Cambrian origin of all skeletalized metazoan phyla—Discovery of Earth’s oldest bryozoans (Upper Cambrian, southern Mexico) Ed Landing1*, Adam English2, and John D. Keppie3 1
New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA Chevron Gulf of Mexico Business Unit, 5750 Johnston Street, Lafayette, Louisiana 70503, USA 3 Instituto de Geologia, Universidad National Automona de Mexico, Ciudad Universitaria, 04510 Coyoacan, D.F., Mexico 2
ABSTRACT Exquisite Pywackia baileyi Landing n. gen. and sp. specimens from the lower Tiñu Formation, southern Mexico, extend the bryozoan record into the Upper Cambrian. They are ~8 m.y. older than the purported oldest bryozoans from South China, and show that all skeletalized metazoan phyla appeared in the Cambrian. The new form differs from similar, twig-like cryptostomes by its shallow autozooecia and an elongate axial zooid, which may be homologous to the stolon in nonmineralized ctenostomes. It may morphologically resemble mineralized stem group bryozoans that retained a stolon-like individual, although an ability to bud was acquired by the feeding individuals (autozooids). The latest Cambrian origin of bryozoans, several mollusk classes (polyplacophorans, cephalopods), and euconodonts was a major evolutionary development and can be considered the onset of the Ordovician radiation of more complex marine communities. INTRODUCTION The Phanerozoic records a series of evolutionary originations, diversifications, and extinctions that led to modern oceanic environments and ecosystems. Perhaps the most intensely dissected of these dramatic biotic diversity changes is the Cambrian evolutionary radiation (e.g., Lipps and Signor, 1992). Definition of a global stratotype for the Cambrian’s base (Landing, 1992, 1994) led to an understanding that the Early Cambrian included an ~24 m.y. interval that transformed marine shelf habitats and communities in a three-stage process (Landing et al., 1989). The first Cambrian stage featured appearance and diversification of metazoans (multicellular animals with tissue-grade organization) with coeloms that produced deep, behaviorally complex burrows. Secondly, the origin and diversification of most modern mineralized metazoans with agglutinated, phosphatic, and calcareous hard parts apparently took place in peritidal habitats vacated by the Ediacaran fauna. These two diversifications form the Placentian Ecologic Evolutionary Unit, which was followed by the full development of the Cambrian Evolutionary Fauna with the offshore, late Early Cambrian origin of trilobites (Landing and Westrop, 2004) (Fig. 1). Sepkoski (1981, 1997) showed that the Early Cambrian was followed by lowered Middle Cambrian–Early Ordovician diversity as high extinction rates depressed genus- and familylevel diversity. But the latter interval did not equate to a time of stasis in higher-level diversity before the radiation of lower-level taxa in the later Ordovician (e.g., Sepkoski, 1995). Indeed, high-level taxa such as the euconodonts, cepha*E-mail:
[email protected].
lopods, and polyplacophorans appeared in the latest Cambrian (Runnegar et al., 1979; Donoghue et al., 2000; Landing and Kröger, 2009). One mineralized group, the phylum Bryozoa, seems to have “missed” the Cambrian radiation. Taylor and Ernst (2004) and Xia et al. (2007) reviewed all purported Cambrian bryozoans
and concluded that the upper Tremadoc of South China has the oldest bryozoans. Xia et al.’s (2007) taxa have complex microstructures, which suggest that these ca. 483 Ma forms (see Landing et al., 1997) had mineralized ancestors. As discussed below, Late Cambrian bryozoans are now known, and have features that suggest they lie near the base of the bryozoan lineage. GEOLOGIC SETTING Landing et al. (2007a) detailed the paleoenvironments, biota, and age of the thin (~72 m) Tiñu Formation in Oaxaca State, southern Mexico. The Tiñu, the only fossiliferous lower Paleozoic unit between northern Mexico and Colombia, nonconformably overlies the middle Proterozoic Grenville orogen and was deposited on the South American margin of West Gondwana (Keppie et al., 2003).
