It's all internal anatomy

When presented with the first large alien fossil ever discovered, the mysterious Megaloxenostoma hyrotrioskjiani, our xenobiologists were baffled. Without any living relatives to look to for clues, it was very hard to piece together what this creature might have been like in life. So naturally we turned to the nemoids for help.

According to their own claims, the nemoids had studied life on a hundred worlds before humans had even figured out selective breeding, so we trusted them to have the knowhow to reconstruct our mystery beast. We were pleasantly surprised when a nemoid xenobiologist told us they knew exactly what we had on our manipulatory organs and offered to restore it for us, free of charge. 

The result was rather startling. According to the nemoid, the creature was a giant sessile filter-feeder that had evolved from freely swimming vertebrate-like animals. When we tried to inquire about some of the more confusing aspects of this restoration, especially pertaining to some of the more curious features of the fossil, the only reply we got was: "It is all internal anatomy." Of course this was just the accepted translation, and because of the convoluted nature of nemoid language an equally valid translation could have been "it is all a figment of imagination."

Since at that time it was believed that nemoids were completely devoid of a sense of humor, we trusted this restoration and it informed our study of other related fossil fragments coming from the same planet, even though many later finds were at odds with the original restoration. Of course a few decades later we found out that the nemoids were actually the most infamous pranksters in the whole galaxy.