monly in hab ited by the small slip per lim pet, Crepidula unguiformis Lamarck. The Crepidula lives sedentarily in the space within the shell, be hind the crab. The lim pet bores a sin gle slit-shaped hole to the ex te rior (Fig. 2) at its an te rior end, pro vid ing

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Fig. 1. Bioerosional traces oc cur ring in plank tonic foraminifera. A. Gen eral view. B. Stellatichnus radiatus Niel sen & Niel sen, 2001.

Fig. 2. Crepidula unguiformis bor ings. A. Crepidula unguiformis bor ings (ar rows) in a pagurized shell, Hexaplex trunculus con tain ing the her mit crab Paguristes oculatus. The smaller bor ings are Entobia isp., the work of a bor ing sponge. B. Two bor ings of Crepidula unguiformis in the shell of Strombus decorus. In the left bor ing, the edge of the shell of the tracemaker is in a char ac ter is tic po si tion (ar -row). Note the heavy scrap ing of the sub strate by hun gry echinoids, pro duc ing the trace fos sil Gnathichnus pentax.

both safety and ac cess to wa ter. A sin gle pagurized shell may con tain nu mer ous in di vid u als of these slip per lim pets.

The bor ing can be re ferred to Oichnus isp. How ever, it has to re main lack ing an ichnospecies des ig na tion, be cause it has not been found in fos sil ma te rial, de spite much search ing. It will be re gret ta ble, if the bor ing can not be given a triv ial name, as it rep re sents an Oichnus with a unique func tion and there fore mer its fur ther re search. How -ever, if the shell is re garded as be com ing fos sil ized on the death of the Crepidula, e.g., when it is col lected, then the prob lem van ishes.

CON CLU SIONS

Trace fos sils may be named only on the ba sis of fos sil -ized type ma te rial. How ever, this is not very sat is fac tory, as the fos sil iza tion bar rier is not de fin able. Nev er the less, there is a pos si ble way out of this prob lem for bioerosional trace fos sils in lithic and hard skel e tal sub strates. In these hard sub strates, lit tle preservational change takes place with burial and time. As a con se quence, traces in woody sub strate should be ex cluded. Bor ings in mod ern shells re sem -ble those in Ce no zoic and even ear lier fos sil sub strates. Thus it is sug gested that the fos sil iza tion bar rier on trace fos sils in lithic and hard skel e tal sub strates may be de fined as the death of the borer.

In bioerosion stud ies us ing mod ern seafloor sed i ments, the age of foraminifera, the sub strates for microborings, is not known. Yet the mode of pres er va tion is closely sim i lar to that of Ce no zoic ma te rial, so in or der that the mi crobor ings can be named, the dead ma te rial is con sid ered to be fos -sil ized.

In the case of small bor ings, re fer able to Oichnus isp. and made by slip per lim pets in mod ern pagurized shells, it is clear that they are not fos sil ized, so pro vid ing them with an ichnospecies name is pro hib ited. A search of Pleis to cene ma te rial has not yielded fos sil ized bor ings of this type. How ever, if the fos sil iza tion bar rier is taken at the death of the borer, the prob lem is solved.

Ac knowl edg ments

The au thors are in debted to Al fred Uchman and Frank Simp-son for help ful sug ges tions. Fur ther, Emma Ruggiero, Jordi Marti- nell and Max Wisshak for their help ful sug ges tion and crit i cal re -view of the manu script

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