SKELETAL STRUCTURE, GROWTH, AND PALEOECOLOGY OF THE PATCH REEF-BlJILDING POLYCHAETE WORM DIPLOCHABTETES MEXICANUS WILSON, 1986 FROM THE OLIGOCENE OF BAJA CALIFORNIA (MEXICO) by RUDOLF FISCHER*, CARLos GALLI OLIVER** &JOACHIM REITNER*** ABSTRACT Worm reeflets discribed here are composed of parallel and straight, curved, or bifurcated tub es, which are up to 10 cm long and have an average diameter of 1.4 mm. Their walls are composed of microgranular calcite and are multilamellar. The reef-builders settled on soft bottoms within the tidal or upper- most zone. They tolerated high water turbulence and a certain rate of sedimentation. Dead parts of the reef were recolonlzed by a new population. The reefs stabilized the sediment, produ- ced biogenic debris, and offered new ecological niches for en- do- and epilithic organlsms. They mark a regional ecologlcal event at the Oligocene/Miocene boundary. Diplochaetetes mexicantlS as weil D. longituhtls demonstrate e10se relation- ships to a modern skeletal-forming polychaete \Vorm, Dodeca- r/(I. Dlplochaetetes Is definltly not a selerosponge and de- monstrates a convergence with the chaetetid basal skeletons of sponges and favositld corals. REsUME Le ver polycMte Diplochaetetes mexicantlS edifie de petits r~cifs sur des substrats sableux de la zone intertidale basse. Ces r~clfs se composent de tubes de forme drolte, courbee ou ramifiee pouvant attelndre 10 cm de long. Les parois des tubes sont constltuees de calcite microgranuleuse. Les constructeurs des recifs tol~rent une haute turbulence de I'eau et un certain taux de sedimentation. Les recifs stabilisent les sediments et offrent de nouveaux milieux de vie po ur des organismes epi- et endolithlques. Hs Indlquent un ev~nement ecologique regional a la limite Oligoc~ne/Mloc~ne en Basse Californie et peut-~tre aussi en Californie. Diplochaetetes mexicantls ainsi que D. /ongituhus presentent des similitudes avec le ver polych~te ac- tuel Dodecaria qul possMe egalement un squelette basal cal- calre agglutlne. Le genre Diplochaetetes n'est pas un spon- giaire coralllen. Son squelette basal chaetetide represente une convergence avec celul de certains spongiaires (p.ex. Acantho- chaetetes) et des coraux favosltldes. ZUSAMMENFASSUNG Der polychäte Wurm Diplochaetetes mexicantls baut auf sandigem Substrat des Gezeiten-Nledrigwasser-Bereichs kleine Riffkörper auf. Sie werden aus bis zu 10 cm langen Röhren, die gerade, gebogen und verzweigt sein können, zusammengesetzt. Die Röhrenwand besteht aus mikrogranulärem Kalzit. Die Riff- bildner tolerieren hohe Wasserturbulenz und eine gewisse Se- dimentüberdeckung. Die Riffe sind Sedimentstabilisatoren und Lebensraum für Epi- und Endolithen. Sie zeigen ein regionales ökologisches Ereignis an der Wende Oligozän/Miozän In Nle- derkallfomlen und womöglich auch in Kalifomien an. Diplo- chaetetes mexicantls als auch D. longituhus zeigen Überein- stimmungen mit dem rezenten polychätem Wurm Dodecaria, der ebenfalls kalkige, agglutinierte Basalskelette aufweiSt. Das Genus Diplochaetetes ist kein coralliner Schwamm. Das chae- te tide Basalskelett ist konvergent zu den chaetetiden Basalske- letten einiger Spongien (z.B. Achtlntocpaetetes) und dem favo- sitider Korallen. , '. f KEY-WORDS : POLYCHAETA, AGGLUTINATED WORM TUBES, "CHAETETIDA", PATCH REEFS, PALEOECOLOGY, OLIGOCENE, SAN GREGORIO FORMATION BAJA CALIFORNIA, MEXICO. MOTS.CLEs : POLYCHAETA, TUBES DE VERS AGGLUllNEs, "CtIAETETIDA", BIO·CONSTRUCTIONS, PALEOECOLOGIE, OLIGOcI!NE, FORMATION SAN GREGORIO BAJA CALI· FORNIE, MEXIQUE. • Prof. Dr. Rudolf Fischer, Institut ffir Geologie und Paläontologie der Universität Hannover, Callinstr. 30, D-3000 Hannover I. .... Dr. Carlos Galli Oliver, Geologia Manna, Univ. auton. de Baja California Sur, Apdo 319·B, MX·23080 La Paz, B.C.S., Mexico . ..... Dr.Joachim Reitner, FU Berlin, Institut für Paläontologie, Schwendenerstr. 8, D·1000 Berlin 33. Geobios, n° 22, fasc. 5 p. 761-775, 2 Hg. , 4 pI. Lyon, decembre 1989 - 762 - INTRODUCTION Between La Paz and a point approximately 60 km farther north, the Oligocene San Gregoiro Formation crops out near the coast of BaJa California (fig. 1). The sequcncc is characterl- zcd by conglomcratcs, siltstoncs, and coarse sands tones Inter- caleted with layers of oolithic phosphorites. Close to the top of the San Gregorlo Formation, probable of Late Oligocene (Apple- gate 1986), only a fcw meters below the youngcr San Isldro Formation, there are smaIl reeflets embedded In the conglome- rates and coarse sands tones (flg. 2), wh ich Wilson 1986 re- gards as belng formed by sclerosponges. Very similar fossils have been reported from the San Lorenzo Series (Eocene to Fig. I - Sketch map of the area of investigation in the southern part of t11e Californla Peninsula. . Carte-eroquls de la region etudlee dans la partie sud de la Penln- sule callfornlenne. Fig. 2 - Stratigraphie seetIon of the San Gregorlo Formation near the Bay of La Paz, BaJa Callfornla. Coupe stratlgraphlque de la Formation San Gregorlo pr~ de la Bale de La Paz, BaJa Callfornla. Ollgocene) of Callfornla (U.S.A.) under the name of "Serpti/a rectiformis n.sp." (CLARK, 1918). Reccnt studies and newly co 1- lected material extend our knowledge about the skeletal struc- ture and growth of the reef formlng organisms, their systematic position, and their habitat and paleoecology. Recently, Applegate (1986) proposed new statrigraphic names and units for the phosphorltic series and the overlaying marine sediments. The "San Gregorio Formation" of our paper colncides wlth the "San HIlario Member" and the "San Isidro- Formation" with the "Cerro Colorado Member" of Applegate "EI eien Formation". ~ • u. .. c c 0 .. 0 u ;; c: 0 0 i' '0 C !Zi ~ 80 e Vl Z c w .~ 30 L "i w ~ c: o c u 0 1. <.:> ~ .~ u • ~ 0 Oi Z 20 Vi o Oi