Heteromorphus is an extinct genus of banffiid from the lower Cambrian Chengjiang lagerstatte. It contains one broadly accepted species, "Heteromorphus confusus", as well as a proposed junior synonym, "Heteromorphus longicaudatus" that may prove to be a separate species as additional specimens are examined. A much smaller species labeled "Form A" is allied with "Heteromorphus" at the class level but has not been formally described or assigned to "Heteromorphus" itself.
Description.
Like "Banffia", "Heteromorphus" has a two-part body with a notable constriction between the parts, and a crossover that effectively reverses the dorsal and ventral sides between the anterior and posterior sections. The posterior portion is segmented, although the common presence of wrinkling makes counting the segments difficult. The anterior body shape ranges from torpedo-like to more rectangular, with a near-vertical anterior edge.
"Heteromorphus" is separated from "Banffia" by the presence of a lateral groove, similar to that found in the Vetulicolida. However, while a 2004 description claimed that four gill openings were present in the groove, later workers have not observed them. The lack of visible gills in banffiids has been speculated to indicate a burrowing lifestyle. A partial "twist" has been described in "Heteromorphus", compared to the well-developed torsion in "Banffia". While the initial description of "H. confusus" indicated an anus halfway along the ventral surface of the posterior section, later descriptions confirmed the terminal location of the anus.
Taxonomy.
"Heteromorphus" has been grouped with "Banffia" and "Skeemella" in the family Banffiidae.
The following cladogram is simplified from Mussini "et al." (2024), using the definition of Vetulicolidae from Li "et al." (2018). It shows "Heteromorphus" as part of the earliest steps, corresponding to the class Banffozoa, in the evolutionary grade leading to extant chordates.
An earlier study in 2014 placed vetulicolians as the sister-group to tunicates, but was unable to resolve any relationships among vetulicolians as a group:
"H. confusus" vs "H. longicaudatus".
"H. confusus" was originally described as "Banffia confusa" in 1997, while "H. longicaudatus" was described and assigned to the new genus "Heteromorphus" in 1999. The discoverers of "B. confusa" referred to it as "H. confusus" in 2002, accepting its assignment to "Heteromorphus" and, according to a comprehensive 2007 review of vetulicolian phylogenetics, indicating that "H. longicaudatus" is its junior synonym. However, the same review also noted substantial variation among "Heteromorphus" specimens, and that more species (including a distinct "H. longicaudatus") may exist.