Figure 1. Oldest occurrences (black boxes) of classes or orders (initial capital) of skeletalized Metazoa phyla (all capitals). Porifera, either metazoans or cell aggregates without tissue-grade organization, are included. Archaeocyatha, considered a phylum by some workers, may be poriferans. Anabaritids and Corallomorpha may be cnidarians; dashed line associates them with the Cnidaria. “Conodontimorphs,” polyphyletic macropredators, are with Pseudoconodontida and Protoconodontida of Landing (1995). First-appearance data are in Landing et al. (1989), Lipps and Signor (1992), and Valentine (2004). Global Cambrian subdivisions are from Zhu et al. (2006) and Landing et al. (2007b); geochronology is in Landing et al. (2000); “Ladatheca” is interpreted as annelid in Landing (1993). Lower, Middle, and Upper Cambrian are informal subsystems (Landing, 2007). Dr.—Drumian; Evol.—Evolutionary; L.—Lower; M—Middle; Ord.—Ordovician; Pai.—Paibian Stage; S—Stage; Ser.—Series; s.s.—sensu stricto; Sten.—Stenolaemata.
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. GEOLOGY, June 2010 Geology, June 2010; v. 38; no. 6; p. 547–550; doi: 10.1130/G30870.1; 3 figures; 1 table.
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The lower Tiñu Formation is the Upper Cambrian Yudachica Member, which is unconformably overlain by the Lower Ordovician (Tremadoc) Río Salinas Member. The Yudachica at the southern, Río Salinas section (locality Tu of Landing et al., 2007a) includes tempestite limestones and gray shale deposited on a wavedominated shelf. The coeval, deeper-water Yudachica at Santiago Ixtaltepec (locality SIx of Landing et al., 2007a) has upper slope debris flows, nodular limestone, and distal tempestites. Yudachica Member limestone samples have conodont elements and phosphatic brachiopod valves at both localities, with much phosphatic debris (0.5 kg per 6 kg sample) at locality Tu. This debris is a spongy phosphatic cement with phosphatized polymerid trilobite sclerites, complete agnostids, gastropod and chancellorid (Archiasterella Sdzuy) steinkerns, and echinoderm columnals at locality Tu. Nearly 150 stem-like fragments of a colonial organism occur in three samples in the lower part of the Upper Cambrian Cordylodus andresi Zone of the lower Yudachica at locality Tu (Table 1).
TABLE 1. PYWACKIA BAILEYI N. GEN. AND SP., UPPER CAMBRIAN CORDYLODUS ANDRESI ZONE, LOWER TIÑU FORMATION, RÍO SALINAS SECTION, OAXACA STATE, SOUTHERN MEXICO
Proximal extremity Medial stem fragments Apical ends
Tu-2.0
Tu-4.3
Tu-4.95
12 53 2
3 26 0
2 35 1
Note: Number after hyphen (i.e., 2.0) is meters above middle Proterozoic–Tiñu nonconformity.
TIÑU BRYOZOANS The Yudachica Member specimens do not resemble any known Cambrian fossil, and appear to be phosphate-replaced fragments of tabulate coral or bryozoan colonies (Fig. 2; see detailed description below). The “cup-like” depressions on the fragments have a bilobed keel at the base of the “cup” on most fragments (Figs. 2K and 2L). The keel shows that the “cups” were formed by a bilaterally symmetrical organism, such as a bryozoan zooid—not a biradially symmetrical corallite nor a radially symmetrical hydrozoan. Keels occur in many tubular Bryozoa in the Paleozoic–Mesozoic (Boardman, 1983, p. 116–117) and are unknown in tabulates. Tabulates often have intercorallite pores, and the Tiñu specimen walls are imperforate. The Tiñu specimens lack septa, which may occur in tabulates and characterize zoantharid corals. Further evidence of bryozoan affinities that rules out a coral relationship include the apparent presence of three types of individuals—those that formed the large “cups” (autozooecia), those
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Figure 2. Pywackia baileyi Landing n. gen. and sp.; paratypes from Tu-4.95, unless otherwise indicated. A: Trilobed basal extremity; bilobed medial costa shows elongate, immature zooecium, NYSM 13508, ×26. B: Trilobed basal extremity with apical autozooecia; arrow shows Figure 2C area, NYSM 13509, ×32, Tu-2.05. C: NYSM 13509 detail shows phosphatized, calcite crystallites near arrow, ×250. D: Specimen with horizontal interzooecial walls on midline, NYSM 13510, ×23. E: Small polymorph near arrow, see Figure 2F, NYSM 13511, ×29. F: Polymorph at arrow, NYSM 13511, ×83. G: Low hemiphragms (arrows), NYSM 13512, ×27. H: Thin dissepiments (d) over phosphate-filled zooecia; arrow shows Figure 2I area, NYSM 13513, ×30. I: Figure 2H detail shows bilamellar wall, NYSM 13513, ×125. J: Apical tip, NYSM 13514, ×29. K: Holotype NYSM 13515, bilobed keels on autozooecia midlines, ×33. L: NYSM 13516, bilobed keels above axial zooecium ×29, Tu-4.3.
