Succinipatopsidae

''Succinipatopsis'' is an extinct genus of animal from Eocene-aged Baltic amber. This animal is known from a single fossil that preserves a body with 10 pairs of stubby appendages, with a hole between the third pair. Due to its poor preservation, the placement of ''Succinipatopsis'' is contested, as there are multiple interpretations of its anatomy. Some researchers consider it an onychophoran (velvet worm) that lost its claws when fossilized. Others reject this affinity due to differences between its cuticle and its apparent lack of onychophoran-specific traits.

Discovery and naming

''Succinipatopsis'' was first mentioned in a 1996 paper by George Poinar Jr., who formally described it four years later. ''Succinipatopsis'' was found in Eocene-aged Baltic amber which dates to roughly 40 million years ago. Its location or even country of origin is unknown.

''Succinipatopsis'' is known from a single fossil that is deposited at the Poinar Amber Collection of Oregon State University in Corvallis, Oregon. The etymology of the animal's species or genus name was not provided in its description.

Description

The ''Succinipatopsis'' holotype is encased in amber and had at least 10 appendages, with the rest being cut off. Visually, the animal was poorly preserved. Its body is covered with a fine deposit and surrounded by small cracks in the amber matrix. This made examining its external appearance difficult, and it is unclear whether ''Succinipatopsis'' had annulations (rings of dermal papillae) along its trunk or appendages. While some cracks looked like spines jutting out from the body, these were probably artifacts of preservation as their placement is irregular.

The holotype had a purplish-black or black tint with no traces its color diffusing into the amber matrix. Organic compounds like alcohols and terpenes occur in tree resin (the precursor to amber) and are capable of dissolving certain pigments. Since the animal's color did not diffuse, the original description considered its pigment to be insoluble in organic solvents.

In the original description, the fossil was interpreted as the anterior (front) part of the animal, for two reasons. First, the underside had a roughly triangular opening between the third pair of visible appendages. This was interpreted as a mouth, as modern velvet worms have a mouth between their second and third segments. Second, in living velvet worms, the anus is located at either the very end of the body or between the last pair of legs. Using the framework above, the first pair of appendages were interpreted as antennae. The second pair of appendages were shorter than the antennae but longer than the rest. Because of their position on the animal, these were interpreted as slime papillae. The remaining appendages were interpreted as legs known as lobopods. These lobopods were described as simple and lacked the distinct foot, claw, and spinous pads typical of modern velvet worms. Based on a length-to-width comparison with extant onychophorans, ''Succinipatopsis'' is estimated to have 18–20 pairs of legs. The animal was also said to have large epidermal cells without protrusions.

Classification

Original classification

When first described by Poinar, ''Succinipatopsis'' was classified using a now-outdated definition of Onychophora that included various Paleozoic lobopodians. This classification is now obsolete, as modern phylogenies label this grouping "Total-Group Onychophora" which lacks many of the traditional members or do not recover it at all.

In its original description, ''Succinipatopsis'' was placed in a variety of newly created taxa. The first of these was a class named Udeonychophora that contained terrestrial Onychophorans with a ventral (bottom-facing) mouth. The class was divided into two new orders, Euonychophora and Ontonychophora. ''Succinipatopsis'' was placed in Ontonychophora due to its members having simple lobopods (no annulations of dermal papillae), potentially having claws, but lacking a specialized terminal foot. Ontonychophora also included ''Helenodora'' (put in the new family Helenodoridae) and the co-described ''Tertiapatus''. Ontonychophora was further subdivided into the superfamily Tertiapatoidea, defined by a lack of claws or spinous cushions. This clade included ''Succinipatopsis'' and ''Tertiapatus'', both placed in their own monotypic families (''Succinipatopsidae'' and ''Tertiapatidae'').

Later critiques

Grimaldi et al. (2002)

Soon after its original description, ''Succinipatopsis''s anatomy and classification came into question. In a 2002 paper, Grimaldi et al. contended the animal's placement in a new family and order, and stated that the vast majority of Baltic amber fossils came from modern families, and despite being older, a fossil it described (''Cretoperipatus'') belonged to the modern family Peripatidae. Because of this, they argued that ''Succinipatopsis'' belonging to an extinct clade was highly improbable, especially since it existed so recently.

The paper also claimed that ''Succinipatopsis''s lack of feet or claws was probably an artifact of preservation. Onychophorans are known to retract their feet, and most specimens preserved in ethanol have their feet retracted with only the claws protruding. Based on these points, the paper synonymized Udeonychophora and Ontonychophora with Euonychophora, lumping ''Succinipatopsis'', ''Tertiapatus'', and ''Helenodora'' with the extant onychophorans.

Garwood et al. (2016)

Disputes about ''Succinipatopsis'' 's classification reemerged more than a decade later in 2016. In a study by Garwood et al. describing ''Antennipatus'', the authors questioned if ''Succinipatopsis'' was an onychophoran in the first place. To justify this, the paper cited how the animal had fine projections on its skin which differed from the onychophoran cuticle and lacked any traits specific to the group.

Grimaldi et al. (2016)

Later in the same year, Grimaldi et al. disagreed with Garwood and instead considered ''Succinipatopsis'' an onychophoran. This paper focused on ''Cretoperipatus'' and the insights gained from redescribing it. While studying new fossils, the authors noticed that claws often detached from the animal's foot and could disperse throughout the amber, sometimes very far away. Because of how frequent this was, the authors reasoned that ''Succinipatopsis'' may have had claws, but that these were either trimmed away or left unnoticed. Ultimately, the study concluded that this animal should eventually be reclassified.

Paleoenvironment

References

{{Taxonbar, from1=Q7632690, from2=Q2362497, from3=Q15137956

Prehistoric onychophorans
Monotypic onychophoran genera
Onychophorans of Europe
Prehistoric protostome genera
Baltic amber
Fossil taxa described in 2000
Taxa named by George Poinar Jr.