Mark A. S. McMenamin

A fossil chimaeroid tooth plate (Ischyodus sp.) from the Cretaceous of Frankstown, Mississippi shows exceptional preservation of tritor microstructure. Diagenetic loss of the petrodentine/whitlockin in this specimen has revealed the ultrastructure of the vascular tubule networks of tritor interior structure. The observation that vascular tubes running through the hard tissues of chimaeroid tritors host odontoblast cells supports the hypothesis of an odontoblastic origin for chimaeroid tritor tissue.

Late Cambrian stromatolites (Cryptozoön; Little Falls Formation, Late Cambrian [ca. 490 Ma], Herkimer County, New York, USA) from the classic Herkimer “diamond” locality host an accumulation of microcoprolites (Coprulus oblongus) in a cryptic cavity between stromatolite columns. The cryptic cavity space is partly delineated by a region of overhanging stromatolitic laminae, a phenomenon known to occur in New York Cryptozoöns. The same rock slab hosts tiny burrows, some of which are partly filled with rounded quartz grains that were introduced into the burrow as backfill sediment. The association of pellet accumulations and microburrows demonstrates that the New York Cryptozoön community had a significant component of small metazoans, comparable to the microburrow nests reported from early Cambrian archaeocyath-calcimicrobe communities. Pervasive burrowing in selected laminae of the New York Cryptozoön communities indicate that these organo-sedimentary structures are best considered as microbial-metazoan constructions or benthic microbial-metazoan communities (BMMC). This result has implications for understanding the origin of animals. Controversial structures in the 890 Ma Little Dal Group, northwestern Canada, are directly comparable to the vermiform structures observed in the New York Cryptozoöns. Coprulus of comparable age (800-900 Ma, Baish Group, Saudia Arabia) was reported in the 1980s. Metazoan may thus date back to the time of Rodinia.

A silicified, thick-shelled, smooth-surfaced nuculanoid bivalve has been recovered using acid maceration of the Late Triassic (Carnian–Norian) strata of the Luning Formation, Nevada. Comparable modern nuculanoid clams inhabit water depths from 525 to 2562 m, and the living clam (an undescribed species of Pseudoneilonella from Caleta Sierra, Coquimbo, Chile) most similar to the fossil lives at 878–933 m. The Triassic nuculanoid clam (possibly a neilonellid) is inferred here to have inhabited marine waters at approximately 1000 m deep during the deposition of the Shaly Limestone Member of the Luning Formation. The acid maceration sample also produced a silicified specimen of an abyssochrysoid gastropod. The most similar living species to the fossil snail is Abyssochrysos brasilianus, an abyssochrysoid known to occur in water depths from 1540 to 620 m. This depth range also suggests an approximate 1000 m depositional depth for the silicified fossil-producing acid maceration sample from the Luning Formation. These new fossil discoveries falsify hypotheses that the ichthyosaurs (Shonisaurus popularis) of Berlin-Ichthyosaur State Park, Nevada, USA, were deposited, respectively, in either shoreline deposits or in strata that accumulated above the storm wave base. Evidence is also presented here for the existence of a giant Triassic cephalopod that, by comparison with the modern Mesonychoteuthis, preferred water depths of approximately 1000 m.

An enigmatic bilaterian from the Chengjiang biota, _Shenzianyuloma yunnanense_, is assigned to an enigmatic Cambrian group, the vetulicolians. In _Shenzianyuloma_, a clear notochord can be observed, as can cone-in-cone myomeres. Unlike other vetulicolians and _Yunnanozoon_, _Shenzianyuloma_ lacks arthropod-like segmentation in the posterior part of its body. Removal of matrix from the _Shenzianyuloma _holotype has revealed a broader, more laterally flattened tail. Also present in the freshly prepared area is evidence that _Shenzianyuloma_ possessed a possible ventral fin ray box, comparable to that of _Branchiostoma_, implying that _Shenzianyuloma_ had an even greater top-to-bottom tail width than hitherto known. A new tail reconstruction for _Shenzianyuloma_ shows greater lateral area than in previous reconstructions, indicating a more efficient posterior organ of propulsion. A possible dorsal fin spine can also now be recognized in _Shenzianyuloma_. The revised reconstruction of _Shenzianyuloma_ provides insight into early chordate fin evolution. Early bilaterians such as yunnanozoans and _Shenzianyuloma _are crucial for assessing the morphological innovations, and combination of traits, associated with the appearance of vertebrates.