apparently associated with small depressions at some autozooecial wall corners (Figs. 2E and 2F), and a tube-like axial zooid (Figs. 3A, 3B, and 3D). The smallest polymorphs lie in depressions of the walls and resemble bryozoan nanozooids in size and simplicity (0.03 mm diameter) (Boardman, 1983, p. 100–103). Axial zooids are common in a number of cryptostomate (rhabdomesinid) Bryozoa (Blake, 1983b, p. 536). Other bryozoan characters include presence of an axial individual, a tapered proximal extremity (Figs. 2A and 2B) for substrate attachment, and bilamellar wall construction (Boardman and Cheetham, 1987, p. 512) (Fig. 2I)—all features of trepostomes and cryptostomes. Shallow autozooecia (i.e., apertural diameter about the same as depth) (Figs. 3A and 3B), infrequent diaphragms (Fig. 2H), low hemiphragms (Fig. 2G), and an axial zooid are known in the Cryptostomata Vine, 1884 (see Blake, 1983a, 1983b).
Bryozoans often have a regular linear or helicoid arrangement of autozooecia with fairly constant shape and size. Variable arrangement (linear to slightly helicoid) of the Tiñu autozooecia and their variable size and shape (Fig. 2) may seem to argue against a bryozoan assignment, but irregular aperture size, shape, and arrangement occur in bryozoans (e.g., Shimer and Shrock, 1944, their Plate 96 and Figs. 10, 16, and 17; Boardman and Cheetham, 1987, their Fig. 29D). Thus, variable aperture shape, size, and arrangement cannot exclude the species’ assignment to the Bryozoa. PRESERVATION AND ONTOGENY Mineralized bryozoan zooaria are calcareous. Recovery of the Tiñu specimens was possible as phosphate replaced a calcareous granular or granular-prismatic microstructure, and allows a “brick-like” structure (not a secondary botryoidal phosphate) to show up in refracted
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Figure 3. Pywackia baileyi Landing n. gen. and sp.; paratypes from Tu-4.95 unless otherwise indicated; az—axial zooecium. A: Section with diaphragm in axial zooecium (arrow), NYSM 13517, ×40. B: Section shows two diaphragms in axial zooecium, NYSM 13518, ×42, Tu-2.05. C: Transverse view shows firstgeneration autozooecia (1) and thick end of interzooecial wall (arrow), NYSM 13519, ×40. D: Transverse view with axial zooecium and first-generation (1) and second-generation (2) autozooecia, NYSM 13520, ×30.
