Fact-checking Planet Dinosaur’s Onchopristis

Onchopristis size chart

A size chart comparing Planet Dinosaur‘s Onchopristis with the real largest specimen. The sawfish-like silhouette is redrawn from Scott (2012).

In 2011, Onchopristis numida made its onscreen debut in the television miniseries Planet Dinosaur. This has been the only significant appearance of a sclerorhynchoid in a documentary.1 As a result, Planet Dinosaur has shaped their public perception for better and worse. It contains a fair amount of widely-repeated information that needs clarifying or correcting. Reviewing the Onchopristis model here would be redundant. It is basically a copy of the largetooth sawfish (Pristis pristis) and I already explained why that is inaccurate in my previous posts. Instead, I will address some of the claims made about Onchopristis in the first episode (Brass & Paterson, 2011) and the companion book (Scott, 2012). These range from minor mistakes to misguided speculations to catastrophic errors.

Onchopristis was called a sawfish in the episode and the book. This common name is incorrect, as sclerorhynchoids are more closely related to skates than sawfishes and should be called sawskates (again, see my previous posts). To be fair, their relationship to skates was not known until 2019 (Villalobos-Segura et al., 2019). However, the idea that they are unrelated to sawfishes was proposed as early as 1974 (Cappetta, 1974).2 The book at least acknowledged this fact, while the episode did not.

“It wasn’t a close relative of the modern sawfish. Onchopristis went extinct at the end of the Cretaceous Period and it was another 10 million years before modern sawfishes evolved.”

The episode showed Onchopristis traveling into freshwater to breed and implied that rivers served as nurseries.

“It’s thought they migrated into freshwater rivers to breed, where the young may be safer, but the adults are exposed to new threats. With their breeding season at its height, these rivers are filled with Onchopristis.”

The book stated that it used spawning, or external fertilization, as its mode of reproduction.

Spinosaurus would wait patiently, ready to pounce as a school of Onchopristis made their way to their spawning grounds.”

This behavior was presumably inspired by the largetooth sawfish. The juveniles of the northwestern Australian population use the Fitzroy River as a nursery for 4-5 years before returning to the ocean. The adults breed and give birth near the river mouth and do not migrate up it like their offspring (Thorburn et al., 2007). Although Onchopristis did inhabit both freshwater and saltwater environments (Villalobos-Segura et al., 2021b), evidence for nurseries has not been found. All cartilaginous fishes use internal fertilization (Conrath & Musick, 2012) and sclerorhynchoids were no exception. Claspers, the male copulatory organs, are known from Ptychotrygon rostrispatula (Villalobos-Segura et al., 2021a). There is an anecdotal report of a Libanopristis hiram with embryos inside its body (Gayet et al., 2003/2012), indicating that sclerorhynchoids gave live birth like most other rays [see addendum]. 

Two specimens were presented in the episode that supposedly provided direct evidence of Spinosaurus predation on Onchopristis.

“In 2005, a spinosaur fossil was found with a sawfish vertebra stuck in a tooth socket. And another discovered in 2008 had a fragment of a sawfish barb apparently embedded in its jaw.”

The first example was also mentioned in the book, but the second was not.

“A fossilized jaw found in 1975 and known as MSNM V4047 has what is thought to be a vertebra of a sawfish known as Onchopristis embedded in the jaw itself. Although the bone was embedded after the dinosaur’s death, it indicates that the bones were deposited in the same place and hence that Spinosaurus and Onchopristis probably lived in the same habitat.”

The second example has never been published to my knowledge. The first was published by Dal Sasso et al. (2005), who described a Spinosaurus rostrum (MSNM V4047) from the Kem Kem Group of Morocco. It had a sclerorhynchoid vertebra wedged between its right 2nd premaxillary socket and tooth. Based on this position, it probably represents a feeding event and not a taphonomic effect. Dal Sasso et al. referred the vertebra to Onchopristis because it is the most common genus in the Kem Kem Group. However, the rarer Marckgrafia libyca is also present in this group (Dutheil, 2000). Since vertebrae are not diagnostic at the generic level for sclerorhynchoids, this specimen should be considered Sclerorhynchoidei indet. Currently, there is no published, direct evidence for Spinosaurus predation on Onchopristis. That being said, it is still highly likely this occurred given their frequent association (Beevor et al., 2021).

Onchopristis rostrum 4

A diagram of the measurements of the largest Onchopristis specimen, modified from Stromer (1925). 

Onchopristis was alleged to be longer than the largetooth sawfish, which can reach 7 m (Robillard & Séret, 2006), in the episode.

“This is Onchopristis, an 8 metre long giant sawfish similar to those alive today. The saw-like rostrum is lined with lethal barbs and is in itself up to 2½ metres in length.”

An even bigger size was claimed in the book, which is equivalent to large basking sharks (McClain et al., 2015).

“The 3 metre (10 feet) long, saw-like rostrum was lined with irregular, ragged-looking teeth. […] Onchopristis grew up to 10 metres (33 feet) long, from tail to the tip of its rostrum.”

The largest, published specimen of Onchopristis was a nearly complete rostrum and partial cranium described and illustrated by Stromer (1925). It was housed in the Bayerische Staatssammlung für Paläontologie und Geologie and was almost certainly destroyed by bombing in World War II.3 Stromer did not specify the exact length of the rostrum, only that it was approximately 1 m. He noted that the width of the rostrum between the posteriormost rostral denticles was about 15 cm. Scaling with this measurement results in a length of 99 cm from the end of the rostrum to the posteriormost rostral denticle. Applying that length to my Onchopristis reconstruction estimates a total length of 3.72 m for this specimen. There are rumors of rostra exceeding 2 m (Underwood et al., 2016), but other published specimens are smaller than Stromer’s (Werner, 1989; Villalobos-Segura et al., 2021b).4

I am begrudgingly grateful to Planet Dinosaur for bringing attention to Onchopristis, but I also think it caused more harm than good. Along with the anatomy of the model, the most egregious problem is the exaggerated size. These inaccuracies created the paleoart meme that Onchopristis was a giant sawfish, when in reality it was neither. I have spent considerable time and effort over the last two years to counteract this misconception. The flaws of Planet Dinosaur are unfortunately symptomatic of paleontology documentaries in general. One is not always explaining which aspects are based on fossil evidence and which are speculative. Another is the treatment of animals that are not ‘charismatic megafauna’; they are either relegated to the background or are overhyped into being megafauna. The only solution I see is to continue researching these animals and informing people about them.

Notes

1Onchopristis also had a brief cameo at the end of the 2014 documentary Bigger Than T. rex (Cohen, 2014).

2Cappetta hypothesized that sclerorhynchoids and sawfishes both descended from guitarfishes, but evolved saws independently.

3This specimen could not be relocated in the BSPG collections by either Werner (1989) or Smith et al. (2006).

4My suspicion is that these rumors came from fragments that were misscaled or from multiple rostra composited together.

Addendum (1/30/2023)

The specimen of Libanopristis with embryos was figured and described by Capasso (2003), a paper which I was unaware of when I first published this post.

References

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