A silicified, thick-shelled, smooth-surfaced nuculanoid bivalve has been recovered by acid maceration of the Late Triassic (Carnian-Norian) strata of the Luning Formation, Nevada. Comparable modern nuculanoid clams inhabit water depths from 525-2,562 meters, and the living clam (an undescribed species of Pseudoneilonella from Caleta Sierra, Coquimbo, Chile) most similar to the fossil lives at 878-933 m. The Triassic nuculanoid clam (possibly a neilonellid) is inferred here to have inhabited marine waters at approximately 1000 m deep during deposition of the Shaly Limestone Member of the Luning Formation. This new fossil discovery falsifies hypotheses that the ichthyosaurs (Shonisaurus popularis) of Berlin-Ichthyosaur State Park, Nevada, USA, were deposited, respectively, in either shoreline deposits or in strata that accumulated above storm wave base.

Reports of diverse vermiform and peloidal structures in Neoproterozoic to Mesozoic open marine to peritidal carbonates include cases interpreted to be keratose sponges. However, living keratose sponges have elaborate, highly elastic skeletons of spongin (a mesoscopic end‐member of a hierarchical assemblage of collagenous structures) lacking spicules, thus have poor preservation potential in contrast to the more easily fossilized spicule‐bearing sponges. Such interpreted fossil keratose sponges comprise diverse layered, network, amalgamated, granular and variegated microfabrics of narrow curved, branching, vesicular–cellular to irregular areas of calcite cement, thought to represent former spongin, embedded in microcrystalline to peloidal carbonate. Interpreted keratose sponges are presented in publications almost entirely in two‐dimensional (thin section) studies, usually displayed normal to bedding, lacking mesoscopic three‐dimensional views in support of a sponge body fossil. For these structures to be keratose sponges critically requires conversion of the spongin skeleton into the calcite cement component, under shallow‐burial conditions and this must have occurred prior to compaction. However, there is no robust petrographic–geochemical evidence that the fine‐grained carbonate component originated from sponge mummification (automicritic body fossils via calcification of structural tissue components) because in the majority of cases the fine‐grained component is homogenous and thus likely to be deposited sediment. Thus, despite numerous studies, verification of fossil keratose sponges is lacking. Although some may be sponges, all can be otherwise explained. Alternatives include: (i) meiofaunal activity; (ii) layered microbial (spongiostromate) accretion; (iii) sedimentary peloidal to clotted micrites; (iv) fluid escape and capture resulting in bird's eye to vuggy porosities; and (v) moulds of siliceous sponge spicules. Uncertainty of keratose sponge identification is fundamental and far‐reaching for understanding: (i) microfacies and diagenesis where they occur; (ii) fossil assemblages; and (iii) wider aspects of origins of animal clades, sponge ecology, evolution and the systemic recovery from mass extinctions. Thus, alternative explanations must be considered.

Permodontodus waurikensis n. gen. n. sp. is a small bony fish, characterized by symmetrically-placed dermal odontode pads, that inhabited Permian freshwater mudflat and seasonal pond-dominated continental lowland depositional environments in Oklahoma, USA. Lower jaw rami have been recovered from three individuals, and they present a type of dentition that appears to be previously unknown among bony fish. The broad, ridged surface of the tooth plate approaches the loxodont condition and may have functioned to scrape algae and other organisms from hard surfaces. The presence of a Meckelian groove suggests that Permodontodus waurikensis n. gen. n. sp. may have been a sarcopterygian.

Silicified ostracods from the Tin Mountain Limestone provide new information regarding the Carboniferous paleontology of the Death Valley Region, California, USA. Acid maceration of marine limestones yielded the following ostracods: Acratia spp., Bairdia quasilecta, Bairdia sp. cf. B. orientalis, Ceratobairdia sp., Kirkbya panamintensis sp. nov., Rectobairdia sp. cf. R. legumen, and Silenites sp. This is the first report of Ceratobairdia and Silenites from the Tin Mountain Limestone. These ostracods occupied a carbonate ramp environment that formed during a major Paleozoic transgression. The ostracods played an important paleoecological role, likely as benthic marine scavengers in a shallow marine biotope along the northern shores of Pangea.

Silicified ostracods from the Lower Carboniferous (Lower Tin Mountain Limestone; Kinderhookian-early Osagean; 350-358.9 Ma) of Lost Burro Gap, Death Valley region, Inyo County, California, USA augment the diversity of Paleozoic ostracods of western North America. Acid maceration of pelmatozoan, micritic and silicate clay-rich micritic marine limestones yielded the following palaeocopid and podocopid ostracods: Acratia spp., Bairdia quasilecta Bushmina, 1975, Bairdia sp. cf. B. orientalis, Ceratobairdia sp., Kirkbya panamintensis sp. nov., Rectobairdia sp. cf. R. legumen, and Silenites sp. This is the first report of Ceratobairdia and Silenites from the Tin Mountain Limestone. These ostracods occupied a Panthalassan carbonate ramp environment, and represent part of a fauna that was widespread in shallow marine waters of Panthalassa.

Early Jurassic pterosaur from Massachusetts