Following the logic behind biological interpretations of the Ediacaran biotas

For almost 150 years, megascopic structures in siliciclastic sequences of terminal Precambrian age have been frustratingly difficult to characterize and classify. As with all other areas of human knowledge, progress with exploration, documentation and understanding is growing at an exponential rate. Nevertheless, there is much to be learned from following the evolution of the logic behind the biological interpretations of these enigmatic fossils. Here, I review the history of discovery as well as some long-established core members of widely recognized clades that are still difficult to graft on to the tree of life. These ‘orphan plesions’ occupy roles that were once held by famous former Problematica, such as archaeocyaths, graptolites and rudist bivalves. In some of those cases, taxonomic enlightenment was brought about by the discovery of new characters; in others it required a better knowledge of their living counterparts. Can we use these approaches to rescue the Ediacaran orphans? Five taxa that are examined in this context are Arborea (Arboreomorpha), Dickinsonia (Dickinsoniomorpha), Pteridinium plus Ernietta (Erniettomorpha) and Kimberella (Bilateria?). With the possible exception of Dickinsonia, all of these organisms may be coelenterate grade eumetazoans.

Rudist bivalves (Hippuritoidea) from the Clifton Limestone (Lower Campanian) of western Jamaica and a reassessment of the genus Vaccinites in the Americas

The lower Campanian (Upper Cretaceous) Clifton Limestone of Jamaica yields three species of hippuritid bivalve: Barrettia ruseae CHUBB, Whitfieldiella luceae sp. nov. and Vaccinites vermunti MAC GILLAVRY, and the plagioptychid: Plagioptychus sp. The hippuritids are described in detail using statistics. Barrettia ruseae is demonstrated to be a more primitive species of Barrettia than B. monilifera WOODWARD or B. multilirata WHITFIELD, and the species Whitfieldiella luceae is shown to be a more primitive species of Whitfieldiella than W. gigas CHUBB. The specimens of Vaccinites from the Clifton Limestone are compared with populations of Vaccinites from elsewhere in the Americas, and five species (probably representing a single evolutionary lineage) are recognized: V. alencasteri sp. nov. (?late Turonian-?Coniacian), V. martini MAC GILLAVRY (probably early to mid Santonian), V. macgillavryi PALMER (probably mid to late Santonian), V. vermunti MAC GILLAVRY (earliest Campanian), and V. temazcali sp. nov. (late early Campanian). The Vaccinites species can be distinguished using statistical techniques. The ages of the Clifton Limestone and the five Vaccinites species are reviewed. This research demonstrates the value of using hippuritids for biostratigraphy in the Upper Cretaceous of the Americas.

Convergent and Parallel Evolutionary Traits in Early Cretaceous Rudist Bivalves (Hippuritidina)

Early Cretaceous Hippuritida clades, requieniide (family Requieniidae) and hippuritide (families Radiolitidae, Polyconitidae,Caprinidae, “Caprinulidae” and Caprinuloideidae), show distinctive myophoral arrangements and shell structures. Nevertheless they share some characters, such as the transverse shell thickening of the myophores of the attached valve which are convergent traits in Lovetchenia (Requieniidae) and Homopleura (Monopleuridae). The bent posterior myophore of the right valve of Pseudotoucasia (Requieniidae) closely resemble the posterior myophore of the left valve of Horiopleura and Polyconites (Polyconitidae). The shell cellular structure is one of the key attributes of the family Radiolitidae (e.g.Eoradiolites) but this structure is also present in some advanced Requieniidae (“Toucasia-Apricardia “group). Canaliculate shell structures are convergent evolutionary traits which are common in the Caprinidae and Caprinuloideidae and also exist in the Polyconitidae and “Caprinulidae”. In most of the foregoing canaliculated groups, two trends are well expressed, reflecting parallel evolution: expansion of canals into the entire shell and increasing complexity of canal architecture. Convergent taxa took some advantages by using former innovations. An Albian peak of convergence coincided with the emergence of new clades, which suggests a reset following the mid-Aptian extinction event.

The Hatillo Limestone, Pueblo Viejo district, Dominican Republic: Marginal reef or impermeable cap?

The Hatillo Limestone and the underlying Los Ranchos Formation are exposed over an east-west distance of 100 km in the eastern Dominican Republic. The lowermost portion of the Hatillo Limestone in the Pueblo Viejo district contains a Late Lower Albian fossil assemblage including corals and rudist bivalves indicative of a near-shore reef environment. Diamond drilling in the Pueblo Viejo district and exposures in the open pits show that the Hatillo Limestone conformably overlies the Early Cretaceous Los Ranchos Formation. Volcanogenic massive sulfide beds, exposed in the Moore pit, provide evidence for an Early Cretaceous, syn-mineralization paleosurface. Altered and mineralized clasts in the epiclastic, sedimentary host-rock section at the Pueblo Viejo mine indicate that the ore deposits were open to erosion during hydrothermal alteration and mineralization. The Hatillo Limestone did not overlie the ore deposits during the mineralizing event and, consequently, could not have acted as an impermeable cap to ascending hydrothermal fluids. Intra-oceanic island arc volcanism (Los Ranchos Formation) overlapped at the Aptian–Albian boundary (112 Ma) with a marginal fringing reef (basal Hatillo Limestone). The marginal reef gradually gave way to deeper-water facies as Hatillo Limestone deposition progressed through the middle Albian. Low-angle reverse faulting, penetrative deformation, and metamorphic recrystallization affected the Hatillo Limestone as well as the Los Ranchos and Maimón formations during the Late Cretaceous. Deformation intensity and metamorphic grade progressed from incipient metamorphism in the Pueblo Viejo district to schists in the Maimón Formation to amphibolite near a faulted contact with the Loma Caribe peridotite.

Exceptionally well-preserved silicified hippuritid rudist bivalves from the lower Maastrichtian of Puerto Rico

Exceptionally well-preserved (silicified) hippuritid rudists occur in the El Rayo Formation (lower Maastrichtian) of south-western Puerto Rico. Three species belonging to three different genera are present: Caribbea muellerreidi (VERMUNT), Laluzia peruviana (GERTH) and Parastroma guitarti (PALMER). Acid digestion of the limestones has resulted in a collection with numerous three-dimensional left and right valves many with the preservation of the minute details of the pore system. The morphological features of each species are described, and many features are illustrated for the first time. The new material, coupled with descriptions from other studies, demonstrates that six genera of endemic hippuritids evolved in two separate radiations in the New World: an older radiation of forms that had pallial canals in their left valves (Barrettia, Whitfieldiella and Parastroma) and a younger radiation of forms lacking pallial canals in their left valves (Laluzia, Caribbea and Praebarrettia). The exquisite preservation also reveals that in these endemic New World hippuritids the sockets for the teeth consisted of slots into which ribs on the teeth fitted; this contrasts with Old World hippuritids that have true sockets formed from upfolds of the tabulae for the teeth. The distinctive morphology of the tooth sockets is here used to define a monophyletic subfamily for which the name Barrettiinae CHUBB is available.