light (Fig. 2C). Granular or granular-prismatic microstructure is primary in some Cystoporata (Utgaard, 1983, p. 358) and locally in the generally laminated Cryptostomata (Blake, 1983b, p. 550). This replacement and the phosphate infill of some autozooecia (Fig. 2H) was likely early in diagenesis, as some fragments (Figs. 2A and 2B) are polished—perhaps by wave excavation and movement of phosphatized specimens before final burial. Bryozoans are usually studied in thin section. However, the Tiñu specimens show transverse and longitudinal sections (Fig. 3) that allow diagnosis of Pywackia baileyi Landing n. gen. and sp., and its comparison with other cryptostomes. The specimens record an ontogeny from growth of a terminal extremity (Figs. 2A and 2B), origin of the first autozooecia, growth of a stem with a rounded apex (Fig. 2J), and the budding of autozooecia (Figs. 3C and 3D). DISCUSSION AND CONCLUSIONS Pywackia baileyi from the lower Cordylodus andresi Zone extends the range of the Bryozoa down from the upper Tremadoc (ca. 481 Ma). Biostratigraphy and U-Pb geochronology indi-
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cate how much older the Mexican specimens are than the South Chinese. Shergold (1988, his Fig. 2) correlated the Peltura scarabaeoides Zone of Baltica and Avalon with the TsinainaPtychaspis Zone of South China. Both the Tsinaina-Ptychaspis Zone and lower Cordylodus andresi Zone of the Tiñu correlate with the Proconodontus muelleri Zone or lower Eoconodontus Zone (conodonts) (Chen and Teichert, 1983; Landing et al., 2007a). A 491 Ma U-Pb date on the P. scarabaeoides Zone (Davidek et al., 1998) shows that the Tiñu bryozoans are ~8 m.y. older than the South Chinese, and several million years older than the end of the Cambrian (i.e., 489 Ma; Landing et al., 2000). Bryozoans are an important Paleozoic–Holocene phylum in substrate stabilization, as a food source and major filter-feeding group, as rock formers, and as a component of a new Late Ordovician habitat—animal-constructed reefs. Late Ordovician bryozoan-coral-stromatoporoid reefs were colonized by high-diversity faunas (e.g., Webby, 2002). These reefs replaced earlier, microbially formed, thrombolite reefs (Kröger and Landing, 2009). The Tiñu Formation shows that Bryozoa, as all other mineralized metazoan phyla, had a Cambrian origin (Fig. 1), although Bryozoa formed only small Early Ordovician reefs (e.g., Xia et al., 2007). Late Cambrian bryozoans provide a new insight to the origin of higher-level metazoans. The Middle Cambrian–lowest Ordovician stasis in lower-level metazoan diversity (Sepkoski, 1995) includes a Late Cambrian interval with first appearances of higher-level taxa. These include euconodonts, cephalopods, polyplacophorans, and, now, bryozoans that are important in the complex communities of the Ordovician radiation. The Tiñu bryozoans also suggest insights into the origin of mineralized bryozoans. Phylogenetic analyses (e.g., Taylor and Ernst, 2004, their Fig. 16.2) derive mineralized bryozoans from ctenostomes—unmineralized forms in which a stolon buds to form a loosely integrated colony. In addition to Pywackia baileyi’s early age, it is a simply constructed “stony bryozoan”—a colony dominated by shallow, irregularly shaped and sized autozooecia and an axial zooecium. Polymorphism occurs in the trepostomes and cryptostomes of the upper Tremadoc (Xia et al., 2007), and P. baileyi shows that specialized zooids appeared early in bryozoan phylogeny. Taylor and Ernst (2004) emphasized that a key transition in bryozoan evolution was a change in musculature that everted and retracted the feeding tentacles—from muscles that compressed and relaxed the flexible zooid wall and fluid-filled body cavity in ctenostomes to development of a muscle that attached to an inflexible, mineralized body wall to retract the tentacles. Pywackia baileyi can be regarded as an early stage in “stony
bryozoans” that followed development of a mineralized, inflexible body wall (perhaps as protection from predators or to reduce dehydration and UV damage in peritidal environments) (e.g., Landing and Westrop, 2004) and origin of a retractor muscle. Such an early “stony bryozoan” could have retained an axial zooid as the homolog of the ctenostome stolon. However, limitation of budding to an axial zooid would produce elongate, fragile (brittle) colonies that could not have fed on richer plankton sources in higher-energy environments above the seafloor. One way to reach a higher level above the seafloor would have been to form a stronger, stemlike colony by autozooecial budding to elongate and thicken the zooarium. SYSTEMATIC PALEONTOLOGY Order CRYPTOSTOMATA? Vine, 