Based primarily on the extremes in the diagram, showing smaller organisms at bottom, and larger at top. The argument is not only one of complexity but also of timing in history. A good many adherents to the theory of evolution posit that aquatic life preceded life on land, hence the shark is placed beneath the horse in the diagram above.
The only argument I am making is that this diagram presents evolution as a progression from smaller to greater, older to newer. The fossil record is hardly so neat as this diagram would lead one to believe. The general forms of these creatures are present to this day and living contemporaneously. The subjectivity represented in this diagram is enormous, but less so if one is predisposed to define evolution as a progression from the simple to the more complex, which is how evolution is usually understood.
It presents it as a progression from older to newer, certainly. That's a progression called time. It certainly does not present it as a progression from smaller to greater.
The fossil record is hardly so neat as this diagram would lead one to believe.
I don't think it's intended to represent the complexity of the fossil record. I think it's intended to show representative creatures from each geological era.