Succinipatopsis

Succinipatopsis Temporal range: Eocene PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Onychophora
Family:	†Succinipatopsidae Poinar, 2000
Genus:	†Succinipatopsis Poinar, 2000
Species:	†S. balticus
Binomial name
†Succinipatopsis balticus Poinar, 2000

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 the 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.

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, as neither were mentioned in the papers that described or originally mentioned it.^[1][2] Succinipatopsis is known from a single fossil, with the holotype being deposited at the Poinar Amber Collection of Oregon State University in Corvallis, Oregon.^[2]

The origin of the animal's species or genus name is unknown, as neither were not mentioned in the original description.^[2]

Succinipatopsis was encased in amber and had at least 10 appendages, with the rest being cut off.^[2] Visually, the animal was poorly preserved. Its body was covered with a fine deposit and small cracks in the amber matrix. This made examining its external appearance difficult, and it is unsure if Succinipatopsis had annulations (rings of dermal papillae) along its trunk or appendages. Some of these cracks looked like spines jutting out from the body. Still, since the spines are inconsistent, these were probably artifacts of preservation.^[2] Additionally, Succinipatopsis had a purplish-black^[1] or black tint with no traces of color diffusion into the amber.^[2] For context, 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 didn’t diffuse, the original description considered its pigment to be insoluble in organic solvents.^[2]

In the original description, the fossil was interpreted as the anterior (front) part of Succinipatopsis. This was done 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 segment. Second, in living velvet worms, the anus is located at either the very end of the body or between the last pair of legs.^[2] Using the framework above, the first pair of appendages were interpreted as antennae.^[2] 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.^[2] 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 a modern velvet worm. Based on a length-to-width comparison with extant onychophorans, Succinipatopsis was estimated to have 18-20 pairs of legs.^[2] The animal was also said to have large epidermal cells without protrusions.^[2]

When first described, Succinipatopsis was classified using an old definition of Onychophora that included various Paleozoic lobopodians.^[2] This classification is now obsolete, as modern phylogenies label this grouping "Total-Group Onychophora" which lacks many of the traditional members^[3][4][5][6] or do not recover it at all.^[7][8][9]

In its original description, Succinipatopsis was placed in a variety of newly created taxa.^[2] 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.^[2] 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.^[2] 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).^[2]

Soon after its original description, Succinipatopsis's anatomy and classification came into question. This started with a 2002 paper that contested its placement in a new family and order. The paper 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.^[10] Because of this, the study argued that Succinipatopsis coming from an extinct clade was highly improbable, especially since it existed so recently.^[10] 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.^[10] Based on these points, the paper synonymized Udeonychophora and Ontonychophora with Euonychophora, lumping Succinipatopsis, Tertiapatus, and Helenodora with the extant onychophorans.^[10]

These disputes reemerged more than a decade later. In 2016, a study describing Antennipatus questioned if it was even an onychophoran in the first place. To justify this, the paper cited how Succinipatopsis had fine projections on its skin which differed from the onychophoran cuticle and lacked any traits specific to the group.^[11] However, a later study from the same year disagreed, considering Succinipatopsis an onychophoran.^[12] This paper mainly focused on Cretoperipatus and the insights gained from redescribing it. When looking at some 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.^[12] Ultimately, the study concluded that this animal should eventually be reclassified.^[12]