1884 Family UNDETERMINED Genus PYWACKIA Landing n. gen. Type species. P. baileyi Landing n. gen. and sp., Upper Cambrian Cordylodus andresi Zone, Tiñu Formation (lower Yudachica Member) near Río Salinas, Oaxaca State, Mexico. Diagnosis. Probable cryptostome with stemlike, erect, unjointed, cylindrical zooaria; axial zooecium with widely spaced diaphragms; distally triangular, apically expanding basal extremity; and shallow autozooecia about as deep as aperture width. Autozooecia develop on basal extremity, bud to form six- to twelve-sided zooarium with broadly elliptical to circular cross section and rounded apical end. Autozooecia with roughly linear to helical arrangement and variable outline, common bilobed median keels, rare diaphragms and hemiphragms; autozooecial walls bilamellar, show granular-prismatic microstructure, thicken at outer margin, are imperforate, have apparent small polymorphs at wall intersections. Occurrence. Presently known only from the type locality. Description. Trilobed basal extremity with low expansion angle (5°–16°; Figs. 2A and 2E), may be elongate and lack mature autozooecia (Figs. 2A and 2B), or autozooecia may appear near base (Figs. 2D–2G). Middle zooarium tubular (Fig. 2K), apically tapers (Figs. 2J and 2L), has circular to broadly elliptical cross section (Figs. 3C and 3D). Zooarium likely unjointed as no ball-and-socket terminations recovered. Autozooecia aperture rectangular–rhomboid– hexagonal–lenticular; lack calcified frontal walls (Figs. 2D, 2K, and 2L); autozooecia differ in size in basal and apical parts of zooarium (Figs. 2D, 2E, and 2J), are more uniform in middle part (Figs. 2K and 2L). Zooecia deepest adapically and/or abapically, depth equivalent to or slightly deeper than width (Figs. 3A and 3B). Walls bilamellar (Figs. 2H and 2I), microgranular-prismatic (Fig. 2C), may thicken distally
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(Fig. 3C, arrow), have rare, apparent tiny polymorphs at wall intersections (Figs. 2E and 2F). Low hemiphragms and thin diaphragms rare (Figs. 2G and 2H); bilobed medial keel common (Figs. 2K and 2L). Axial zooecium has widely spaced diaphragms (Figs. 3A and 3B). Autozooids bud to lengthen and broaden zooarium (Fig. 2D, midline; Fig. 3D). Discussion and comparison. Pywackia n. gen. is assigned to the Cryptostomata? based on the presence of taxa with shallow autozooecia and axial zooecia in the order (Blake, 1983b). The assignment means that cryptostomates likely originated by the Late Cambrian. Pywackia differs from similar arthrostylid cryptostomes (see Blake, 1983b) in having very shallow autozooecia, rare hemiphragms, and a prominent axial zooecium with dissepiments. Etymology. Pywackia (L.), named for Pywacket (1992–), EL’s tough, friendly white cat. PYWACKIA BAILEYI Landing n. gen. and sp. Figures 2 and 3 Holotype. NYSM 13515 (Fig. 2K) from sample Tu-4.95 of the lower Tiñu Formation. Diagnosis, description, and discussion. As Pywackia is presently monospecific, the diagnosis, description, and discussion of P. baileyi n. gen. and sp. are the same as for the genus. Etymology. baileyi (L.), named for Bailey (1992–1996), a gentle, tawny cat. ACKNOWLEDGMENTS F. Mannolini photographed specimens with an auto-montage system. All specimens are in the New York State Museum (NYSM) Paleontology Collection. D. Blake gave constructive comments. J. Pachut and two anonymous reviewers are thanked. REFERENCES CITED Blake, D.B., 1983a, The order Cryptostomata, in Boardman, R.S., et al., eds., Part G Bryozoa (revised): Treatise on invertebrate paleontology: Lawrence, Kansas, Geological Society of America and University of Kansas, p. 440–452. Blake, D.B., 1983b, Systematic descriptions for the suborder Rhabdomesina, in Boardman, R.S., et al., eds., Part G Bryozoa (revised): Treatise on invertebrate paleontology: Lawrence, Kansas, Geological Society of America and University of Kansas, p. 550–592. Boardman, R.S., 1983, General features of the class Stenolaemata, in Boardman, R.S., et al., eds., Part G Bryozoa (revised): Treatise on invertebrate paleontology: Lawrence, Kansas, Geological Society of America and University of Kansas, p. 48–137. Boardman, R.S., and Cheetham, A.H., 1987, Phylum Bryozoa, in Boardman, R.S., et al., eds., Fossil invertebrates: Oxford, UK, Blackwell Scientific Publications, 713 p. Chen, J.-Y., and Teichert, C., 1983, Cambrian Cephalopoda in China: Palaeontographica Abteilung A, v. 181, 102 p. Davidek, K., Landing, E., Bowring, S.A., Westrop, S.R., Rushton, A.W.A., Fortey, R.A., and Adrain, J.M., 1998, New uppermost Cambrian U-Pb date from Avalonian Wales and age of the
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