Cambrocyathellus: the double-walled archaeocyath with porous plates

Cambrocyathellus is an early Cambrian archaeocyath with two porous walls and vertical perforated plates between them. The plates, called pseudosepta, cross the intervallum but do not divide it into horizontal floors. Its type species, C. tschuranicus, was described from the Pestrotsvet Formation near Churan on the Lena River; the holotype PIN 1161 is now reported as unlocated. A separate phosphatised fragment preserves part of a wall and probable plate traces, but it is assigned only to Cambrocyathellus sp. These different evidence levels make the genus a revealing entry in the Cambrian animal catalogue.

Quick facts

The missing holotype and the published record

Irina Zhuravleva established Cambrocyathellus in 1960 in her monograph on archaeocyaths of the Siberian Platform. She designated C. tschuranicus as the type species. The name refers to Churan village on the Lena River, where the fossils came from the Tommotian-age Pestrotsvet Formation. The holotype, PIN 1161, was illustrated in transverse section in the original work.

A modern taxonomic account notes that the holotype is currently unlocated. This means that researchers cannot simply request that exact object for new imaging or sectioning. The published illustration and other specimens from the region still document the taxon, but a new fragment cannot be substituted for the name-bearing type. Keeping that distinction visible is important when a later specimen is assigned to the genus.

Walls, pseudosepta and the absence of floors

The cup has an outer and an inner wall. Pores pass through both, and the space between them is crossed by large-pored vertical plates termed pseudosepta. These plates do not join into a regular network of horizontal floors. In other words, the intervallum was not a stack of chambers like a building with storeys. A transverse section can show the two walls and the radial elements; a longitudinal section helps establish that the horizontal tabulae are absent.

The outer wall has simple rounded or polygonal pores that are relatively even in size, while the interior structure carries additional diagnostic features. A single surface view can miss the inner wall entirely. For this reason, the taxonomy relies on sections through the skeleton rather than a generic image of a porous cone. It also explains why a fragment with only one wall can support a genus-level possibility without confirming a species.

A rare phosphatised fragment

One unusually informative piece, SMNH Sp11543, comes from the Tyuser Formation in the lower Lena region. It is a phosphatised fragment rather than a complete cup. Its outer wall contains irregularly placed pores about 80–140 micrometres in diameter. Ridges on the inner surface were interpreted as remains of vertical plate-like structures. The size and arrangement are compatible with Cambrocyathellus.

The authors kept the identification at Cambrocyathellus sp. because the fragment does not preserve the complete combination of inner wall, outer wall and pseudosepta needed to name a species. This restraint is informative. The fossil expands what is known about the genus's wall texture, but it does not prove that the fragment is C. tschuranicus or that every Cambrian wall with similar pores belongs to this genus.

Age and geographic range

The type species is from Tommotian strata in Siberia. Regional summaries place Cambrocyathellus in the Tommotian and lower Atdabanian, intervals correlated with parts of Cambrian Stages 2 and 3. Published global records extend across additional areas, including Mongolia, Kazakhstan and Australia. These ranges combine multiple species and regional identifications rather than one population crossing a continuous sea.

Older Siberian stage names do not line up exactly with the international numbered stages. A record should therefore be tied to the formation and biozone in which it was found. The rare Tyuser fragment is associated with the Dokidocyathus regularis Zone and correlated with the upper part of Stage 2. That contextual information is stronger than assigning an age from the genus name alone.

How it fitted into early reefs

In some Mongolian and Siberian carbonate buildups, archaeocyath cups occur together with microbial carbonate, other sponges and early cement. Cambrocyathellus could grow as an individual cup or as part of closely packed and modular arrangements, depending on the species and setting. The skeleton could create small pockets and surfaces that trapped sediment. Those effects contributed to a community-built framework, not a reef made by this genus alone.

The orientation of cups and their spacing can show whether they grew exposed, crowded or within protected spaces. But the rock may combine successive growth and burial events. A branching pattern in a section describes the mineralised modules; it does not by itself demonstrate how long living tissue connected them or how reproduction occurred.

Filtering is plausible, but the pump is inferred

As in other archaeocyaths, a porous double wall is compatible with water entering the intervallum and passing toward the central cavity. The animal likely captured suspended food with soft tissue lining part of that route. Three-dimensional studies of other archaeocyaths show that skeleton geometry can support active water movement, not merely a passive sieve. Applying that model to Cambrocyathellus is reasonable by comparison, but its precise flow rate has not been measured from this fossil.

No feeding cells, gut contents, larvae or soft reproductive structures are known for the genus. A reconstruction should show the two walls and vertical plates as the evidence-based part; colour, tissue and current remain illustrative. This distinction is especially important when a broken cup is used to explain a filter-feeding animal whose living anatomy is absent.

Comparing its architecture with other archaeocyaths

The presence of two walls alone is not enough to identify Cambrocyathellus. Other archaeocyaths also have an outer wall, an inner wall and an intervallum. The diagnostic work compares how the pores line up with the internal plates, whether those plates connect to one another, and whether horizontal tabulae cross the cup. In Cambrocyathellus, the large-pored pseudosepta are vertical and do not form a regular floor system. In genera with true septa or a net-like intervallar skeleton, the section has a different pattern.

This is why the phosphatised specimen is useful even though it is incomplete. Its outer wall retains pore sizes and the inner surface preserves ridges where vertical elements were attached. Those ridges are not the entire plate system, so the authors kept the identification at genus level. The fragment supplements the classical thin sections rather than replacing the type material or proving every detail of the cup's construction.

What reef positions reveal

Archaeocyath cups in Siberian and Mongolian carbonate buildups have been described in exposed growth positions, crowded clusters and protected spaces within reef frameworks. These categories come from where the skeletons occur relative to microbial layers, sediment and other cups. They help describe the physical habitat, but they do not give a direct record of how the living animal responded to currents or competition.

When several cups lie close together, their relationship may represent growth, repeated settlement or the accumulation of separate remains. Branching modules can be traced in a section, yet the fossil cannot show whether all modules shared one continuous soft body. The safest reconstruction uses the observed geometry and labels the biological interpretation as an inference.